1	//===- MachineBlockPlacement.cpp - Basic Block Code Layout optimization ---===//
2	//
3	// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4	// See https://llvm.org/LICENSE.txt for license information.
5	// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6	//
7	//===----------------------------------------------------------------------===//
8	//
9	// This file implements basic block placement transformations using the CFG
10	// structure and branch probability estimates.
11	//
12	// The pass strives to preserve the structure of the CFG (that is, retain
13	// a topological ordering of basic blocks) in the absence of a *strong* signal
14	// to the contrary from probabilities. However, within the CFG structure, it
15	// attempts to choose an ordering which favors placing more likely sequences of
16	// blocks adjacent to each other.
17	//
18	// The algorithm works from the inner-most loop within a function outward, and
19	// at each stage walks through the basic blocks, trying to coalesce them into
20	// sequential chains where allowed by the CFG (or demanded by heavy
21	// probabilities). Finally, it walks the blocks in topological order, and the
22	// first time it reaches a chain of basic blocks, it schedules them in the
23	// function in-order.
24	//
25	//===----------------------------------------------------------------------===//
26
27	#include "BranchFolding.h"
28	#include "llvm/ADT/ArrayRef.h"
29	#include "llvm/ADT/DenseMap.h"
30	#include "llvm/ADT/STLExtras.h"
31	#include "llvm/ADT/SetVector.h"
32	#include "llvm/ADT/SmallPtrSet.h"
33	#include "llvm/ADT/SmallVector.h"
34	#include "llvm/ADT/Statistic.h"
35	#include "llvm/Analysis/BlockFrequencyInfoImpl.h"
36	#include "llvm/Analysis/ProfileSummaryInfo.h"
37	#include "llvm/CodeGen/MBFIWrapper.h"
38	#include "llvm/CodeGen/MachineBasicBlock.h"
39	#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
40	#include "llvm/CodeGen/MachineBranchProbabilityInfo.h"
41	#include "llvm/CodeGen/MachineFunction.h"
42	#include "llvm/CodeGen/MachineFunctionPass.h"
43	#include "llvm/CodeGen/MachineLoopInfo.h"
44	#include "llvm/CodeGen/MachinePostDominators.h"
45	#include "llvm/CodeGen/MachineSizeOpts.h"
46	#include "llvm/CodeGen/TailDuplicator.h"
47	#include "llvm/CodeGen/TargetInstrInfo.h"
48	#include "llvm/CodeGen/TargetLowering.h"
49	#include "llvm/CodeGen/TargetPassConfig.h"
50	#include "llvm/CodeGen/TargetSubtargetInfo.h"
51	#include "llvm/IR/DebugLoc.h"
52	#include "llvm/IR/Function.h"
53	#include "llvm/IR/PrintPasses.h"
54	#include "llvm/InitializePasses.h"
55	#include "llvm/Pass.h"
56	#include "llvm/Support/Allocator.h"
57	#include "llvm/Support/BlockFrequency.h"
58	#include "llvm/Support/BranchProbability.h"
59	#include "llvm/Support/CodeGen.h"
60	#include "llvm/Support/CommandLine.h"
61	#include "llvm/Support/Compiler.h"
62	#include "llvm/Support/Debug.h"
63	#include "llvm/Support/raw_ostream.h"
64	#include "llvm/Target/TargetMachine.h"
65	#include "llvm/Transforms/Utils/CodeLayout.h"
66	#include <algorithm>
67	#include <cassert>
68	#include <cstdint>
69	#include <iterator>
70	#include <memory>
71	#include <string>
72	#include <tuple>
73	#include <utility>
74	#include <vector>
75
76	using namespace llvm;
77
78	#define DEBUG_TYPE "block-placement"
79
80	STATISTIC(NumCondBranches, "Number of conditional branches");
81	STATISTIC(NumUncondBranches, "Number of unconditional branches");
82	STATISTIC(CondBranchTakenFreq,
83	"Potential frequency of taking conditional branches");
84	STATISTIC(UncondBranchTakenFreq,
85	"Potential frequency of taking unconditional branches");
86
87	static cl::opt<unsigned> AlignAllBlock(
88	"align-all-blocks",
89	cl::desc("Force the alignment of all blocks in the function in log2 format "
90	"(e.g 4 means align on 16B boundaries)."),
91	cl::init(Val: 0), cl::Hidden);
92
93	static cl::opt<unsigned> AlignAllNonFallThruBlocks(
94	"align-all-nofallthru-blocks",
95	cl::desc("Force the alignment of all blocks that have no fall-through "
96	"predecessors (i.e. don't add nops that are executed). In log2 "
97	"format (e.g 4 means align on 16B boundaries)."),
98	cl::init(Val: 0), cl::Hidden);
99
100	static cl::opt<unsigned> MaxBytesForAlignmentOverride(
101	"max-bytes-for-alignment",
102	cl::desc("Forces the maximum bytes allowed to be emitted when padding for "
103	"alignment"),
104	cl::init(Val: 0), cl::Hidden);
105
106	// FIXME: Find a good default for this flag and remove the flag.
107	static cl::opt<unsigned> ExitBlockBias(
108	"block-placement-exit-block-bias",
109	cl::desc("Block frequency percentage a loop exit block needs "
110	"over the original exit to be considered the new exit."),
111	cl::init(Val: 0), cl::Hidden);
112
113	// Definition:
114	// - Outlining: placement of a basic block outside the chain or hot path.
115
116	static cl::opt<unsigned> LoopToColdBlockRatio(
117	"loop-to-cold-block-ratio",
118	cl::desc("Outline loop blocks from loop chain if (frequency of loop) / "
119	"(frequency of block) is greater than this ratio"),
120	cl::init(Val: 5), cl::Hidden);
121
122	static cl::opt<bool> ForceLoopColdBlock(
123	"force-loop-cold-block",
124	cl::desc("Force outlining cold blocks from loops."),
125	cl::init(Val: false), cl::Hidden);
126
127	static cl::opt<bool>
128	PreciseRotationCost("precise-rotation-cost",
129	cl::desc("Model the cost of loop rotation more "
130	"precisely by using profile data."),
131	cl::init(Val: false), cl::Hidden);
132
133	static cl::opt<bool>
134	ForcePreciseRotationCost("force-precise-rotation-cost",
135	cl::desc("Force the use of precise cost "
136	"loop rotation strategy."),
137	cl::init(Val: false), cl::Hidden);
138
139	static cl::opt<unsigned> MisfetchCost(
140	"misfetch-cost",
141	cl::desc("Cost that models the probabilistic risk of an instruction "
142	"misfetch due to a jump comparing to falling through, whose cost "
143	"is zero."),
144	cl::init(Val: 1), cl::Hidden);
145
146	static cl::opt<unsigned> JumpInstCost("jump-inst-cost",
147	cl::desc("Cost of jump instructions."),
148	cl::init(Val: 1), cl::Hidden);
149	static cl::opt<bool>
150	TailDupPlacement("tail-dup-placement",
151	cl::desc("Perform tail duplication during placement. "
152	"Creates more fallthrough opportunites in "
153	"outline branches."),
154	cl::init(Val: true), cl::Hidden);
155
156	static cl::opt<bool>
157	BranchFoldPlacement("branch-fold-placement",
158	cl::desc("Perform branch folding during placement. "
159	"Reduces code size."),
160	cl::init(Val: true), cl::Hidden);
161
162	// Heuristic for tail duplication.
163	static cl::opt<unsigned> TailDupPlacementThreshold(
164	"tail-dup-placement-threshold",
165	cl::desc("Instruction cutoff for tail duplication during layout. "
166	"Tail merging during layout is forced to have a threshold "
167	"that won't conflict."), cl::init(Val: 2),
168	cl::Hidden);
169
170	// Heuristic for aggressive tail duplication.
171	static cl::opt<unsigned> TailDupPlacementAggressiveThreshold(
172	"tail-dup-placement-aggressive-threshold",
173	cl::desc("Instruction cutoff for aggressive tail duplication during "
174	"layout. Used at -O3. Tail merging during layout is forced to "
175	"have a threshold that won't conflict."), cl::init(Val: 4),
176	cl::Hidden);
177
178	// Heuristic for tail duplication.
179	static cl::opt<unsigned> TailDupPlacementPenalty(
180	"tail-dup-placement-penalty",
181	cl::desc("Cost penalty for blocks that can avoid breaking CFG by copying. "
182	"Copying can increase fallthrough, but it also increases icache "
183	"pressure. This parameter controls the penalty to account for that. "
184	"Percent as integer."),
185	cl::init(Val: 2),
186	cl::Hidden);
187
188	// Heuristic for tail duplication if profile count is used in cost model.
189	static cl::opt<unsigned> TailDupProfilePercentThreshold(
190	"tail-dup-profile-percent-threshold",
191	cl::desc("If profile count information is used in tail duplication cost "
192	"model, the gained fall through number from tail duplication "
193	"should be at least this percent of hot count."),
194	cl::init(Val: 50), cl::Hidden);
195
196	// Heuristic for triangle chains.
197	static cl::opt<unsigned> TriangleChainCount(
198	"triangle-chain-count",
199	cl::desc("Number of triangle-shaped-CFG's that need to be in a row for the "
200	"triangle tail duplication heuristic to kick in. 0 to disable."),
201	cl::init(Val: 2),
202	cl::Hidden);
203
204	// Use case: When block layout is visualized after MBP pass, the basic blocks
205	// are labeled in layout order; meanwhile blocks could be numbered in a
206	// different order. It's hard to map between the graph and pass output.
207	// With this option on, the basic blocks are renumbered in function layout
208	// order. For debugging only.
209	static cl::opt<bool> RenumberBlocksBeforeView(
210	"renumber-blocks-before-view",
211	cl::desc(
212	"If true, basic blocks are re-numbered before MBP layout is printed "
213	"into a dot graph. Only used when a function is being printed."),
214	cl::init(Val: false), cl::Hidden);
215
216	namespace llvm {
217	extern cl::opt<bool> EnableExtTspBlockPlacement;
218	extern cl::opt<bool> ApplyExtTspWithoutProfile;
219	extern cl::opt<unsigned> StaticLikelyProb;
220	extern cl::opt<unsigned> ProfileLikelyProb;
221
222	// Internal option used to control BFI display only after MBP pass.
223	// Defined in CodeGen/MachineBlockFrequencyInfo.cpp:
224	// -view-block-layout-with-bfi=
225	extern cl::opt<GVDAGType> ViewBlockLayoutWithBFI;
226
227	// Command line option to specify the name of the function for CFG dump
228	// Defined in Analysis/BlockFrequencyInfo.cpp: -view-bfi-func-name=
229	extern cl::opt<std::string> ViewBlockFreqFuncName;
230	} // namespace llvm
231
232	namespace {
233
234	class BlockChain;
235
236	/// Type for our function-wide basic block -> block chain mapping.
237	using BlockToChainMapType = DenseMap<const MachineBasicBlock *, BlockChain *>;
238
239	/// A chain of blocks which will be laid out contiguously.
240	///
241	/// This is the datastructure representing a chain of consecutive blocks that
242	/// are profitable to layout together in order to maximize fallthrough
243	/// probabilities and code locality. We also can use a block chain to represent
244	/// a sequence of basic blocks which have some external (correctness)
245	/// requirement for sequential layout.
246	///
247	/// Chains can be built around a single basic block and can be merged to grow
248	/// them. They participate in a block-to-chain mapping, which is updated
249	/// automatically as chains are merged together.
250	class BlockChain {
251	/// The sequence of blocks belonging to this chain.
252	///
253	/// This is the sequence of blocks for a particular chain. These will be laid
254	/// out in-order within the function.
255	SmallVector<MachineBasicBlock *, 4> Blocks;
256
257	/// A handle to the function-wide basic block to block chain mapping.
258	///
259	/// This is retained in each block chain to simplify the computation of child
260	/// block chains for SCC-formation and iteration. We store the edges to child
261	/// basic blocks, and map them back to their associated chains using this
262	/// structure.
263	BlockToChainMapType &BlockToChain;
264
265	public:
266	/// Construct a new BlockChain.
267	///
268	/// This builds a new block chain representing a single basic block in the
269	/// function. It also registers itself as the chain that block participates
270	/// in with the BlockToChain mapping.
271	BlockChain(BlockToChainMapType &BlockToChain, MachineBasicBlock *BB)
272	: Blocks(1, BB), BlockToChain(BlockToChain) {
273	assert(BB && "Cannot create a chain with a null basic block");
274	BlockToChain[BB] = this;
275	}
276
277	/// Iterator over blocks within the chain.
278	using iterator = SmallVectorImpl<MachineBasicBlock *>::iterator;
279	using const_iterator = SmallVectorImpl<MachineBasicBlock *>::const_iterator;
280
281	/// Beginning of blocks within the chain.
282	iterator begin() { return Blocks.begin(); }
283	const_iterator begin() const { return Blocks.begin(); }
284
285	/// End of blocks within the chain.
286	iterator end() { return Blocks.end(); }
287	const_iterator end() const { return Blocks.end(); }
288
289	bool remove(MachineBasicBlock* BB) {
290	for(iterator i = begin(); i != end(); ++i) {
291	if (*i == BB) {
292	Blocks.erase(CI: i);
293	return true;
294	}
295	}
296	return false;
297	}
298
299	/// Merge a block chain into this one.
300	///
301	/// This routine merges a block chain into this one. It takes care of forming
302	/// a contiguous sequence of basic blocks, updating the edge list, and
303	/// updating the block -> chain mapping. It does not free or tear down the
304	/// old chain, but the old chain's block list is no longer valid.
305	void merge(MachineBasicBlock *BB, BlockChain *Chain) {
306	assert(BB && "Can't merge a null block.");
307	assert(!Blocks.empty() && "Can't merge into an empty chain.");
308
309	// Fast path in case we don't have a chain already.
310	if (!Chain) {
311	assert(!BlockToChain[BB] &&
312	"Passed chain is null, but BB has entry in BlockToChain.");
313	Blocks.push_back(Elt: BB);
314	BlockToChain[BB] = this;
315	return;
316	}
317
318	assert(BB == *Chain->begin() && "Passed BB is not head of Chain.");
319	assert(Chain->begin() != Chain->end());
320
321	// Update the incoming blocks to point to this chain, and add them to the
322	// chain structure.
323	for (MachineBasicBlock *ChainBB : *Chain) {
324	Blocks.push_back(Elt: ChainBB);
325	assert(BlockToChain[ChainBB] == Chain && "Incoming blocks not in chain.");
326	BlockToChain[ChainBB] = this;
327	}
328	}
329
330	#ifndef NDEBUG
331	/// Dump the blocks in this chain.
332	LLVM_DUMP_METHOD void dump() {
333	for (MachineBasicBlock *MBB : *this)
334	MBB->dump();
335	}
336	#endif // NDEBUG
337
338	/// Count of predecessors of any block within the chain which have not
339	/// yet been scheduled. In general, we will delay scheduling this chain
340	/// until those predecessors are scheduled (or we find a sufficiently good
341	/// reason to override this heuristic.) Note that when forming loop chains,
342	/// blocks outside the loop are ignored and treated as if they were already
343	/// scheduled.
344	///
345	/// Note: This field is reinitialized multiple times - once for each loop,
346	/// and then once for the function as a whole.
347	unsigned UnscheduledPredecessors = 0;
348	};
349
350	class MachineBlockPlacement : public MachineFunctionPass {
351	/// A type for a block filter set.
352	using BlockFilterSet = SmallSetVector<const MachineBasicBlock *, 16>;
353
354	/// Pair struct containing basic block and taildup profitability
355	struct BlockAndTailDupResult {
356	MachineBasicBlock *BB = nullptr;
357	bool ShouldTailDup;
358	};
359
360	/// Triple struct containing edge weight and the edge.
361	struct WeightedEdge {
362	BlockFrequency Weight;
363	MachineBasicBlock *Src = nullptr;
364	MachineBasicBlock *Dest = nullptr;
365	};
366
367	/// work lists of blocks that are ready to be laid out
368	SmallVector<MachineBasicBlock *, 16> BlockWorkList;
369	SmallVector<MachineBasicBlock *, 16> EHPadWorkList;
370
371	/// Edges that have already been computed as optimal.
372	DenseMap<const MachineBasicBlock *, BlockAndTailDupResult> ComputedEdges;
373
374	/// Machine Function
375	MachineFunction *F = nullptr;
376
377	/// A handle to the branch probability pass.
378	const MachineBranchProbabilityInfo *MBPI = nullptr;
379
380	/// A handle to the function-wide block frequency pass.
381	std::unique_ptr<MBFIWrapper> MBFI;
382
383	/// A handle to the loop info.
384	MachineLoopInfo *MLI = nullptr;
385
386	/// Preferred loop exit.
387	/// Member variable for convenience. It may be removed by duplication deep
388	/// in the call stack.
389	MachineBasicBlock *PreferredLoopExit = nullptr;
390
391	/// A handle to the target's instruction info.
392	const TargetInstrInfo *TII = nullptr;
393
394	/// A handle to the target's lowering info.
395	const TargetLoweringBase *TLI = nullptr;
396
397	/// A handle to the post dominator tree.
398	MachinePostDominatorTree *MPDT = nullptr;
399
400	ProfileSummaryInfo *PSI = nullptr;
401
402	/// Duplicator used to duplicate tails during placement.
403	///
404	/// Placement decisions can open up new tail duplication opportunities, but
405	/// since tail duplication affects placement decisions of later blocks, it
406	/// must be done inline.
407	TailDuplicator TailDup;
408
409	/// Partial tail duplication threshold.
410	BlockFrequency DupThreshold;
411
412	/// True: use block profile count to compute tail duplication cost.
413	/// False: use block frequency to compute tail duplication cost.
414	bool UseProfileCount = false;
415
416	/// Allocator and owner of BlockChain structures.
417	///
418	/// We build BlockChains lazily while processing the loop structure of
419	/// a function. To reduce malloc traffic, we allocate them using this
420	/// slab-like allocator, and destroy them after the pass completes. An
421	/// important guarantee is that this allocator produces stable pointers to
422	/// the chains.
423	SpecificBumpPtrAllocator<BlockChain> ChainAllocator;
424
425	/// Function wide BasicBlock to BlockChain mapping.
426	///
427	/// This mapping allows efficiently moving from any given basic block to the
428	/// BlockChain it participates in, if any. We use it to, among other things,
429	/// allow implicitly defining edges between chains as the existing edges
430	/// between basic blocks.
431	DenseMap<const MachineBasicBlock *, BlockChain *> BlockToChain;
432
433	#ifndef NDEBUG
434	/// The set of basic blocks that have terminators that cannot be fully
435	/// analyzed. These basic blocks cannot be re-ordered safely by
436	/// MachineBlockPlacement, and we must preserve physical layout of these
437	/// blocks and their successors through the pass.
438	SmallPtrSet<MachineBasicBlock *, 4> BlocksWithUnanalyzableExits;
439	#endif
440
441	/// Get block profile count or frequency according to UseProfileCount.
442	/// The return value is used to model tail duplication cost.
443	BlockFrequency getBlockCountOrFrequency(const MachineBasicBlock *BB) {
444	if (UseProfileCount) {
445	auto Count = MBFI->getBlockProfileCount(MBB: BB);
446	if (Count)
447	return BlockFrequency(*Count);
448	else
449	return BlockFrequency(0);
450	} else
451	return MBFI->getBlockFreq(MBB: BB);
452	}
453
454	/// Scale the DupThreshold according to basic block size.
455	BlockFrequency scaleThreshold(MachineBasicBlock *BB);
456	void initDupThreshold();
457
458	/// Decrease the UnscheduledPredecessors count for all blocks in chain, and
459	/// if the count goes to 0, add them to the appropriate work list.
460	void markChainSuccessors(
461	const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB,
462	const BlockFilterSet *BlockFilter = nullptr);
463
464	/// Decrease the UnscheduledPredecessors count for a single block, and
465	/// if the count goes to 0, add them to the appropriate work list.
466	void markBlockSuccessors(
467	const BlockChain &Chain, const MachineBasicBlock *BB,
468	const MachineBasicBlock *LoopHeaderBB,
469	const BlockFilterSet *BlockFilter = nullptr);
470
471	BranchProbability
472	collectViableSuccessors(
473	const MachineBasicBlock *BB, const BlockChain &Chain,
474	const BlockFilterSet *BlockFilter,
475	SmallVector<MachineBasicBlock *, 4> &Successors);
476	bool isBestSuccessor(MachineBasicBlock *BB, MachineBasicBlock *Pred,
477	BlockFilterSet *BlockFilter);
478	void findDuplicateCandidates(SmallVectorImpl<MachineBasicBlock *> &Candidates,
479	MachineBasicBlock *BB,
480	BlockFilterSet *BlockFilter);
481	bool repeatedlyTailDuplicateBlock(
482	MachineBasicBlock *BB, MachineBasicBlock *&LPred,
483	const MachineBasicBlock *LoopHeaderBB,
484	BlockChain &Chain, BlockFilterSet *BlockFilter,
485	MachineFunction::iterator &PrevUnplacedBlockIt);
486	bool maybeTailDuplicateBlock(
487	MachineBasicBlock *BB, MachineBasicBlock *LPred,
488	BlockChain &Chain, BlockFilterSet *BlockFilter,
489	MachineFunction::iterator &PrevUnplacedBlockIt,
490	bool &DuplicatedToLPred);
491	bool hasBetterLayoutPredecessor(
492	const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
493	const BlockChain &SuccChain, BranchProbability SuccProb,
494	BranchProbability RealSuccProb, const BlockChain &Chain,
495	const BlockFilterSet *BlockFilter);
496	BlockAndTailDupResult selectBestSuccessor(
497	const MachineBasicBlock *BB, const BlockChain &Chain,
498	const BlockFilterSet *BlockFilter);
499	MachineBasicBlock *selectBestCandidateBlock(
500	const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList);
501	MachineBasicBlock *getFirstUnplacedBlock(
502	const BlockChain &PlacedChain,
503	MachineFunction::iterator &PrevUnplacedBlockIt,
504	const BlockFilterSet *BlockFilter);
505
506	/// Add a basic block to the work list if it is appropriate.
507	///
508	/// If the optional parameter BlockFilter is provided, only MBB
509	/// present in the set will be added to the worklist. If nullptr
510	/// is provided, no filtering occurs.
511	void fillWorkLists(const MachineBasicBlock *MBB,
512	SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
513	const BlockFilterSet *BlockFilter);
514
515	void buildChain(const MachineBasicBlock *BB, BlockChain &Chain,
516	BlockFilterSet *BlockFilter = nullptr);
517	bool canMoveBottomBlockToTop(const MachineBasicBlock *BottomBlock,
518	const MachineBasicBlock *OldTop);
519	bool hasViableTopFallthrough(const MachineBasicBlock *Top,
520	const BlockFilterSet &LoopBlockSet);
521	BlockFrequency TopFallThroughFreq(const MachineBasicBlock *Top,
522	const BlockFilterSet &LoopBlockSet);
523	BlockFrequency FallThroughGains(const MachineBasicBlock *NewTop,
524	const MachineBasicBlock *OldTop,
525	const MachineBasicBlock *ExitBB,
526	const BlockFilterSet &LoopBlockSet);
527	MachineBasicBlock *findBestLoopTopHelper(MachineBasicBlock *OldTop,
528	const MachineLoop &L, const BlockFilterSet &LoopBlockSet);
529	MachineBasicBlock *findBestLoopTop(
530	const MachineLoop &L, const BlockFilterSet &LoopBlockSet);
531	MachineBasicBlock *findBestLoopExit(
532	const MachineLoop &L, const BlockFilterSet &LoopBlockSet,
533	BlockFrequency &ExitFreq);
534	BlockFilterSet collectLoopBlockSet(const MachineLoop &L);
535	void buildLoopChains(const MachineLoop &L);
536	void rotateLoop(
537	BlockChain &LoopChain, const MachineBasicBlock *ExitingBB,
538	BlockFrequency ExitFreq, const BlockFilterSet &LoopBlockSet);
539	void rotateLoopWithProfile(
540	BlockChain &LoopChain, const MachineLoop &L,
541	const BlockFilterSet &LoopBlockSet);
542	void buildCFGChains();
543	void optimizeBranches();
544	void alignBlocks();
545	/// Returns true if a block should be tail-duplicated to increase fallthrough
546	/// opportunities.
547	bool shouldTailDuplicate(MachineBasicBlock *BB);
548	/// Check the edge frequencies to see if tail duplication will increase
549	/// fallthroughs.
550	bool isProfitableToTailDup(
551	const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
552	BranchProbability QProb,
553	const BlockChain &Chain, const BlockFilterSet *BlockFilter);
554
555	/// Check for a trellis layout.
556	bool isTrellis(const MachineBasicBlock *BB,
557	const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
558	const BlockChain &Chain, const BlockFilterSet *BlockFilter);
559
560	/// Get the best successor given a trellis layout.
561	BlockAndTailDupResult getBestTrellisSuccessor(
562	const MachineBasicBlock *BB,
563	const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
564	BranchProbability AdjustedSumProb, const BlockChain &Chain,
565	const BlockFilterSet *BlockFilter);
566
567	/// Get the best pair of non-conflicting edges.
568	static std::pair<WeightedEdge, WeightedEdge> getBestNonConflictingEdges(
569	const MachineBasicBlock *BB,
570	MutableArrayRef<SmallVector<WeightedEdge, 8>> Edges);
571
572	/// Returns true if a block can tail duplicate into all unplaced
573	/// predecessors. Filters based on loop.
574	bool canTailDuplicateUnplacedPreds(
575	const MachineBasicBlock *BB, MachineBasicBlock *Succ,
576	const BlockChain &Chain, const BlockFilterSet *BlockFilter);
577
578	/// Find chains of triangles to tail-duplicate where a global analysis works,
579	/// but a local analysis would not find them.
580	void precomputeTriangleChains();
581
582	/// Apply a post-processing step optimizing block placement.
583	void applyExtTsp();
584
585	/// Modify the existing block placement in the function and adjust all jumps.
586	void assignBlockOrder(const std::vector<const MachineBasicBlock *> &NewOrder);
587
588	/// Create a single CFG chain from the current block order.
589	void createCFGChainExtTsp();
590
591	public:
592	static char ID; // Pass identification, replacement for typeid
593
594	MachineBlockPlacement() : MachineFunctionPass(ID) {
595	initializeMachineBlockPlacementPass(*PassRegistry::getPassRegistry());
596	}
597
598	bool runOnMachineFunction(MachineFunction &F) override;
599
600	bool allowTailDupPlacement() const {
601	assert(F);
602	return TailDupPlacement && !F->getTarget().requiresStructuredCFG();
603	}
604
605	void getAnalysisUsage(AnalysisUsage &AU) const override {
606	AU.addRequired<MachineBranchProbabilityInfo>();
607	AU.addRequired<MachineBlockFrequencyInfo>();
608	if (TailDupPlacement)
609	AU.addRequired<MachinePostDominatorTree>();
610	AU.addRequired<MachineLoopInfo>();
611	AU.addRequired<ProfileSummaryInfoWrapperPass>();
612	AU.addRequired<TargetPassConfig>();
613	MachineFunctionPass::getAnalysisUsage(AU);
614	}
615	};
616
617	} // end anonymous namespace
618
619	char MachineBlockPlacement::ID = 0;
620
621	char &llvm::MachineBlockPlacementID = MachineBlockPlacement::ID;
622
623	INITIALIZE_PASS_BEGIN(MachineBlockPlacement, DEBUG_TYPE,
624	"Branch Probability Basic Block Placement", false, false)
625	INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
626	INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
627	INITIALIZE_PASS_DEPENDENCY(MachinePostDominatorTree)
628	INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
629	INITIALIZE_PASS_DEPENDENCY(ProfileSummaryInfoWrapperPass)
630	INITIALIZE_PASS_END(MachineBlockPlacement, DEBUG_TYPE,
631	"Branch Probability Basic Block Placement", false, false)
632
633	#ifndef NDEBUG
634	/// Helper to print the name of a MBB.
635	///
636	/// Only used by debug logging.
637	static std::string getBlockName(const MachineBasicBlock *BB) {
638	std::string Result;
639	raw_string_ostream OS(Result);
640	OS << printMBBReference(MBB: *BB);
641	OS << " ('" << BB->getName() << "')";
642	OS.flush();
643	return Result;
644	}
645	#endif
646
647	/// Mark a chain's successors as having one fewer preds.
648	///
649	/// When a chain is being merged into the "placed" chain, this routine will
650	/// quickly walk the successors of each block in the chain and mark them as
651	/// having one fewer active predecessor. It also adds any successors of this
652	/// chain which reach the zero-predecessor state to the appropriate worklist.
653	void MachineBlockPlacement::markChainSuccessors(
654	const BlockChain &Chain, const MachineBasicBlock *LoopHeaderBB,
655	const BlockFilterSet *BlockFilter) {
656	// Walk all the blocks in this chain, marking their successors as having
657	// a predecessor placed.
658	for (MachineBasicBlock *MBB : Chain) {
659	markBlockSuccessors(Chain, BB: MBB, LoopHeaderBB, BlockFilter);
660	}
661	}
662
663	/// Mark a single block's successors as having one fewer preds.
664	///
665	/// Under normal circumstances, this is only called by markChainSuccessors,
666	/// but if a block that was to be placed is completely tail-duplicated away,
667	/// and was duplicated into the chain end, we need to redo markBlockSuccessors
668	/// for just that block.
669	void MachineBlockPlacement::markBlockSuccessors(
670	const BlockChain &Chain, const MachineBasicBlock *MBB,
671	const MachineBasicBlock *LoopHeaderBB, const BlockFilterSet *BlockFilter) {
672	// Add any successors for which this is the only un-placed in-loop
673	// predecessor to the worklist as a viable candidate for CFG-neutral
674	// placement. No subsequent placement of this block will violate the CFG
675	// shape, so we get to use heuristics to choose a favorable placement.
676	for (MachineBasicBlock *Succ : MBB->successors()) {
677	if (BlockFilter && !BlockFilter->count(key: Succ))
678	continue;
679	BlockChain &SuccChain = *BlockToChain[Succ];
680	// Disregard edges within a fixed chain, or edges to the loop header.
681	if (&Chain == &SuccChain || Succ == LoopHeaderBB)
682	continue;
683
684	// This is a cross-chain edge that is within the loop, so decrement the
685	// loop predecessor count of the destination chain.
686	if (SuccChain.UnscheduledPredecessors == 0 ||
687	--SuccChain.UnscheduledPredecessors > 0)
688	continue;
689
690	auto *NewBB = *SuccChain.begin();
691	if (NewBB->isEHPad())
692	EHPadWorkList.push_back(Elt: NewBB);
693	else
694	BlockWorkList.push_back(Elt: NewBB);
695	}
696	}
697
698	/// This helper function collects the set of successors of block
699	/// \p BB that are allowed to be its layout successors, and return
700	/// the total branch probability of edges from \p BB to those
701	/// blocks.
702	BranchProbability MachineBlockPlacement::collectViableSuccessors(
703	const MachineBasicBlock *BB, const BlockChain &Chain,
704	const BlockFilterSet *BlockFilter,
705	SmallVector<MachineBasicBlock *, 4> &Successors) {
706	// Adjust edge probabilities by excluding edges pointing to blocks that is
707	// either not in BlockFilter or is already in the current chain. Consider the
708	// following CFG:
709	//
710	// --->A
711	// | / \
712	// | B C
713	// | \ / \
714	// ----D E
715	//
716	// Assume A->C is very hot (>90%), and C->D has a 50% probability, then after
717	// A->C is chosen as a fall-through, D won't be selected as a successor of C
718	// due to CFG constraint (the probability of C->D is not greater than
719	// HotProb to break topo-order). If we exclude E that is not in BlockFilter
720	// when calculating the probability of C->D, D will be selected and we
721	// will get A C D B as the layout of this loop.
722	auto AdjustedSumProb = BranchProbability::getOne();
723	for (MachineBasicBlock *Succ : BB->successors()) {
724	bool SkipSucc = false;
725	if (Succ->isEHPad() || (BlockFilter && !BlockFilter->count(key: Succ))) {
726	SkipSucc = true;
727	} else {
728	BlockChain *SuccChain = BlockToChain[Succ];
729	if (SuccChain == &Chain) {
730	SkipSucc = true;
731	} else if (Succ != *SuccChain->begin()) {
732	LLVM_DEBUG(dbgs() << " " << getBlockName(Succ)
733	<< " -> Mid chain!\n");
734	continue;
735	}
736	}
737	if (SkipSucc)
738	AdjustedSumProb -= MBPI->getEdgeProbability(Src: BB, Dst: Succ);
739	else
740	Successors.push_back(Elt: Succ);
741	}
742
743	return AdjustedSumProb;
744	}
745
746	/// The helper function returns the branch probability that is adjusted
747	/// or normalized over the new total \p AdjustedSumProb.
748	static BranchProbability
749	getAdjustedProbability(BranchProbability OrigProb,
750	BranchProbability AdjustedSumProb) {
751	BranchProbability SuccProb;
752	uint32_t SuccProbN = OrigProb.getNumerator();
753	uint32_t SuccProbD = AdjustedSumProb.getNumerator();
754	if (SuccProbN >= SuccProbD)
755	SuccProb = BranchProbability::getOne();
756	else
757	SuccProb = BranchProbability(SuccProbN, SuccProbD);
758
759	return SuccProb;
760	}
761
762	/// Check if \p BB has exactly the successors in \p Successors.
763	static bool
764	hasSameSuccessors(MachineBasicBlock &BB,
765	SmallPtrSetImpl<const MachineBasicBlock *> &Successors) {
766	if (BB.succ_size() != Successors.size())
767	return false;
768	// We don't want to count self-loops
769	if (Successors.count(Ptr: &BB))
770	return false;
771	for (MachineBasicBlock *Succ : BB.successors())
772	if (!Successors.count(Ptr: Succ))
773	return false;
774	return true;
775	}
776
777	/// Check if a block should be tail duplicated to increase fallthrough
778	/// opportunities.
779	/// \p BB Block to check.
780	bool MachineBlockPlacement::shouldTailDuplicate(MachineBasicBlock *BB) {
781	// Blocks with single successors don't create additional fallthrough
782	// opportunities. Don't duplicate them. TODO: When conditional exits are
783	// analyzable, allow them to be duplicated.
784	bool IsSimple = TailDup.isSimpleBB(TailBB: BB);
785
786	if (BB->succ_size() == 1)
787	return false;
788	return TailDup.shouldTailDuplicate(IsSimple, TailBB&: *BB);
789	}
790
791	/// Compare 2 BlockFrequency's with a small penalty for \p A.
792	/// In order to be conservative, we apply a X% penalty to account for
793	/// increased icache pressure and static heuristics. For small frequencies
794	/// we use only the numerators to improve accuracy. For simplicity, we assume the
795	/// penalty is less than 100%
796	/// TODO(iteratee): Use 64-bit fixed point edge frequencies everywhere.
797	static bool greaterWithBias(BlockFrequency A, BlockFrequency B,
798	BlockFrequency EntryFreq) {
799	BranchProbability ThresholdProb(TailDupPlacementPenalty, 100);
800	BlockFrequency Gain = A - B;
801	return (Gain / ThresholdProb) >= EntryFreq;
802	}
803
804	/// Check the edge frequencies to see if tail duplication will increase
805	/// fallthroughs. It only makes sense to call this function when
806	/// \p Succ would not be chosen otherwise. Tail duplication of \p Succ is
807	/// always locally profitable if we would have picked \p Succ without
808	/// considering duplication.
809	bool MachineBlockPlacement::isProfitableToTailDup(
810	const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
811	BranchProbability QProb,
812	const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
813	// We need to do a probability calculation to make sure this is profitable.
814	// First: does succ have a successor that post-dominates? This affects the
815	// calculation. The 2 relevant cases are:
816	// BB BB
817	// | \Qout | \Qout
818	// P| C |P C
819	// = C' = C'
820	// | /Qin | /Qin
821	// | / | /
822	// Succ Succ
823	// / \ | \ V
824	// U/ =V |U \
825	// / \ = D
826	// D E | /
827	// | /
828	// |/
829	// PDom
830	// '=' : Branch taken for that CFG edge
831	// In the second case, Placing Succ while duplicating it into C prevents the
832	// fallthrough of Succ into either D or PDom, because they now have C as an
833	// unplaced predecessor
834
835	// Start by figuring out which case we fall into
836	MachineBasicBlock *PDom = nullptr;
837	SmallVector<MachineBasicBlock *, 4> SuccSuccs;
838	// Only scan the relevant successors
839	auto AdjustedSuccSumProb =
840	collectViableSuccessors(BB: Succ, Chain, BlockFilter, Successors&: SuccSuccs);
841	BranchProbability PProb = MBPI->getEdgeProbability(Src: BB, Dst: Succ);
842	auto BBFreq = MBFI->getBlockFreq(MBB: BB);
843	auto SuccFreq = MBFI->getBlockFreq(MBB: Succ);
844	BlockFrequency P = BBFreq * PProb;
845	BlockFrequency Qout = BBFreq * QProb;
846	BlockFrequency EntryFreq = MBFI->getEntryFreq();
847	// If there are no more successors, it is profitable to copy, as it strictly
848	// increases fallthrough.
849	if (SuccSuccs.size() == 0)
850	return greaterWithBias(A: P, B: Qout, EntryFreq);
851
852	auto BestSuccSucc = BranchProbability::getZero();
853	// Find the PDom or the best Succ if no PDom exists.
854	for (MachineBasicBlock *SuccSucc : SuccSuccs) {
855	auto Prob = MBPI->getEdgeProbability(Src: Succ, Dst: SuccSucc);
856	if (Prob > BestSuccSucc)
857	BestSuccSucc = Prob;
858	if (PDom == nullptr)
859	if (MPDT->dominates(A: SuccSucc, B: Succ)) {
860	PDom = SuccSucc;
861	break;
862	}
863	}
864	// For the comparisons, we need to know Succ's best incoming edge that isn't
865	// from BB.
866	auto SuccBestPred = BlockFrequency(0);
867	for (MachineBasicBlock *SuccPred : Succ->predecessors()) {
868	if (SuccPred == Succ || SuccPred == BB
869	|| BlockToChain[SuccPred] == &Chain
870	|| (BlockFilter && !BlockFilter->count(key: SuccPred)))
871	continue;
872	auto Freq = MBFI->getBlockFreq(MBB: SuccPred)
873	* MBPI->getEdgeProbability(Src: SuccPred, Dst: Succ);
874	if (Freq > SuccBestPred)
875	SuccBestPred = Freq;
876	}
877	// Qin is Succ's best unplaced incoming edge that isn't BB
878	BlockFrequency Qin = SuccBestPred;
879	// If it doesn't have a post-dominating successor, here is the calculation:
880	// BB BB
881	// | \Qout | \
882	// P| C | =
883	// = C' | C
884	// | /Qin | |
885	// | / | C' (+Succ)
886	// Succ Succ /|
887	// / \ | \/ |
888	// U/ =V | == |
889	// / \ | / \|
890	// D E D E
891	// '=' : Branch taken for that CFG edge
892	// Cost in the first case is: P + V
893	// For this calculation, we always assume P > Qout. If Qout > P
894	// The result of this function will be ignored at the caller.
895	// Let F = SuccFreq - Qin
896	// Cost in the second case is: Qout + min(Qin, F) * U + max(Qin, F) * V
897
898	if (PDom == nullptr || !Succ->isSuccessor(MBB: PDom)) {
899	BranchProbability UProb = BestSuccSucc;
900	BranchProbability VProb = AdjustedSuccSumProb - UProb;
901	BlockFrequency F = SuccFreq - Qin;
902	BlockFrequency V = SuccFreq * VProb;
903	BlockFrequency QinU = std::min(a: Qin, b: F) * UProb;
904	BlockFrequency BaseCost = P + V;
905	BlockFrequency DupCost = Qout + QinU + std::max(a: Qin, b: F) * VProb;
906	return greaterWithBias(A: BaseCost, B: DupCost, EntryFreq);
907	}
908	BranchProbability UProb = MBPI->getEdgeProbability(Src: Succ, Dst: PDom);
909	BranchProbability VProb = AdjustedSuccSumProb - UProb;
910	BlockFrequency U = SuccFreq * UProb;
911	BlockFrequency V = SuccFreq * VProb;
912	BlockFrequency F = SuccFreq - Qin;
913	// If there is a post-dominating successor, here is the calculation:
914	// BB BB BB BB
915	// | \Qout | \ | \Qout | \
916	// |P C | = |P C | =
917	// = C' |P C = C' |P C
918	// | /Qin | | | /Qin | |
919	// | / | C' (+Succ) | / | C' (+Succ)
920	// Succ Succ /| Succ Succ /|
921	// | \ V | \/ | | \ V | \/ |
922	// |U \ |U /\ =? |U = |U /\ |
923	// = D = = =?| | D | = =|
924	// | / |/ D | / |/ D
925	// | / | / | = | /
926	// |/ | / |/ | =
927	// Dom Dom Dom Dom
928	// '=' : Branch taken for that CFG edge
929	// The cost for taken branches in the first case is P + U
930	// Let F = SuccFreq - Qin
931	// The cost in the second case (assuming independence), given the layout:
932	// BB, Succ, (C+Succ), D, Dom or the layout:
933	// BB, Succ, D, Dom, (C+Succ)
934	// is Qout + max(F, Qin) * U + min(F, Qin)
935	// compare P + U vs Qout + P * U + Qin.
936	//
937	// The 3rd and 4th cases cover when Dom would be chosen to follow Succ.
938	//
939	// For the 3rd case, the cost is P + 2 * V
940	// For the 4th case, the cost is Qout + min(Qin, F) * U + max(Qin, F) * V + V
941	// We choose 4 over 3 when (P + V) > Qout + min(Qin, F) * U + max(Qin, F) * V
942	if (UProb > AdjustedSuccSumProb / 2 &&
943	!hasBetterLayoutPredecessor(BB: Succ, Succ: PDom, SuccChain: *BlockToChain[PDom], SuccProb: UProb, RealSuccProb: UProb,
944	Chain, BlockFilter))
945	// Cases 3 & 4
946	return greaterWithBias(
947	A: (P + V), B: (Qout + std::max(a: Qin, b: F) * VProb + std::min(a: Qin, b: F) * UProb),
948	EntryFreq);
949	// Cases 1 & 2
950	return greaterWithBias(A: (P + U),
951	B: (Qout + std::min(a: Qin, b: F) * AdjustedSuccSumProb +
952	std::max(a: Qin, b: F) * UProb),
953	EntryFreq);
954	}
955
956	/// Check for a trellis layout. \p BB is the upper part of a trellis if its
957	/// successors form the lower part of a trellis. A successor set S forms the
958	/// lower part of a trellis if all of the predecessors of S are either in S or
959	/// have all of S as successors. We ignore trellises where BB doesn't have 2
960	/// successors because for fewer than 2, it's trivial, and for 3 or greater they
961	/// are very uncommon and complex to compute optimally. Allowing edges within S
962	/// is not strictly a trellis, but the same algorithm works, so we allow it.
963	bool MachineBlockPlacement::isTrellis(
964	const MachineBasicBlock *BB,
965	const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
966	const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
967	// Technically BB could form a trellis with branching factor higher than 2.
968	// But that's extremely uncommon.
969	if (BB->succ_size() != 2 || ViableSuccs.size() != 2)
970	return false;
971
972	SmallPtrSet<const MachineBasicBlock *, 2> Successors(BB->succ_begin(),
973	BB->succ_end());
974	// To avoid reviewing the same predecessors twice.
975	SmallPtrSet<const MachineBasicBlock *, 8> SeenPreds;
976
977	for (MachineBasicBlock *Succ : ViableSuccs) {
978	int PredCount = 0;
979	for (auto *SuccPred : Succ->predecessors()) {
980	// Allow triangle successors, but don't count them.
981	if (Successors.count(Ptr: SuccPred)) {
982	// Make sure that it is actually a triangle.
983	for (MachineBasicBlock *CheckSucc : SuccPred->successors())
984	if (!Successors.count(Ptr: CheckSucc))
985	return false;
986	continue;
987	}
988	const BlockChain *PredChain = BlockToChain[SuccPred];
989	if (SuccPred == BB || (BlockFilter && !BlockFilter->count(key: SuccPred)) ||
990	PredChain == &Chain || PredChain == BlockToChain[Succ])
991	continue;
992	++PredCount;
993	// Perform the successor check only once.
994	if (!SeenPreds.insert(Ptr: SuccPred).second)
995	continue;
996	if (!hasSameSuccessors(BB&: *SuccPred, Successors))
997	return false;
998	}
999	// If one of the successors has only BB as a predecessor, it is not a
1000	// trellis.
1001	if (PredCount < 1)
1002	return false;
1003	}
1004	return true;
1005	}
1006
1007	/// Pick the highest total weight pair of edges that can both be laid out.
1008	/// The edges in \p Edges[0] are assumed to have a different destination than
1009	/// the edges in \p Edges[1]. Simple counting shows that the best pair is either
1010	/// the individual highest weight edges to the 2 different destinations, or in
1011	/// case of a conflict, one of them should be replaced with a 2nd best edge.
1012	std::pair<MachineBlockPlacement::WeightedEdge,
1013	MachineBlockPlacement::WeightedEdge>
1014	MachineBlockPlacement::getBestNonConflictingEdges(
1015	const MachineBasicBlock *BB,
1016	MutableArrayRef<SmallVector<MachineBlockPlacement::WeightedEdge, 8>>
1017	Edges) {
1018	// Sort the edges, and then for each successor, find the best incoming
1019	// predecessor. If the best incoming predecessors aren't the same,
1020	// then that is clearly the best layout. If there is a conflict, one of the
1021	// successors will have to fallthrough from the second best predecessor. We
1022	// compare which combination is better overall.
1023
1024	// Sort for highest frequency.
1025	auto Cmp = [](WeightedEdge A, WeightedEdge B) { return A.Weight > B.Weight; };
1026
1027	llvm::stable_sort(Range&: Edges[0], C: Cmp);
1028	llvm::stable_sort(Range&: Edges[1], C: Cmp);
1029	auto BestA = Edges[0].begin();
1030	auto BestB = Edges[1].begin();
1031	// Arrange for the correct answer to be in BestA and BestB
1032	// If the 2 best edges don't conflict, the answer is already there.
1033	if (BestA->Src == BestB->Src) {
1034	// Compare the total fallthrough of (Best + Second Best) for both pairs
1035	auto SecondBestA = std::next(x: BestA);
1036	auto SecondBestB = std::next(x: BestB);
1037	BlockFrequency BestAScore = BestA->Weight + SecondBestB->Weight;
1038	BlockFrequency BestBScore = BestB->Weight + SecondBestA->Weight;
1039	if (BestAScore < BestBScore)
1040	BestA = SecondBestA;
1041	else
1042	BestB = SecondBestB;
1043	}
1044	// Arrange for the BB edge to be in BestA if it exists.
1045	if (BestB->Src == BB)
1046	std::swap(a&: BestA, b&: BestB);
1047	return std::make_pair(x&: *BestA, y&: *BestB);
1048	}
1049
1050	/// Get the best successor from \p BB based on \p BB being part of a trellis.
1051	/// We only handle trellises with 2 successors, so the algorithm is
1052	/// straightforward: Find the best pair of edges that don't conflict. We find
1053	/// the best incoming edge for each successor in the trellis. If those conflict,
1054	/// we consider which of them should be replaced with the second best.
1055	/// Upon return the two best edges will be in \p BestEdges. If one of the edges
1056	/// comes from \p BB, it will be in \p BestEdges[0]
1057	MachineBlockPlacement::BlockAndTailDupResult
1058	MachineBlockPlacement::getBestTrellisSuccessor(
1059	const MachineBasicBlock *BB,
1060	const SmallVectorImpl<MachineBasicBlock *> &ViableSuccs,
1061	BranchProbability AdjustedSumProb, const BlockChain &Chain,
1062	const BlockFilterSet *BlockFilter) {
1063
1064	BlockAndTailDupResult Result = {.BB: nullptr, .ShouldTailDup: false};
1065	SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(),
1066	BB->succ_end());
1067
1068	// We assume size 2 because it's common. For general n, we would have to do
1069	// the Hungarian algorithm, but it's not worth the complexity because more
1070	// than 2 successors is fairly uncommon, and a trellis even more so.
1071	if (Successors.size() != 2 || ViableSuccs.size() != 2)
1072	return Result;
1073
1074	// Collect the edge frequencies of all edges that form the trellis.
1075	SmallVector<WeightedEdge, 8> Edges[2];
1076	int SuccIndex = 0;
1077	for (auto *Succ : ViableSuccs) {
1078	for (MachineBasicBlock *SuccPred : Succ->predecessors()) {
1079	// Skip any placed predecessors that are not BB
1080	if (SuccPred != BB)
1081	if ((BlockFilter && !BlockFilter->count(key: SuccPred)) ||
1082	BlockToChain[SuccPred] == &Chain ||
1083	BlockToChain[SuccPred] == BlockToChain[Succ])
1084	continue;
1085	BlockFrequency EdgeFreq = MBFI->getBlockFreq(MBB: SuccPred) *
1086	MBPI->getEdgeProbability(Src: SuccPred, Dst: Succ);
1087	Edges[SuccIndex].push_back(Elt: {.Weight: EdgeFreq, .Src: SuccPred, .Dest: Succ});
1088	}
1089	++SuccIndex;
1090	}
1091
1092	// Pick the best combination of 2 edges from all the edges in the trellis.
1093	WeightedEdge BestA, BestB;
1094	std::tie(args&: BestA, args&: BestB) = getBestNonConflictingEdges(BB, Edges);
1095
1096	if (BestA.Src != BB) {
1097	// If we have a trellis, and BB doesn't have the best fallthrough edges,
1098	// we shouldn't choose any successor. We've already looked and there's a
1099	// better fallthrough edge for all the successors.
1100	LLVM_DEBUG(dbgs() << "Trellis, but not one of the chosen edges.\n");
1101	return Result;
1102	}
1103
1104	// Did we pick the triangle edge? If tail-duplication is profitable, do
1105	// that instead. Otherwise merge the triangle edge now while we know it is
1106	// optimal.
1107	if (BestA.Dest == BestB.Src) {
1108	// The edges are BB->Succ1->Succ2, and we're looking to see if BB->Succ2
1109	// would be better.
1110	MachineBasicBlock *Succ1 = BestA.Dest;
1111	MachineBasicBlock *Succ2 = BestB.Dest;
1112	// Check to see if tail-duplication would be profitable.
1113	if (allowTailDupPlacement() && shouldTailDuplicate(BB: Succ2) &&
1114	canTailDuplicateUnplacedPreds(BB, Succ: Succ2, Chain, BlockFilter) &&
1115	isProfitableToTailDup(BB, Succ: Succ2, QProb: MBPI->getEdgeProbability(Src: BB, Dst: Succ1),
1116	Chain, BlockFilter)) {
1117	LLVM_DEBUG(BranchProbability Succ2Prob = getAdjustedProbability(
1118	MBPI->getEdgeProbability(BB, Succ2), AdjustedSumProb);
1119	dbgs() << " Selected: " << getBlockName(Succ2)
1120	<< ", probability: " << Succ2Prob
1121	<< " (Tail Duplicate)\n");
1122	Result.BB = Succ2;
1123	Result.ShouldTailDup = true;
1124	return Result;
1125	}
1126	}
1127	// We have already computed the optimal edge for the other side of the
1128	// trellis.
1129	ComputedEdges[BestB.Src] = { .BB: BestB.Dest, .ShouldTailDup: false };
1130
1131	auto TrellisSucc = BestA.Dest;
1132	LLVM_DEBUG(BranchProbability SuccProb = getAdjustedProbability(
1133	MBPI->getEdgeProbability(BB, TrellisSucc), AdjustedSumProb);
1134	dbgs() << " Selected: " << getBlockName(TrellisSucc)
1135	<< ", probability: " << SuccProb << " (Trellis)\n");
1136	Result.BB = TrellisSucc;
1137	return Result;
1138	}
1139
1140	/// When the option allowTailDupPlacement() is on, this method checks if the
1141	/// fallthrough candidate block \p Succ (of block \p BB) can be tail-duplicated
1142	/// into all of its unplaced, unfiltered predecessors, that are not BB.
1143	bool MachineBlockPlacement::canTailDuplicateUnplacedPreds(
1144	const MachineBasicBlock *BB, MachineBasicBlock *Succ,
1145	const BlockChain &Chain, const BlockFilterSet *BlockFilter) {
1146	if (!shouldTailDuplicate(BB: Succ))
1147	return false;
1148
1149	// The result of canTailDuplicate.
1150	bool Duplicate = true;
1151	// Number of possible duplication.
1152	unsigned int NumDup = 0;
1153
1154	// For CFG checking.
1155	SmallPtrSet<const MachineBasicBlock *, 4> Successors(BB->succ_begin(),
1156	BB->succ_end());
1157	for (MachineBasicBlock *Pred : Succ->predecessors()) {
1158	// Make sure all unplaced and unfiltered predecessors can be
1159	// tail-duplicated into.
1160	// Skip any blocks that are already placed or not in this loop.
1161	if (Pred == BB || (BlockFilter && !BlockFilter->count(key: Pred))
1162	|| (BlockToChain[Pred] == &Chain && !Succ->succ_empty()))
1163	continue;
1164	if (!TailDup.canTailDuplicate(TailBB: Succ, PredBB: Pred)) {
1165	if (Successors.size() > 1 && hasSameSuccessors(BB&: *Pred, Successors))
1166	// This will result in a trellis after tail duplication, so we don't
1167	// need to copy Succ into this predecessor. In the presence
1168	// of a trellis tail duplication can continue to be profitable.
1169	// For example:
1170	// A A
1171	// |\ |\
1172	// | \ | \
1173	// | C | C+BB
1174	// | / | |
1175	// |/ | |
1176	// BB => BB |
1177	// |\ |\/|
1178	// | \ |/\|
1179	// | D | D
1180	// | / | /
1181	// |/ |/
1182	// Succ Succ
1183	//
1184	// After BB was duplicated into C, the layout looks like the one on the
1185	// right. BB and C now have the same successors. When considering
1186	// whether Succ can be duplicated into all its unplaced predecessors, we
1187	// ignore C.
1188	// We can do this because C already has a profitable fallthrough, namely
1189	// D. TODO(iteratee): ignore sufficiently cold predecessors for
1190	// duplication and for this test.
1191	//
1192	// This allows trellises to be laid out in 2 separate chains
1193	// (A,B,Succ,...) and later (C,D,...) This is a reasonable heuristic
1194	// because it allows the creation of 2 fallthrough paths with links
1195	// between them, and we correctly identify the best layout for these
1196	// CFGs. We want to extend trellises that the user created in addition
1197	// to trellises created by tail-duplication, so we just look for the
1198	// CFG.
1199	continue;
1200	Duplicate = false;
1201	continue;
1202	}
1203	NumDup++;
1204	}
1205
1206	// No possible duplication in current filter set.
1207	if (NumDup == 0)
1208	return false;
1209
1210	// If profile information is available, findDuplicateCandidates can do more
1211	// precise benefit analysis.
1212	if (F->getFunction().hasProfileData())
1213	return true;
1214
1215	// This is mainly for function exit BB.
1216	// The integrated tail duplication is really designed for increasing
1217	// fallthrough from predecessors from Succ to its successors. We may need
1218	// other machanism to handle different cases.
1219	if (Succ->succ_empty())
1220	return true;
1221
1222	// Plus the already placed predecessor.
1223	NumDup++;
1224
1225	// If the duplication candidate has more unplaced predecessors than
1226	// successors, the extra duplication can't bring more fallthrough.
1227	//
1228	// Pred1 Pred2 Pred3
1229	// \ | /
1230	// \ | /
1231	// \ | /
1232	// Dup
1233	// / \
1234	// / \
1235	// Succ1 Succ2
1236	//
1237	// In this example Dup has 2 successors and 3 predecessors, duplication of Dup
1238	// can increase the fallthrough from Pred1 to Succ1 and from Pred2 to Succ2,
1239	// but the duplication into Pred3 can't increase fallthrough.
1240	//
1241	// A small number of extra duplication may not hurt too much. We need a better
1242	// heuristic to handle it.
1243	if ((NumDup > Succ->succ_size()) || !Duplicate)
1244	return false;
1245
1246	return true;
1247	}
1248
1249	/// Find chains of triangles where we believe it would be profitable to
1250	/// tail-duplicate them all, but a local analysis would not find them.
1251	/// There are 3 ways this can be profitable:
1252	/// 1) The post-dominators marked 50% are actually taken 55% (This shrinks with
1253	/// longer chains)
1254	/// 2) The chains are statically correlated. Branch probabilities have a very
1255	/// U-shaped distribution.
1256	/// [http://nrs.harvard.edu/urn-3:HUL.InstRepos:24015805]
1257	/// If the branches in a chain are likely to be from the same side of the
1258	/// distribution as their predecessor, but are independent at runtime, this
1259	/// transformation is profitable. (Because the cost of being wrong is a small
1260	/// fixed cost, unlike the standard triangle layout where the cost of being
1261	/// wrong scales with the # of triangles.)
1262	/// 3) The chains are dynamically correlated. If the probability that a previous
1263	/// branch was taken positively influences whether the next branch will be
1264	/// taken
1265	/// We believe that 2 and 3 are common enough to justify the small margin in 1.
1266	void MachineBlockPlacement::precomputeTriangleChains() {
1267	struct TriangleChain {
1268	std::vector<MachineBasicBlock *> Edges;
1269
1270	TriangleChain(MachineBasicBlock *src, MachineBasicBlock *dst)
1271	: Edges({src, dst}) {}
1272
1273	void append(MachineBasicBlock *dst) {
1274	assert(getKey()->isSuccessor(dst) &&
1275	"Attempting to append a block that is not a successor.");
1276	Edges.push_back(x: dst);
1277	}
1278
1279	unsigned count() const { return Edges.size() - 1; }
1280
1281	MachineBasicBlock *getKey() const {
1282	return Edges.back();
1283	}
1284	};
1285
1286	if (TriangleChainCount == 0)
1287	return;
1288
1289	LLVM_DEBUG(dbgs() << "Pre-computing triangle chains.\n");
1290	// Map from last block to the chain that contains it. This allows us to extend
1291	// chains as we find new triangles.
1292	DenseMap<const MachineBasicBlock *, TriangleChain> TriangleChainMap;
1293	for (MachineBasicBlock &BB : *F) {
1294	// If BB doesn't have 2 successors, it doesn't start a triangle.
1295	if (BB.succ_size() != 2)
1296	continue;
1297	MachineBasicBlock *PDom = nullptr;
1298	for (MachineBasicBlock *Succ : BB.successors()) {
1299	if (!MPDT->dominates(A: Succ, B: &BB))
1300	continue;
1301	PDom = Succ;
1302	break;
1303	}
1304	// If BB doesn't have a post-dominating successor, it doesn't form a
1305	// triangle.
1306	if (PDom == nullptr)
1307	continue;
1308	// If PDom has a hint that it is low probability, skip this triangle.
1309	if (MBPI->getEdgeProbability(Src: &BB, Dst: PDom) < BranchProbability(50, 100))
1310	continue;
1311	// If PDom isn't eligible for duplication, this isn't the kind of triangle
1312	// we're looking for.
1313	if (!shouldTailDuplicate(BB: PDom))
1314	continue;
1315	bool CanTailDuplicate = true;
1316	// If PDom can't tail-duplicate into it's non-BB predecessors, then this
1317	// isn't the kind of triangle we're looking for.
1318	for (MachineBasicBlock* Pred : PDom->predecessors()) {
1319	if (Pred == &BB)
1320	continue;
1321	if (!TailDup.canTailDuplicate(TailBB: PDom, PredBB: Pred)) {
1322	CanTailDuplicate = false;
1323	break;
1324	}
1325	}
1326	// If we can't tail-duplicate PDom to its predecessors, then skip this
1327	// triangle.
1328	if (!CanTailDuplicate)
1329	continue;
1330
1331	// Now we have an interesting triangle. Insert it if it's not part of an
1332	// existing chain.
1333	// Note: This cannot be replaced with a call insert() or emplace() because
1334	// the find key is BB, but the insert/emplace key is PDom.
1335	auto Found = TriangleChainMap.find(Val: &BB);
1336	// If it is, remove the chain from the map, grow it, and put it back in the
1337	// map with the end as the new key.
1338	if (Found != TriangleChainMap.end()) {
1339	TriangleChain Chain = std::move(Found->second);
1340	TriangleChainMap.erase(I: Found);
1341	Chain.append(dst: PDom);
1342	TriangleChainMap.insert(KV: std::make_pair(x: Chain.getKey(), y: std::move(Chain)));
1343	} else {
1344	auto InsertResult = TriangleChainMap.try_emplace(Key: PDom, Args: &BB, Args&: PDom);
1345	assert(InsertResult.second && "Block seen twice.");
1346	(void)InsertResult;
1347	}
1348	}
1349
1350	// Iterating over a DenseMap is safe here, because the only thing in the body
1351	// of the loop is inserting into another DenseMap (ComputedEdges).
1352	// ComputedEdges is never iterated, so this doesn't lead to non-determinism.
1353	for (auto &ChainPair : TriangleChainMap) {
1354	TriangleChain &Chain = ChainPair.second;
1355	// Benchmarking has shown that due to branch correlation duplicating 2 or
1356	// more triangles is profitable, despite the calculations assuming
1357	// independence.
1358	if (Chain.count() < TriangleChainCount)
1359	continue;
1360	MachineBasicBlock *dst = Chain.Edges.back();
1361	Chain.Edges.pop_back();
1362	for (MachineBasicBlock *src : reverse(C&: Chain.Edges)) {
1363	LLVM_DEBUG(dbgs() << "Marking edge: " << getBlockName(src) << "->"
1364	<< getBlockName(dst)
1365	<< " as pre-computed based on triangles.\n");
1366
1367	auto InsertResult = ComputedEdges.insert(KV: {src, {.BB: dst, .ShouldTailDup: true}});
1368	assert(InsertResult.second && "Block seen twice.");
1369	(void)InsertResult;
1370
1371	dst = src;
1372	}
1373	}
1374	}
1375
1376	// When profile is not present, return the StaticLikelyProb.
1377	// When profile is available, we need to handle the triangle-shape CFG.
1378	static BranchProbability getLayoutSuccessorProbThreshold(
1379	const MachineBasicBlock *BB) {
1380	if (!BB->getParent()->getFunction().hasProfileData())
1381	return BranchProbability(StaticLikelyProb, 100);
1382	if (BB->succ_size() == 2) {
1383	const MachineBasicBlock *Succ1 = *BB->succ_begin();
1384	const MachineBasicBlock *Succ2 = *(BB->succ_begin() + 1);
1385	if (Succ1->isSuccessor(MBB: Succ2) || Succ2->isSuccessor(MBB: Succ1)) {
1386	/* See case 1 below for the cost analysis. For BB->Succ to
1387	* be taken with smaller cost, the following needs to hold:
1388	* Prob(BB->Succ) > 2 * Prob(BB->Pred)
1389	* So the threshold T in the calculation below
1390	* (1-T) * Prob(BB->Succ) > T * Prob(BB->Pred)
1391	* So T / (1 - T) = 2, Yielding T = 2/3
1392	* Also adding user specified branch bias, we have
1393	* T = (2/3)*(ProfileLikelyProb/50)
1394	* = (2*ProfileLikelyProb)/150)
1395	*/
1396	return BranchProbability(2 * ProfileLikelyProb, 150);
1397	}
1398	}
1399	return BranchProbability(ProfileLikelyProb, 100);
1400	}
1401
1402	/// Checks to see if the layout candidate block \p Succ has a better layout
1403	/// predecessor than \c BB. If yes, returns true.
1404	/// \p SuccProb: The probability adjusted for only remaining blocks.
1405	/// Only used for logging
1406	/// \p RealSuccProb: The un-adjusted probability.
1407	/// \p Chain: The chain that BB belongs to and Succ is being considered for.
1408	/// \p BlockFilter: if non-null, the set of blocks that make up the loop being
1409	/// considered
1410	bool MachineBlockPlacement::hasBetterLayoutPredecessor(
1411	const MachineBasicBlock *BB, const MachineBasicBlock *Succ,
1412	const BlockChain &SuccChain, BranchProbability SuccProb,
1413	BranchProbability RealSuccProb, const BlockChain &Chain,
1414	const BlockFilterSet *BlockFilter) {
1415
1416	// There isn't a better layout when there are no unscheduled predecessors.
1417	if (SuccChain.UnscheduledPredecessors == 0)
1418	return false;
1419
1420	// There are two basic scenarios here:
1421	// -------------------------------------
1422	// Case 1: triangular shape CFG (if-then):
1423	// BB
1424	// | \
1425	// | \
1426	// | Pred
1427	// | /
1428	// Succ
1429	// In this case, we are evaluating whether to select edge -> Succ, e.g.
1430	// set Succ as the layout successor of BB. Picking Succ as BB's
1431	// successor breaks the CFG constraints (FIXME: define these constraints).
1432	// With this layout, Pred BB
1433	// is forced to be outlined, so the overall cost will be cost of the
1434	// branch taken from BB to Pred, plus the cost of back taken branch
1435	// from Pred to Succ, as well as the additional cost associated
1436	// with the needed unconditional jump instruction from Pred To Succ.
1437
1438	// The cost of the topological order layout is the taken branch cost
1439	// from BB to Succ, so to make BB->Succ a viable candidate, the following
1440	// must hold:
1441	// 2 * freq(BB->Pred) * taken_branch_cost + unconditional_jump_cost
1442	// < freq(BB->Succ) * taken_branch_cost.
1443	// Ignoring unconditional jump cost, we get
1444	// freq(BB->Succ) > 2 * freq(BB->Pred), i.e.,
1445	// prob(BB->Succ) > 2 * prob(BB->Pred)
1446	//
1447	// When real profile data is available, we can precisely compute the
1448	// probability threshold that is needed for edge BB->Succ to be considered.
1449	// Without profile data, the heuristic requires the branch bias to be
1450	// a lot larger to make sure the signal is very strong (e.g. 80% default).
1451	// -----------------------------------------------------------------
1452	// Case 2: diamond like CFG (if-then-else):
1453	// S
1454	// / \
1455	// | \
1456	// BB Pred
1457	// \ /
1458	// Succ
1459	// ..
1460	//
1461	// The current block is BB and edge BB->Succ is now being evaluated.
1462	// Note that edge S->BB was previously already selected because
1463	// prob(S->BB) > prob(S->Pred).
1464	// At this point, 2 blocks can be placed after BB: Pred or Succ. If we
1465	// choose Pred, we will have a topological ordering as shown on the left
1466	// in the picture below. If we choose Succ, we have the solution as shown
1467	// on the right:
1468	//
1469	// topo-order:
1470	//
1471	// S----- ---S
1472	// | | | |
1473	// ---BB | | BB
1474	// | | | |
1475	// | Pred-- | Succ--
1476	// | | | |
1477	// ---Succ ---Pred--
1478	//
1479	// cost = freq(S->Pred) + freq(BB->Succ) cost = 2 * freq (S->Pred)
1480	// = freq(S->Pred) + freq(S->BB)
1481	//
1482	// If we have profile data (i.e, branch probabilities can be trusted), the
1483	// cost (number of taken branches) with layout S->BB->Succ->Pred is 2 *
1484	// freq(S->Pred) while the cost of topo order is freq(S->Pred) + freq(S->BB).
1485	// We know Prob(S->BB) > Prob(S->Pred), so freq(S->BB) > freq(S->Pred), which
1486	// means the cost of topological order is greater.
1487	// When profile data is not available, however, we need to be more
1488	// conservative. If the branch prediction is wrong, breaking the topo-order
1489	// will actually yield a layout with large cost. For this reason, we need
1490	// strong biased branch at block S with Prob(S->BB) in order to select
1491	// BB->Succ. This is equivalent to looking the CFG backward with backward
1492	// edge: Prob(Succ->BB) needs to >= HotProb in order to be selected (without
1493	// profile data).
1494	// --------------------------------------------------------------------------
1495	// Case 3: forked diamond
1496	// S
1497	// / \
1498	// / \
1499	// BB Pred
1500	// | \ / |
1501	// | \ / |
1502	// | X |
1503	// | / \ |
1504	// | / \ |
1505	// S1 S2
1506	//
1507	// The current block is BB and edge BB->S1 is now being evaluated.
1508	// As above S->BB was already selected because
1509	// prob(S->BB) > prob(S->Pred). Assume that prob(BB->S1) >= prob(BB->S2).
1510	//
1511	// topo-order:
1512	//
1513	// S-------| ---S
1514	// | | | |
1515	// ---BB | | BB
1516	// | | | |
1517	// | Pred----| | S1----
1518	// | | | |
1519	// --(S1 or S2) ---Pred--
1520	// |
1521	// S2
1522	//
1523	// topo-cost = freq(S->Pred) + freq(BB->S1) + freq(BB->S2)
1524	// + min(freq(Pred->S1), freq(Pred->S2))
1525	// Non-topo-order cost:
1526	// non-topo-cost = 2 * freq(S->Pred) + freq(BB->S2).
1527	// To be conservative, we can assume that min(freq(Pred->S1), freq(Pred->S2))
1528	// is 0. Then the non topo layout is better when
1529	// freq(S->Pred) < freq(BB->S1).
1530	// This is exactly what is checked below.
1531	// Note there are other shapes that apply (Pred may not be a single block,
1532	// but they all fit this general pattern.)
1533	BranchProbability HotProb = getLayoutSuccessorProbThreshold(BB);
1534
1535	// Make sure that a hot successor doesn't have a globally more
1536	// important predecessor.
1537	BlockFrequency CandidateEdgeFreq = MBFI->getBlockFreq(MBB: BB) * RealSuccProb;
1538	bool BadCFGConflict = false;
1539
1540	for (MachineBasicBlock *Pred : Succ->predecessors()) {
1541	BlockChain *PredChain = BlockToChain[Pred];
1542	if (Pred == Succ || PredChain == &SuccChain ||
1543	(BlockFilter && !BlockFilter->count(key: Pred)) ||
1544	PredChain == &Chain || Pred != *std::prev(x: PredChain->end()) ||
1545	// This check is redundant except for look ahead. This function is
1546	// called for lookahead by isProfitableToTailDup when BB hasn't been
1547	// placed yet.
1548	(Pred == BB))
1549	continue;
1550	// Do backward checking.
1551	// For all cases above, we need a backward checking to filter out edges that
1552	// are not 'strongly' biased.
1553	// BB Pred
1554	// \ /
1555	// Succ
1556	// We select edge BB->Succ if
1557	// freq(BB->Succ) > freq(Succ) * HotProb
1558	// i.e. freq(BB->Succ) > freq(BB->Succ) * HotProb + freq(Pred->Succ) *
1559	// HotProb
1560	// i.e. freq((BB->Succ) * (1 - HotProb) > freq(Pred->Succ) * HotProb
1561	// Case 1 is covered too, because the first equation reduces to:
1562	// prob(BB->Succ) > HotProb. (freq(Succ) = freq(BB) for a triangle)
1563	BlockFrequency PredEdgeFreq =
1564	MBFI->getBlockFreq(MBB: Pred) * MBPI->getEdgeProbability(Src: Pred, Dst: Succ);
1565	if (PredEdgeFreq * HotProb >= CandidateEdgeFreq * HotProb.getCompl()) {
1566	BadCFGConflict = true;
1567	break;
1568	}
1569	}
1570
1571	if (BadCFGConflict) {
1572	LLVM_DEBUG(dbgs() << " Not a candidate: " << getBlockName(Succ) << " -> "
1573	<< SuccProb << " (prob) (non-cold CFG conflict)\n");
1574	return true;
1575	}
1576
1577	return false;
1578	}
1579
1580	/// Select the best successor for a block.
1581	///
1582	/// This looks across all successors of a particular block and attempts to
1583	/// select the "best" one to be the layout successor. It only considers direct
1584	/// successors which also pass the block filter. It will attempt to avoid
1585	/// breaking CFG structure, but cave and break such structures in the case of
1586	/// very hot successor edges.
1587	///
1588	/// \returns The best successor block found, or null if none are viable, along
1589	/// with a boolean indicating if tail duplication is necessary.
1590	MachineBlockPlacement::BlockAndTailDupResult
1591	MachineBlockPlacement::selectBestSuccessor(
1592	const MachineBasicBlock *BB, const BlockChain &Chain,
1593	const BlockFilterSet *BlockFilter) {
1594	const BranchProbability HotProb(StaticLikelyProb, 100);
1595
1596	BlockAndTailDupResult BestSucc = { .BB: nullptr, .ShouldTailDup: false };
1597	auto BestProb = BranchProbability::getZero();
1598
1599	SmallVector<MachineBasicBlock *, 4> Successors;
1600	auto AdjustedSumProb =
1601	collectViableSuccessors(BB, Chain, BlockFilter, Successors);
1602
1603	LLVM_DEBUG(dbgs() << "Selecting best successor for: " << getBlockName(BB)
1604	<< "\n");
1605
1606	// if we already precomputed the best successor for BB, return that if still
1607	// applicable.
1608	auto FoundEdge = ComputedEdges.find(Val: BB);
1609	if (FoundEdge != ComputedEdges.end()) {
1610	MachineBasicBlock *Succ = FoundEdge->second.BB;
1611	ComputedEdges.erase(I: FoundEdge);
1612	BlockChain *SuccChain = BlockToChain[Succ];
1613	if (BB->isSuccessor(MBB: Succ) && (!BlockFilter || BlockFilter->count(key: Succ)) &&
1614	SuccChain != &Chain && Succ == *SuccChain->begin())
1615	return FoundEdge->second;
1616	}
1617
1618	// if BB is part of a trellis, Use the trellis to determine the optimal
1619	// fallthrough edges
1620	if (isTrellis(BB, ViableSuccs: Successors, Chain, BlockFilter))
1621	return getBestTrellisSuccessor(BB, ViableSuccs: Successors, AdjustedSumProb, Chain,
1622	BlockFilter);
1623
1624	// For blocks with CFG violations, we may be able to lay them out anyway with
1625	// tail-duplication. We keep this vector so we can perform the probability
1626	// calculations the minimum number of times.
1627	SmallVector<std::pair<BranchProbability, MachineBasicBlock *>, 4>
1628	DupCandidates;
1629	for (MachineBasicBlock *Succ : Successors) {
1630	auto RealSuccProb = MBPI->getEdgeProbability(Src: BB, Dst: Succ);
1631	BranchProbability SuccProb =
1632	getAdjustedProbability(OrigProb: RealSuccProb, AdjustedSumProb);
1633
1634	BlockChain &SuccChain = *BlockToChain[Succ];
1635	// Skip the edge \c BB->Succ if block \c Succ has a better layout
1636	// predecessor that yields lower global cost.
1637	if (hasBetterLayoutPredecessor(BB, Succ, SuccChain, SuccProb, RealSuccProb,
1638	Chain, BlockFilter)) {
1639	// If tail duplication would make Succ profitable, place it.
1640	if (allowTailDupPlacement() && shouldTailDuplicate(BB: Succ))
1641	DupCandidates.emplace_back(Args&: SuccProb, Args&: Succ);
1642	continue;
1643	}
1644
1645	LLVM_DEBUG(
1646	dbgs() << " Candidate: " << getBlockName(Succ)
1647	<< ", probability: " << SuccProb
1648	<< (SuccChain.UnscheduledPredecessors != 0 ? " (CFG break)" : "")
1649	<< "\n");
1650
1651	if (BestSucc.BB && BestProb >= SuccProb) {
1652	LLVM_DEBUG(dbgs() << " Not the best candidate, continuing\n");
1653	continue;
1654	}
1655
1656	LLVM_DEBUG(dbgs() << " Setting it as best candidate\n");
1657	BestSucc.BB = Succ;
1658	BestProb = SuccProb;
1659	}
1660	// Handle the tail duplication candidates in order of decreasing probability.
1661	// Stop at the first one that is profitable. Also stop if they are less
1662	// profitable than BestSucc. Position is important because we preserve it and
1663	// prefer first best match. Here we aren't comparing in order, so we capture
1664	// the position instead.
1665	llvm::stable_sort(Range&: DupCandidates,
1666	C: [](std::tuple<BranchProbability, MachineBasicBlock *> L,
1667	std::tuple<BranchProbability, MachineBasicBlock *> R) {
1668	return std::get<0>(t&: L) > std::get<0>(t&: R);
1669	});
1670	for (auto &Tup : DupCandidates) {
1671	BranchProbability DupProb;
1672	MachineBasicBlock *Succ;
1673	std::tie(args&: DupProb, args&: Succ) = Tup;
1674	if (DupProb < BestProb)
1675	break;
1676	if (canTailDuplicateUnplacedPreds(BB, Succ, Chain, BlockFilter)
1677	&& (isProfitableToTailDup(BB, Succ, QProb: BestProb, Chain, BlockFilter))) {
1678	LLVM_DEBUG(dbgs() << " Candidate: " << getBlockName(Succ)
1679	<< ", probability: " << DupProb
1680	<< " (Tail Duplicate)\n");
1681	BestSucc.BB = Succ;
1682	BestSucc.ShouldTailDup = true;
1683	break;
1684	}
1685	}
1686
1687	if (BestSucc.BB)
1688	LLVM_DEBUG(dbgs() << " Selected: " << getBlockName(BestSucc.BB) << "\n");
1689
1690	return BestSucc;
1691	}
1692
1693	/// Select the best block from a worklist.
1694	///
1695	/// This looks through the provided worklist as a list of candidate basic
1696	/// blocks and select the most profitable one to place. The definition of
1697	/// profitable only really makes sense in the context of a loop. This returns
1698	/// the most frequently visited block in the worklist, which in the case of
1699	/// a loop, is the one most desirable to be physically close to the rest of the
1700	/// loop body in order to improve i-cache behavior.
1701	///
1702	/// \returns The best block found, or null if none are viable.
1703	MachineBasicBlock *MachineBlockPlacement::selectBestCandidateBlock(
1704	const BlockChain &Chain, SmallVectorImpl<MachineBasicBlock *> &WorkList) {
1705	// Once we need to walk the worklist looking for a candidate, cleanup the
1706	// worklist of already placed entries.
1707	// FIXME: If this shows up on profiles, it could be folded (at the cost of
1708	// some code complexity) into the loop below.
1709	llvm::erase_if(C&: WorkList, P: [&](MachineBasicBlock *BB) {
1710	return BlockToChain.lookup(Val: BB) == &Chain;
1711	});
1712
1713	if (WorkList.empty())
1714	return nullptr;
1715
1716	bool IsEHPad = WorkList[0]->isEHPad();
1717
1718	MachineBasicBlock *BestBlock = nullptr;
1719	BlockFrequency BestFreq;
1720	for (MachineBasicBlock *MBB : WorkList) {
1721	assert(MBB->isEHPad() == IsEHPad &&
1722	"EHPad mismatch between block and work list.");
1723
1724	BlockChain &SuccChain = *BlockToChain[MBB];
1725	if (&SuccChain == &Chain)
1726	continue;
1727
1728	assert(SuccChain.UnscheduledPredecessors == 0 &&
1729	"Found CFG-violating block");
1730
1731	BlockFrequency CandidateFreq = MBFI->getBlockFreq(MBB);
1732	LLVM_DEBUG(dbgs() << " " << getBlockName(MBB) << " -> "
1733	<< printBlockFreq(MBFI->getMBFI(), CandidateFreq)
1734	<< " (freq)\n");
1735
1736	// For ehpad, we layout the least probable first as to avoid jumping back
1737	// from least probable landingpads to more probable ones.
1738	//
1739	// FIXME: Using probability is probably (!) not the best way to achieve
1740	// this. We should probably have a more principled approach to layout
1741	// cleanup code.
1742	//
1743	// The goal is to get:
1744	//
1745	// +--------------------------+
1746	// | V
1747	// InnerLp -> InnerCleanup OuterLp -> OuterCleanup -> Resume
1748	//
1749	// Rather than:
1750	//
1751	// +-------------------------------------+
1752	// V |
1753	// OuterLp -> OuterCleanup -> Resume InnerLp -> InnerCleanup
1754	if (BestBlock && (IsEHPad ^ (BestFreq >= CandidateFreq)))
1755	continue;
1756
1757	BestBlock = MBB;
1758	BestFreq = CandidateFreq;
1759	}
1760
1761	return BestBlock;
1762	}
1763
1764	/// Retrieve the first unplaced basic block.
1765	///
1766	/// This routine is called when we are unable to use the CFG to walk through
1767	/// all of the basic blocks and form a chain due to unnatural loops in the CFG.
1768	/// We walk through the function's blocks in order, starting from the
1769	/// LastUnplacedBlockIt. We update this iterator on each call to avoid
1770	/// re-scanning the entire sequence on repeated calls to this routine.
1771	MachineBasicBlock *MachineBlockPlacement::getFirstUnplacedBlock(
1772	const BlockChain &PlacedChain,
1773	MachineFunction::iterator &PrevUnplacedBlockIt,
1774	const BlockFilterSet *BlockFilter) {
1775	for (MachineFunction::iterator I = PrevUnplacedBlockIt, E = F->end(); I != E;
1776	++I) {
1777	if (BlockFilter && !BlockFilter->count(key: &*I))
1778	continue;
1779	if (BlockToChain[&*I] != &PlacedChain) {
1780	PrevUnplacedBlockIt = I;
1781	// Now select the head of the chain to which the unplaced block belongs
1782	// as the block to place. This will force the entire chain to be placed,
1783	// and satisfies the requirements of merging chains.
1784	return *BlockToChain[&*I]->begin();
1785	}
1786	}
1787	return nullptr;
1788	}
1789
1790	void MachineBlockPlacement::fillWorkLists(
1791	const MachineBasicBlock *MBB,
1792	SmallPtrSetImpl<BlockChain *> &UpdatedPreds,
1793	const BlockFilterSet *BlockFilter = nullptr) {
1794	BlockChain &Chain = *BlockToChain[MBB];
1795	if (!UpdatedPreds.insert(Ptr: &Chain).second)
1796	return;
1797
1798	assert(
1799	Chain.UnscheduledPredecessors == 0 &&
1800	"Attempting to place block with unscheduled predecessors in worklist.");
1801	for (MachineBasicBlock *ChainBB : Chain) {
1802	assert(BlockToChain[ChainBB] == &Chain &&
1803	"Block in chain doesn't match BlockToChain map.");
1804	for (MachineBasicBlock *Pred : ChainBB->predecessors()) {
1805	if (BlockFilter && !BlockFilter->count(key: Pred))
1806	continue;
1807	if (BlockToChain[Pred] == &Chain)
1808	continue;
1809	++Chain.UnscheduledPredecessors;
1810	}
1811	}
1812
1813	if (Chain.UnscheduledPredecessors != 0)
1814	return;
1815
1816	MachineBasicBlock *BB = *Chain.begin();
1817	if (BB->isEHPad())
1818	EHPadWorkList.push_back(Elt: BB);
1819	else
1820	BlockWorkList.push_back(Elt: BB);
1821	}
1822
1823	void MachineBlockPlacement::buildChain(
1824	const MachineBasicBlock *HeadBB, BlockChain &Chain,
1825	BlockFilterSet *BlockFilter) {
1826	assert(HeadBB && "BB must not be null.\n");
1827	assert(BlockToChain[HeadBB] == &Chain && "BlockToChainMap mis-match.\n");
1828	MachineFunction::iterator PrevUnplacedBlockIt = F->begin();
1829
1830	const MachineBasicBlock *LoopHeaderBB = HeadBB;
1831	markChainSuccessors(Chain, LoopHeaderBB, BlockFilter);
1832	MachineBasicBlock *BB = *std::prev(x: Chain.end());
1833	while (true) {
1834	assert(BB && "null block found at end of chain in loop.");
1835	assert(BlockToChain[BB] == &Chain && "BlockToChainMap mis-match in loop.");
1836	assert(*std::prev(Chain.end()) == BB && "BB Not found at end of chain.");
1837
1838
1839	// Look for the best viable successor if there is one to place immediately
1840	// after this block.
1841	auto Result = selectBestSuccessor(BB, Chain, BlockFilter);
1842	MachineBasicBlock* BestSucc = Result.BB;
1843	bool ShouldTailDup = Result.ShouldTailDup;
1844	if (allowTailDupPlacement())
1845	ShouldTailDup |= (BestSucc && canTailDuplicateUnplacedPreds(BB, Succ: BestSucc,
1846	Chain,
1847	BlockFilter));
1848
1849	// If an immediate successor isn't available, look for the best viable
1850	// block among those we've identified as not violating the loop's CFG at
1851	// this point. This won't be a fallthrough, but it will increase locality.
1852	if (!BestSucc)
1853	BestSucc = selectBestCandidateBlock(Chain, WorkList&: BlockWorkList);
1854	if (!BestSucc)
1855	BestSucc = selectBestCandidateBlock(Chain, WorkList&: EHPadWorkList);
1856
1857	if (!BestSucc) {
1858	BestSucc = getFirstUnplacedBlock(PlacedChain: Chain, PrevUnplacedBlockIt, BlockFilter);
1859	if (!BestSucc)
1860	break;
1861
1862	LLVM_DEBUG(dbgs() << "Unnatural loop CFG detected, forcibly merging the "
1863	"layout successor until the CFG reduces\n");
1864	}
1865
1866	// Placement may have changed tail duplication opportunities.
1867	// Check for that now.
1868	if (allowTailDupPlacement() && BestSucc && ShouldTailDup) {
1869	repeatedlyTailDuplicateBlock(BB: BestSucc, LPred&: BB, LoopHeaderBB, Chain,
1870	BlockFilter, PrevUnplacedBlockIt);
1871	// If the chosen successor was duplicated into BB, don't bother laying
1872	// it out, just go round the loop again with BB as the chain end.
1873	if (!BB->isSuccessor(MBB: BestSucc))
1874	continue;
1875	}
1876
1877	// Place this block, updating the datastructures to reflect its placement.
1878	BlockChain &SuccChain = *BlockToChain[BestSucc];
1879	// Zero out UnscheduledPredecessors for the successor we're about to merge in case
1880	// we selected a successor that didn't fit naturally into the CFG.
1881	SuccChain.UnscheduledPredecessors = 0;
1882	LLVM_DEBUG(dbgs() << "Merging from " << getBlockName(BB) << " to "
1883	<< getBlockName(BestSucc) << "\n");
1884	markChainSuccessors(Chain: SuccChain, LoopHeaderBB, BlockFilter);
1885	Chain.merge(BB: BestSucc, Chain: &SuccChain);
1886	BB = *std::prev(x: Chain.end());
1887	}
1888
1889	LLVM_DEBUG(dbgs() << "Finished forming chain for header block "
1890	<< getBlockName(*Chain.begin()) << "\n");
1891	}
1892
1893	// If bottom of block BB has only one successor OldTop, in most cases it is
1894	// profitable to move it before OldTop, except the following case:
1895	//
1896	// -->OldTop<-
1897	// | . |
1898	// | . |
1899	// | . |
1900	// ---Pred |
1901	// | |
1902	// BB-----
1903	//
1904	// If BB is moved before OldTop, Pred needs a taken branch to BB, and it can't
1905	// layout the other successor below it, so it can't reduce taken branch.
1906	// In this case we keep its original layout.
1907	bool
1908	MachineBlockPlacement::canMoveBottomBlockToTop(
1909	const MachineBasicBlock *BottomBlock,
1910	const MachineBasicBlock *OldTop) {
1911	if (BottomBlock->pred_size() != 1)
1912	return true;
1913	MachineBasicBlock *Pred = *BottomBlock->pred_begin();
1914	if (Pred->succ_size() != 2)
1915	return true;
1916
1917	MachineBasicBlock *OtherBB = *Pred->succ_begin();
1918	if (OtherBB == BottomBlock)
1919	OtherBB = *Pred->succ_rbegin();
1920	if (OtherBB == OldTop)
1921	return false;
1922
1923	return true;
1924	}
1925
1926	// Find out the possible fall through frequence to the top of a loop.
1927	BlockFrequency
1928	MachineBlockPlacement::TopFallThroughFreq(
1929	const MachineBasicBlock *Top,
1930	const BlockFilterSet &LoopBlockSet) {
1931	BlockFrequency MaxFreq = BlockFrequency(0);
1932	for (MachineBasicBlock *Pred : Top->predecessors()) {
1933	BlockChain *PredChain = BlockToChain[Pred];
1934	if (!LoopBlockSet.count(key: Pred) &&
1935	(!PredChain || Pred == *std::prev(x: PredChain->end()))) {
1936	// Found a Pred block can be placed before Top.
1937	// Check if Top is the best successor of Pred.
1938	auto TopProb = MBPI->getEdgeProbability(Src: Pred, Dst: Top);
1939	bool TopOK = true;
1940	for (MachineBasicBlock *Succ : Pred->successors()) {
1941	auto SuccProb = MBPI->getEdgeProbability(Src: Pred, Dst: Succ);
1942	BlockChain *SuccChain = BlockToChain[Succ];
1943	// Check if Succ can be placed after Pred.
1944	// Succ should not be in any chain, or it is the head of some chain.
1945	if (!LoopBlockSet.count(key: Succ) && (SuccProb > TopProb) &&
1946	(!SuccChain || Succ == *SuccChain->begin())) {
1947	TopOK = false;
1948	break;
1949	}
1950	}
1951	if (TopOK) {
1952	BlockFrequency EdgeFreq = MBFI->getBlockFreq(MBB: Pred) *
1953	MBPI->getEdgeProbability(Src: Pred, Dst: Top);
1954	if (EdgeFreq > MaxFreq)
1955	MaxFreq = EdgeFreq;
1956	}
1957	}
1958	}
1959	return MaxFreq;
1960	}
1961
1962	// Compute the fall through gains when move NewTop before OldTop.
1963	//
1964	// In following diagram, edges marked as "-" are reduced fallthrough, edges
1965	// marked as "+" are increased fallthrough, this function computes
1966	//
1967	// SUM(increased fallthrough) - SUM(decreased fallthrough)
1968	//
1969	// |
1970	// | -
1971	// V
1972	// --->OldTop
1973	// | .
1974	// | .
1975	// +| . +
1976	// | Pred --->
1977	// | |-
1978	// | V
1979	// --- NewTop <---
1980	// |-
1981	// V
1982	//
1983	BlockFrequency
1984	MachineBlockPlacement::FallThroughGains(
1985	const MachineBasicBlock *NewTop,
1986	const MachineBasicBlock *OldTop,
1987	const MachineBasicBlock *ExitBB,
1988	const BlockFilterSet &LoopBlockSet) {
1989	BlockFrequency FallThrough2Top = TopFallThroughFreq(Top: OldTop, LoopBlockSet);
1990	BlockFrequency FallThrough2Exit = BlockFrequency(0);
1991	if (ExitBB)
1992	FallThrough2Exit = MBFI->getBlockFreq(MBB: NewTop) *
1993	MBPI->getEdgeProbability(Src: NewTop, Dst: ExitBB);
1994	BlockFrequency BackEdgeFreq = MBFI->getBlockFreq(MBB: NewTop) *
1995	MBPI->getEdgeProbability(Src: NewTop, Dst: OldTop);
1996
1997	// Find the best Pred of NewTop.
1998	MachineBasicBlock *BestPred = nullptr;
1999	BlockFrequency FallThroughFromPred = BlockFrequency(0);
2000	for (MachineBasicBlock *Pred : NewTop->predecessors()) {
2001	if (!LoopBlockSet.count(key: Pred))
2002	continue;
2003	BlockChain *PredChain = BlockToChain[Pred];
2004	if (!PredChain || Pred == *std::prev(x: PredChain->end())) {
2005	BlockFrequency EdgeFreq =
2006	MBFI->getBlockFreq(MBB: Pred) * MBPI->getEdgeProbability(Src: Pred, Dst: NewTop);
2007	if (EdgeFreq > FallThroughFromPred) {
2008	FallThroughFromPred = EdgeFreq;
2009	BestPred = Pred;
2010	}
2011	}
2012	}
2013
2014	// If NewTop is not placed after Pred, another successor can be placed
2015	// after Pred.
2016	BlockFrequency NewFreq = BlockFrequency(0);
2017	if (BestPred) {
2018	for (MachineBasicBlock *Succ : BestPred->successors()) {
2019	if ((Succ == NewTop) || (Succ == BestPred) || !LoopBlockSet.count(key: Succ))
2020	continue;
2021	if (ComputedEdges.contains(Val: Succ))
2022	continue;
2023	BlockChain *SuccChain = BlockToChain[Succ];
2024	if ((SuccChain && (Succ != *SuccChain->begin())) ||
2025	(SuccChain == BlockToChain[BestPred]))
2026	continue;
2027	BlockFrequency EdgeFreq = MBFI->getBlockFreq(MBB: BestPred) *
2028	MBPI->getEdgeProbability(Src: BestPred, Dst: Succ);
2029	if (EdgeFreq > NewFreq)
2030	NewFreq = EdgeFreq;
2031	}
2032	BlockFrequency OrigEdgeFreq = MBFI->getBlockFreq(MBB: BestPred) *
2033	MBPI->getEdgeProbability(Src: BestPred, Dst: NewTop);
2034	if (NewFreq > OrigEdgeFreq) {
2035	// If NewTop is not the best successor of Pred, then Pred doesn't
2036	// fallthrough to NewTop. So there is no FallThroughFromPred and
2037	// NewFreq.
2038	NewFreq = BlockFrequency(0);
2039	FallThroughFromPred = BlockFrequency(0);
2040	}
2041	}
2042
2043	BlockFrequency Result = BlockFrequency(0);
2044	BlockFrequency Gains = BackEdgeFreq + NewFreq;
2045	BlockFrequency Lost =
2046	FallThrough2Top + FallThrough2Exit + FallThroughFromPred;
2047	if (Gains > Lost)
2048	Result = Gains - Lost;
2049	return Result;
2050	}
2051
2052	/// Helper function of findBestLoopTop. Find the best loop top block
2053	/// from predecessors of old top.
2054	///
2055	/// Look for a block which is strictly better than the old top for laying
2056	/// out before the old top of the loop. This looks for only two patterns:
2057	///
2058	/// 1. a block has only one successor, the old loop top
2059	///
2060	/// Because such a block will always result in an unconditional jump,
2061	/// rotating it in front of the old top is always profitable.
2062	///
2063	/// 2. a block has two successors, one is old top, another is exit
2064	/// and it has more than one predecessors
2065	///
2066	/// If it is below one of its predecessors P, only P can fall through to
2067	/// it, all other predecessors need a jump to it, and another conditional
2068	/// jump to loop header. If it is moved before loop header, all its
2069	/// predecessors jump to it, then fall through to loop header. So all its
2070	/// predecessors except P can reduce one taken branch.
2071	/// At the same time, move it before old top increases the taken branch
2072	/// to loop exit block, so the reduced taken branch will be compared with
2073	/// the increased taken branch to the loop exit block.
2074	MachineBasicBlock *
2075	MachineBlockPlacement::findBestLoopTopHelper(
2076	MachineBasicBlock *OldTop,
2077	const MachineLoop &L,
2078	const BlockFilterSet &LoopBlockSet) {
2079	// Check that the header hasn't been fused with a preheader block due to
2080	// crazy branches. If it has, we need to start with the header at the top to
2081	// prevent pulling the preheader into the loop body.
2082	BlockChain &HeaderChain = *BlockToChain[OldTop];
2083	if (!LoopBlockSet.count(key: *HeaderChain.begin()))
2084	return OldTop;
2085	if (OldTop != *HeaderChain.begin())
2086	return OldTop;
2087
2088	LLVM_DEBUG(dbgs() << "Finding best loop top for: " << getBlockName(OldTop)
2089	<< "\n");
2090
2091	BlockFrequency BestGains = BlockFrequency(0);
2092	MachineBasicBlock *BestPred = nullptr;
2093	for (MachineBasicBlock *Pred : OldTop->predecessors()) {
2094	if (!LoopBlockSet.count(key: Pred))
2095	continue;
2096	if (Pred == L.getHeader())
2097	continue;
2098	LLVM_DEBUG(dbgs() << " old top pred: " << getBlockName(Pred) << ", has "
2099	<< Pred->succ_size() << " successors, "
2100	<< printBlockFreq(MBFI->getMBFI(), *Pred) << " freq\n");
2101	if (Pred->succ_size() > 2)
2102	continue;
2103
2104	MachineBasicBlock *OtherBB = nullptr;
2105	if (Pred->succ_size() == 2) {
2106	OtherBB = *Pred->succ_begin();
2107	if (OtherBB == OldTop)
2108	OtherBB = *Pred->succ_rbegin();
2109	}
2110
2111	if (!canMoveBottomBlockToTop(BottomBlock: Pred, OldTop))
2112	continue;
2113
2114	BlockFrequency Gains = FallThroughGains(NewTop: Pred, OldTop, ExitBB: OtherBB,
2115	LoopBlockSet);
2116	if ((Gains > BlockFrequency(0)) &&
2117	(Gains > BestGains ||
2118	((Gains == BestGains) && Pred->isLayoutSuccessor(MBB: OldTop)))) {
2119	BestPred = Pred;
2120	BestGains = Gains;
2121	}
2122	}
2123
2124	// If no direct predecessor is fine, just use the loop header.
2125	if (!BestPred) {
2126	LLVM_DEBUG(dbgs() << " final top unchanged\n");
2127	return OldTop;
2128	}
2129
2130	// Walk backwards through any straight line of predecessors.
2131	while (BestPred->pred_size() == 1 &&
2132	(*BestPred->pred_begin())->succ_size() == 1 &&
2133	*BestPred->pred_begin() != L.getHeader())
2134	BestPred = *BestPred->pred_begin();
2135
2136	LLVM_DEBUG(dbgs() << " final top: " << getBlockName(BestPred) << "\n");
2137	return BestPred;
2138	}
2139
2140	/// Find the best loop top block for layout.
2141	///
2142	/// This function iteratively calls findBestLoopTopHelper, until no new better
2143	/// BB can be found.
2144	MachineBasicBlock *
2145	MachineBlockPlacement::findBestLoopTop(const MachineLoop &L,
2146	const BlockFilterSet &LoopBlockSet) {
2147	// Placing the latch block before the header may introduce an extra branch
2148	// that skips this block the first time the loop is executed, which we want
2149	// to avoid when optimising for size.
2150	// FIXME: in theory there is a case that does not introduce a new branch,
2151	// i.e. when the layout predecessor does not fallthrough to the loop header.
2152	// In practice this never happens though: there always seems to be a preheader
2153	// that can fallthrough and that is also placed before the header.
2154	bool OptForSize = F->getFunction().hasOptSize() ||
2155	llvm::shouldOptimizeForSize(MBB: L.getHeader(), PSI, MBFIWrapper: MBFI.get());
2156	if (OptForSize)
2157	return L.getHeader();
2158
2159	MachineBasicBlock *OldTop = nullptr;
2160	MachineBasicBlock *NewTop = L.getHeader();
2161	while (NewTop != OldTop) {
2162	OldTop = NewTop;
2163	NewTop = findBestLoopTopHelper(OldTop, L, LoopBlockSet);
2164	if (NewTop != OldTop)
2165	ComputedEdges[NewTop] = { .BB: OldTop, .ShouldTailDup: false };
2166	}
2167	return NewTop;
2168	}
2169
2170	/// Find the best loop exiting block for layout.
2171	///
2172	/// This routine implements the logic to analyze the loop looking for the best
2173	/// block to layout at the top of the loop. Typically this is done to maximize
2174	/// fallthrough opportunities.
2175	MachineBasicBlock *
2176	MachineBlockPlacement::findBestLoopExit(const MachineLoop &L,
2177	const BlockFilterSet &LoopBlockSet,
2178	BlockFrequency &ExitFreq) {
2179	// We don't want to layout the loop linearly in all cases. If the loop header
2180	// is just a normal basic block in the loop, we want to look for what block
2181	// within the loop is the best one to layout at the top. However, if the loop
2182	// header has be pre-merged into a chain due to predecessors not having
2183	// analyzable branches, *and* the predecessor it is merged with is *not* part
2184	// of the loop, rotating the header into the middle of the loop will create
2185	// a non-contiguous range of blocks which is Very Bad. So start with the
2186	// header and only rotate if safe.
2187	BlockChain &HeaderChain = *BlockToChain[L.getHeader()];
2188	if (!LoopBlockSet.count(key: *HeaderChain.begin()))
2189	return nullptr;
2190
2191	BlockFrequency BestExitEdgeFreq;
2192	unsigned BestExitLoopDepth = 0;
2193	MachineBasicBlock *ExitingBB = nullptr;
2194	// If there are exits to outer loops, loop rotation can severely limit
2195	// fallthrough opportunities unless it selects such an exit. Keep a set of
2196	// blocks where rotating to exit with that block will reach an outer loop.
2197	SmallPtrSet<MachineBasicBlock *, 4> BlocksExitingToOuterLoop;
2198
2199	LLVM_DEBUG(dbgs() << "Finding best loop exit for: "
2200	<< getBlockName(L.getHeader()) << "\n");
2201	for (MachineBasicBlock *MBB : L.getBlocks()) {
2202	BlockChain &Chain = *BlockToChain[MBB];
2203	// Ensure that this block is at the end of a chain; otherwise it could be
2204	// mid-way through an inner loop or a successor of an unanalyzable branch.
2205	if (MBB != *std::prev(x: Chain.end()))
2206	continue;
2207
2208	// Now walk the successors. We need to establish whether this has a viable
2209	// exiting successor and whether it has a viable non-exiting successor.
2210	// We store the old exiting state and restore it if a viable looping
2211	// successor isn't found.
2212	MachineBasicBlock *OldExitingBB = ExitingBB;
2213	BlockFrequency OldBestExitEdgeFreq = BestExitEdgeFreq;
2214	bool HasLoopingSucc = false;
2215	for (MachineBasicBlock *Succ : MBB->successors()) {
2216	if (Succ->isEHPad())
2217	continue;
2218	if (Succ == MBB)
2219	continue;
2220	BlockChain &SuccChain = *BlockToChain[Succ];
2221	// Don't split chains, either this chain or the successor's chain.
2222	if (&Chain == &SuccChain) {
2223	LLVM_DEBUG(dbgs() << " exiting: " << getBlockName(MBB) << " -> "
2224	<< getBlockName(Succ) << " (chain conflict)\n");
2225	continue;
2226	}
2227
2228	auto SuccProb = MBPI->getEdgeProbability(Src: MBB, Dst: Succ);
2229	if (LoopBlockSet.count(key: Succ)) {
2230	LLVM_DEBUG(dbgs() << " looping: " << getBlockName(MBB) << " -> "
2231	<< getBlockName(Succ) << " (" << SuccProb << ")\n");
2232	HasLoopingSucc = true;
2233	continue;
2234	}
2235
2236	unsigned SuccLoopDepth = 0;
2237	if (MachineLoop *ExitLoop = MLI->getLoopFor(BB: Succ)) {
2238	SuccLoopDepth = ExitLoop->getLoopDepth();
2239	if (ExitLoop->contains(L: &L))
2240	BlocksExitingToOuterLoop.insert(Ptr: MBB);
2241	}
2242
2243	BlockFrequency ExitEdgeFreq = MBFI->getBlockFreq(MBB) * SuccProb;
2244	LLVM_DEBUG(
2245	dbgs() << " exiting: " << getBlockName(MBB) << " -> "
2246	<< getBlockName(Succ) << " [L:" << SuccLoopDepth << "] ("
2247	<< printBlockFreq(MBFI->getMBFI(), ExitEdgeFreq) << ")\n");
2248	// Note that we bias this toward an existing layout successor to retain
2249	// incoming order in the absence of better information. The exit must have
2250	// a frequency higher than the current exit before we consider breaking
2251	// the layout.
2252	BranchProbability Bias(100 - ExitBlockBias, 100);
2253	if (!ExitingBB || SuccLoopDepth > BestExitLoopDepth ||
2254	ExitEdgeFreq > BestExitEdgeFreq ||
2255	(MBB->isLayoutSuccessor(MBB: Succ) &&
2256	!(ExitEdgeFreq < BestExitEdgeFreq * Bias))) {
2257	BestExitEdgeFreq = ExitEdgeFreq;
2258	ExitingBB = MBB;
2259	}
2260	}
2261
2262	if (!HasLoopingSucc) {
2263	// Restore the old exiting state, no viable looping successor was found.
2264	ExitingBB = OldExitingBB;
2265	BestExitEdgeFreq = OldBestExitEdgeFreq;
2266	}
2267	}
2268	// Without a candidate exiting block or with only a single block in the
2269	// loop, just use the loop header to layout the loop.
2270	if (!ExitingBB) {
2271	LLVM_DEBUG(
2272	dbgs() << " No other candidate exit blocks, using loop header\n");
2273	return nullptr;
2274	}
2275	if (L.getNumBlocks() == 1) {
2276	LLVM_DEBUG(dbgs() << " Loop has 1 block, using loop header as exit\n");
2277	return nullptr;
2278	}
2279
2280	// Also, if we have exit blocks which lead to outer loops but didn't select
2281	// one of them as the exiting block we are rotating toward, disable loop
2282	// rotation altogether.
2283	if (!BlocksExitingToOuterLoop.empty() &&
2284	!BlocksExitingToOuterLoop.count(Ptr: ExitingBB))
2285	return nullptr;
2286
2287	LLVM_DEBUG(dbgs() << " Best exiting block: " << getBlockName(ExitingBB)
2288	<< "\n");
2289	ExitFreq = BestExitEdgeFreq;
2290	return ExitingBB;
2291	}
2292
2293	/// Check if there is a fallthrough to loop header Top.
2294	///
2295	/// 1. Look for a Pred that can be layout before Top.
2296	/// 2. Check if Top is the most possible successor of Pred.
2297	bool
2298	MachineBlockPlacement::hasViableTopFallthrough(
2299	const MachineBasicBlock *Top,
2300	const BlockFilterSet &LoopBlockSet) {
2301	for (MachineBasicBlock *Pred : Top->predecessors()) {
2302	BlockChain *PredChain = BlockToChain[Pred];
2303	if (!LoopBlockSet.count(key: Pred) &&
2304	(!PredChain || Pred == *std::prev(x: PredChain->end()))) {
2305	// Found a Pred block can be placed before Top.
2306	// Check if Top is the best successor of Pred.
2307	auto TopProb = MBPI->getEdgeProbability(Src: Pred, Dst: Top);
2308	bool TopOK = true;
2309	for (MachineBasicBlock *Succ : Pred->successors()) {
2310	auto SuccProb = MBPI->getEdgeProbability(Src: Pred, Dst: Succ);
2311	BlockChain *SuccChain = BlockToChain[Succ];
2312	// Check if Succ can be placed after Pred.
2313	// Succ should not be in any chain, or it is the head of some chain.
2314	if ((!SuccChain || Succ == *SuccChain->begin()) && SuccProb > TopProb) {
2315	TopOK = false;
2316	break;
2317	}
2318	}
2319	if (TopOK)
2320	return true;
2321	}
2322	}
2323	return false;
2324	}
2325
2326	/// Attempt to rotate an exiting block to the bottom of the loop.
2327	///
2328	/// Once we have built a chain, try to rotate it to line up the hot exit block
2329	/// with fallthrough out of the loop if doing so doesn't introduce unnecessary
2330	/// branches. For example, if the loop has fallthrough into its header and out
2331	/// of its bottom already, don't rotate it.
2332	void MachineBlockPlacement::rotateLoop(BlockChain &LoopChain,
2333	const MachineBasicBlock *ExitingBB,
2334	BlockFrequency ExitFreq,
2335	const BlockFilterSet &LoopBlockSet) {
2336	if (!ExitingBB)
2337	return;
2338
2339	MachineBasicBlock *Top = *LoopChain.begin();
2340	MachineBasicBlock *Bottom = *std::prev(x: LoopChain.end());
2341
2342	// If ExitingBB is already the last one in a chain then nothing to do.
2343	if (Bottom == ExitingBB)
2344	return;
2345
2346	// The entry block should always be the first BB in a function.
2347	if (Top->isEntryBlock())
2348	return;
2349
2350	bool ViableTopFallthrough = hasViableTopFallthrough(Top, LoopBlockSet);
2351
2352	// If the header has viable fallthrough, check whether the current loop
2353	// bottom is a viable exiting block. If so, bail out as rotating will
2354	// introduce an unnecessary branch.
2355	if (ViableTopFallthrough) {
2356	for (MachineBasicBlock *Succ : Bottom->successors()) {
2357	BlockChain *SuccChain = BlockToChain[Succ];
2358	if (!LoopBlockSet.count(key: Succ) &&
2359	(!SuccChain || Succ == *SuccChain->begin()))
2360	return;
2361	}
2362
2363	// Rotate will destroy the top fallthrough, we need to ensure the new exit
2364	// frequency is larger than top fallthrough.
2365	BlockFrequency FallThrough2Top = TopFallThroughFreq(Top, LoopBlockSet);
2366	if (FallThrough2Top >= ExitFreq)
2367	return;
2368	}
2369
2370	BlockChain::iterator ExitIt = llvm::find(Range&: LoopChain, Val: ExitingBB);
2371	if (ExitIt == LoopChain.end())
2372	return;
2373
2374	// Rotating a loop exit to the bottom when there is a fallthrough to top
2375	// trades the entry fallthrough for an exit fallthrough.
2376	// If there is no bottom->top edge, but the chosen exit block does have
2377	// a fallthrough, we break that fallthrough for nothing in return.
2378
2379	// Let's consider an example. We have a built chain of basic blocks
2380	// B1, B2, ..., Bn, where Bk is a ExitingBB - chosen exit block.
2381	// By doing a rotation we get
2382	// Bk+1, ..., Bn, B1, ..., Bk
2383	// Break of fallthrough to B1 is compensated by a fallthrough from Bk.
2384	// If we had a fallthrough Bk -> Bk+1 it is broken now.
2385	// It might be compensated by fallthrough Bn -> B1.
2386	// So we have a condition to avoid creation of extra branch by loop rotation.
2387	// All below must be true to avoid loop rotation:
2388	// If there is a fallthrough to top (B1)
2389	// There was fallthrough from chosen exit block (Bk) to next one (Bk+1)
2390	// There is no fallthrough from bottom (Bn) to top (B1).
2391	// Please note that there is no exit fallthrough from Bn because we checked it
2392	// above.
2393	if (ViableTopFallthrough) {
2394	assert(std::next(ExitIt) != LoopChain.end() &&
2395	"Exit should not be last BB");
2396	MachineBasicBlock *NextBlockInChain = *std::next(x: ExitIt);
2397	if (ExitingBB->isSuccessor(MBB: NextBlockInChain))
2398	if (!Bottom->isSuccessor(MBB: Top))
2399	return;
2400	}
2401
2402	LLVM_DEBUG(dbgs() << "Rotating loop to put exit " << getBlockName(ExitingBB)
2403	<< " at bottom\n");
2404	std::rotate(first: LoopChain.begin(), middle: std::next(x: ExitIt), last: LoopChain.end());
2405	}
2406
2407	/// Attempt to rotate a loop based on profile data to reduce branch cost.
2408	///
2409	/// With profile data, we can determine the cost in terms of missed fall through
2410	/// opportunities when rotating a loop chain and select the best rotation.
2411	/// Basically, there are three kinds of cost to consider for each rotation:
2412	/// 1. The possibly missed fall through edge (if it exists) from BB out of
2413	/// the loop to the loop header.
2414	/// 2. The possibly missed fall through edges (if they exist) from the loop
2415	/// exits to BB out of the loop.
2416	/// 3. The missed fall through edge (if it exists) from the last BB to the
2417	/// first BB in the loop chain.
2418	/// Therefore, the cost for a given rotation is the sum of costs listed above.
2419	/// We select the best rotation with the smallest cost.
2420	void MachineBlockPlacement::rotateLoopWithProfile(
2421	BlockChain &LoopChain, const MachineLoop &L,
2422	const BlockFilterSet &LoopBlockSet) {
2423	auto RotationPos = LoopChain.end();
2424	MachineBasicBlock *ChainHeaderBB = *LoopChain.begin();
2425
2426	// The entry block should always be the first BB in a function.
2427	if (ChainHeaderBB->isEntryBlock())
2428	return;
2429
2430	BlockFrequency SmallestRotationCost = BlockFrequency::max();
2431
2432	// A utility lambda that scales up a block frequency by dividing it by a
2433	// branch probability which is the reciprocal of the scale.
2434	auto ScaleBlockFrequency = [](BlockFrequency Freq,
2435	unsigned Scale) -> BlockFrequency {
2436	if (Scale == 0)
2437	return BlockFrequency(0);
2438	// Use operator / between BlockFrequency and BranchProbability to implement
2439	// saturating multiplication.
2440	return Freq / BranchProbability(1, Scale);
2441	};
2442
2443	// Compute the cost of the missed fall-through edge to the loop header if the
2444	// chain head is not the loop header. As we only consider natural loops with
2445	// single header, this computation can be done only once.
2446	BlockFrequency HeaderFallThroughCost(0);
2447	for (auto *Pred : ChainHeaderBB->predecessors()) {
2448	BlockChain *PredChain = BlockToChain[Pred];
2449	if (!LoopBlockSet.count(key: Pred) &&
2450	(!PredChain || Pred == *std::prev(x: PredChain->end()))) {
2451	auto EdgeFreq = MBFI->getBlockFreq(MBB: Pred) *
2452	MBPI->getEdgeProbability(Src: Pred, Dst: ChainHeaderBB);
2453	auto FallThruCost = ScaleBlockFrequency(EdgeFreq, MisfetchCost);
2454	// If the predecessor has only an unconditional jump to the header, we
2455	// need to consider the cost of this jump.
2456	if (Pred->succ_size() == 1)
2457	FallThruCost += ScaleBlockFrequency(EdgeFreq, JumpInstCost);
2458	HeaderFallThroughCost = std::max(a: HeaderFallThroughCost, b: FallThruCost);
2459	}
2460	}
2461
2462	// Here we collect all exit blocks in the loop, and for each exit we find out
2463	// its hottest exit edge. For each loop rotation, we define the loop exit cost
2464	// as the sum of frequencies of exit edges we collect here, excluding the exit
2465	// edge from the tail of the loop chain.
2466	SmallVector<std::pair<MachineBasicBlock *, BlockFrequency>, 4> ExitsWithFreq;
2467	for (auto *BB : LoopChain) {
2468	auto LargestExitEdgeProb = BranchProbability::getZero();
2469	for (auto *Succ : BB->successors()) {
2470	BlockChain *SuccChain = BlockToChain[Succ];
2471	if (!LoopBlockSet.count(key: Succ) &&
2472	(!SuccChain || Succ == *SuccChain->begin())) {
2473	auto SuccProb = MBPI->getEdgeProbability(Src: BB, Dst: Succ);
2474	LargestExitEdgeProb = std::max(a: LargestExitEdgeProb, b: SuccProb);
2475	}
2476	}
2477	if (LargestExitEdgeProb > BranchProbability::getZero()) {
2478	auto ExitFreq = MBFI->getBlockFreq(MBB: BB) * LargestExitEdgeProb;
2479	ExitsWithFreq.emplace_back(Args&: BB, Args&: ExitFreq);
2480	}
2481	}
2482
2483	// In this loop we iterate every block in the loop chain and calculate the
2484	// cost assuming the block is the head of the loop chain. When the loop ends,
2485	// we should have found the best candidate as the loop chain's head.
2486	for (auto Iter = LoopChain.begin(), TailIter = std::prev(x: LoopChain.end()),
2487	EndIter = LoopChain.end();
2488	Iter != EndIter; Iter++, TailIter++) {
2489	// TailIter is used to track the tail of the loop chain if the block we are
2490	// checking (pointed by Iter) is the head of the chain.
2491	if (TailIter == LoopChain.end())
2492	TailIter = LoopChain.begin();
2493
2494	auto TailBB = *TailIter;
2495
2496	// Calculate the cost by putting this BB to the top.
2497	BlockFrequency Cost = BlockFrequency(0);
2498
2499	// If the current BB is the loop header, we need to take into account the
2500	// cost of the missed fall through edge from outside of the loop to the
2501	// header.
2502	if (Iter != LoopChain.begin())
2503	Cost += HeaderFallThroughCost;
2504
2505	// Collect the loop exit cost by summing up frequencies of all exit edges
2506	// except the one from the chain tail.
2507	for (auto &ExitWithFreq : ExitsWithFreq)
2508	if (TailBB != ExitWithFreq.first)
2509	Cost += ExitWithFreq.second;
2510
2511	// The cost of breaking the once fall-through edge from the tail to the top
2512	// of the loop chain. Here we need to consider three cases:
2513	// 1. If the tail node has only one successor, then we will get an
2514	// additional jmp instruction. So the cost here is (MisfetchCost +
2515	// JumpInstCost) * tail node frequency.
2516	// 2. If the tail node has two successors, then we may still get an
2517	// additional jmp instruction if the layout successor after the loop
2518	// chain is not its CFG successor. Note that the more frequently executed
2519	// jmp instruction will be put ahead of the other one. Assume the
2520	// frequency of those two branches are x and y, where x is the frequency
2521	// of the edge to the chain head, then the cost will be
2522	// (x * MisfetechCost + min(x, y) * JumpInstCost) * tail node frequency.
2523	// 3. If the tail node has more than two successors (this rarely happens),
2524	// we won't consider any additional cost.
2525	if (TailBB->isSuccessor(MBB: *Iter)) {
2526	auto TailBBFreq = MBFI->getBlockFreq(MBB: TailBB);
2527	if (TailBB->succ_size() == 1)
2528	Cost += ScaleBlockFrequency(TailBBFreq, MisfetchCost + JumpInstCost);
2529	else if (TailBB->succ_size() == 2) {
2530	auto TailToHeadProb = MBPI->getEdgeProbability(Src: TailBB, Dst: *Iter);
2531	auto TailToHeadFreq = TailBBFreq * TailToHeadProb;
2532	auto ColderEdgeFreq = TailToHeadProb > BranchProbability(1, 2)
2533	? TailBBFreq * TailToHeadProb.getCompl()
2534	: TailToHeadFreq;
2535	Cost += ScaleBlockFrequency(TailToHeadFreq, MisfetchCost) +
2536	ScaleBlockFrequency(ColderEdgeFreq, JumpInstCost);
2537	}
2538	}
2539
2540	LLVM_DEBUG(dbgs() << "The cost of loop rotation by making "
2541	<< getBlockName(*Iter) << " to the top: "
2542	<< printBlockFreq(MBFI->getMBFI(), Cost) << "\n");
2543
2544	if (Cost < SmallestRotationCost) {
2545	SmallestRotationCost = Cost;
2546	RotationPos = Iter;
2547	}
2548	}
2549
2550	if (RotationPos != LoopChain.end()) {
2551	LLVM_DEBUG(dbgs() << "Rotate loop by making " << getBlockName(*RotationPos)
2552	<< " to the top\n");
2553	std::rotate(first: LoopChain.begin(), middle: RotationPos, last: LoopChain.end());
2554	}
2555	}
2556
2557	/// Collect blocks in the given loop that are to be placed.
2558	///
2559	/// When profile data is available, exclude cold blocks from the returned set;
2560	/// otherwise, collect all blocks in the loop.
2561	MachineBlockPlacement::BlockFilterSet
2562	MachineBlockPlacement::collectLoopBlockSet(const MachineLoop &L) {
2563	BlockFilterSet LoopBlockSet;
2564
2565	// Filter cold blocks off from LoopBlockSet when profile data is available.
2566	// Collect the sum of frequencies of incoming edges to the loop header from
2567	// outside. If we treat the loop as a super block, this is the frequency of
2568	// the loop. Then for each block in the loop, we calculate the ratio between
2569	// its frequency and the frequency of the loop block. When it is too small,
2570	// don't add it to the loop chain. If there are outer loops, then this block
2571	// will be merged into the first outer loop chain for which this block is not
2572	// cold anymore. This needs precise profile data and we only do this when
2573	// profile data is available.
2574	if (F->getFunction().hasProfileData() || ForceLoopColdBlock) {
2575	BlockFrequency LoopFreq(0);
2576	for (auto *LoopPred : L.getHeader()->predecessors())
2577	if (!L.contains(BB: LoopPred))
2578	LoopFreq += MBFI->getBlockFreq(MBB: LoopPred) *
2579	MBPI->getEdgeProbability(Src: LoopPred, Dst: L.getHeader());
2580
2581	for (MachineBasicBlock *LoopBB : L.getBlocks()) {
2582	if (LoopBlockSet.count(key: LoopBB))
2583	continue;
2584	auto Freq = MBFI->getBlockFreq(MBB: LoopBB).getFrequency();
2585	if (Freq == 0 || LoopFreq.getFrequency() / Freq > LoopToColdBlockRatio)
2586	continue;
2587	BlockChain *Chain = BlockToChain[LoopBB];
2588	for (MachineBasicBlock *ChainBB : *Chain)
2589	LoopBlockSet.insert(X: ChainBB);
2590	}
2591	} else
2592	LoopBlockSet.insert(Start: L.block_begin(), End: L.block_end());
2593
2594	return LoopBlockSet;
2595	}
2596
2597	/// Forms basic block chains from the natural loop structures.
2598	///
2599	/// These chains are designed to preserve the existing *structure* of the code
2600	/// as much as possible. We can then stitch the chains together in a way which
2601	/// both preserves the topological structure and minimizes taken conditional
2602	/// branches.
2603	void MachineBlockPlacement::buildLoopChains(const MachineLoop &L) {
2604	// First recurse through any nested loops, building chains for those inner
2605	// loops.
2606	for (const MachineLoop *InnerLoop : L)
2607	buildLoopChains(L: *InnerLoop);
2608
2609	assert(BlockWorkList.empty() &&
2610	"BlockWorkList not empty when starting to build loop chains.");
2611	assert(EHPadWorkList.empty() &&
2612	"EHPadWorkList not empty when starting to build loop chains.");
2613	BlockFilterSet LoopBlockSet = collectLoopBlockSet(L);
2614
2615	// Check if we have profile data for this function. If yes, we will rotate
2616	// this loop by modeling costs more precisely which requires the profile data
2617	// for better layout.
2618	bool RotateLoopWithProfile =
2619	ForcePreciseRotationCost ||
2620	(PreciseRotationCost && F->getFunction().hasProfileData());
2621
2622	// First check to see if there is an obviously preferable top block for the
2623	// loop. This will default to the header, but may end up as one of the
2624	// predecessors to the header if there is one which will result in strictly
2625	// fewer branches in the loop body.
2626	MachineBasicBlock *LoopTop = findBestLoopTop(L, LoopBlockSet);
2627
2628	// If we selected just the header for the loop top, look for a potentially
2629	// profitable exit block in the event that rotating the loop can eliminate
2630	// branches by placing an exit edge at the bottom.
2631	//
2632	// Loops are processed innermost to uttermost, make sure we clear
2633	// PreferredLoopExit before processing a new loop.
2634	PreferredLoopExit = nullptr;
2635	BlockFrequency ExitFreq;
2636	if (!RotateLoopWithProfile && LoopTop == L.getHeader())
2637	PreferredLoopExit = findBestLoopExit(L, LoopBlockSet, ExitFreq);
2638
2639	BlockChain &LoopChain = *BlockToChain[LoopTop];
2640
2641	// FIXME: This is a really lame way of walking the chains in the loop: we
2642	// walk the blocks, and use a set to prevent visiting a particular chain
2643	// twice.
2644	SmallPtrSet<BlockChain *, 4> UpdatedPreds;
2645	assert(LoopChain.UnscheduledPredecessors == 0 &&
2646	"LoopChain should not have unscheduled predecessors.");
2647	UpdatedPreds.insert(Ptr: &LoopChain);
2648
2649	for (const MachineBasicBlock *LoopBB : LoopBlockSet)
2650	fillWorkLists(MBB: LoopBB, UpdatedPreds, BlockFilter: &LoopBlockSet);
2651
2652	buildChain(HeadBB: LoopTop, Chain&: LoopChain, BlockFilter: &LoopBlockSet);
2653
2654	if (RotateLoopWithProfile)
2655	rotateLoopWithProfile(LoopChain, L, LoopBlockSet);
2656	else
2657	rotateLoop(LoopChain, ExitingBB: PreferredLoopExit, ExitFreq, LoopBlockSet);
2658
2659	LLVM_DEBUG({
2660	// Crash at the end so we get all of the debugging output first.
2661	bool BadLoop = false;
2662	if (LoopChain.UnscheduledPredecessors) {
2663	BadLoop = true;
2664	dbgs() << "Loop chain contains a block without its preds placed!\n"
2665	<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
2666	<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n";
2667	}
2668	for (MachineBasicBlock *ChainBB : LoopChain) {
2669	dbgs() << " ... " << getBlockName(ChainBB) << "\n";
2670	if (!LoopBlockSet.remove(ChainBB)) {
2671	// We don't mark the loop as bad here because there are real situations
2672	// where this can occur. For example, with an unanalyzable fallthrough
2673	// from a loop block to a non-loop block or vice versa.
2674	dbgs() << "Loop chain contains a block not contained by the loop!\n"
2675	<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
2676	<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
2677	<< " Bad block: " << getBlockName(ChainBB) << "\n";
2678	}
2679	}
2680
2681	if (!LoopBlockSet.empty()) {
2682	BadLoop = true;
2683	for (const MachineBasicBlock *LoopBB : LoopBlockSet)
2684	dbgs() << "Loop contains blocks never placed into a chain!\n"
2685	<< " Loop header: " << getBlockName(*L.block_begin()) << "\n"
2686	<< " Chain header: " << getBlockName(*LoopChain.begin()) << "\n"
2687	<< " Bad block: " << getBlockName(LoopBB) << "\n";
2688	}
2689	assert(!BadLoop && "Detected problems with the placement of this loop.");
2690	});
2691
2692	BlockWorkList.clear();
2693	EHPadWorkList.clear();
2694	}
2695
2696	void MachineBlockPlacement::buildCFGChains() {
2697	// Ensure that every BB in the function has an associated chain to simplify
2698	// the assumptions of the remaining algorithm.
2699	SmallVector<MachineOperand, 4> Cond; // For analyzeBranch.
2700	for (MachineFunction::iterator FI = F->begin(), FE = F->end(); FI != FE;
2701	++FI) {
2702	MachineBasicBlock *BB = &*FI;
2703	BlockChain *Chain =
2704	new (ChainAllocator.Allocate()) BlockChain(BlockToChain, BB);
2705	// Also, merge any blocks which we cannot reason about and must preserve
2706	// the exact fallthrough behavior for.
2707	while (true) {
2708	Cond.clear();
2709	MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
2710	if (!TII->analyzeBranch(MBB&: *BB, TBB, FBB, Cond) || !FI->canFallThrough())
2711	break;
2712
2713	MachineFunction::iterator NextFI = std::next(x: FI);
2714	MachineBasicBlock *NextBB = &*NextFI;
2715	// Ensure that the layout successor is a viable block, as we know that
2716	// fallthrough is a possibility.
2717	assert(NextFI != FE && "Can't fallthrough past the last block.");
2718	LLVM_DEBUG(dbgs() << "Pre-merging due to unanalyzable fallthrough: "
2719	<< getBlockName(BB) << " -> " << getBlockName(NextBB)
2720	<< "\n");
2721	Chain->merge(BB: NextBB, Chain: nullptr);
2722	#ifndef NDEBUG
2723	BlocksWithUnanalyzableExits.insert(Ptr: &*BB);
2724	#endif
2725	FI = NextFI;
2726	BB = NextBB;
2727	}
2728	}
2729
2730	// Build any loop-based chains.
2731	PreferredLoopExit = nullptr;
2732	for (MachineLoop *L : *MLI)
2733	buildLoopChains(L: *L);
2734
2735	assert(BlockWorkList.empty() &&
2736	"BlockWorkList should be empty before building final chain.");
2737	assert(EHPadWorkList.empty() &&
2738	"EHPadWorkList should be empty before building final chain.");
2739
2740	SmallPtrSet<BlockChain *, 4> UpdatedPreds;
2741	for (MachineBasicBlock &MBB : *F)
2742	fillWorkLists(MBB: &MBB, UpdatedPreds);
2743
2744	BlockChain &FunctionChain = *BlockToChain[&F->front()];
2745	buildChain(HeadBB: &F->front(), Chain&: FunctionChain);
2746
2747	#ifndef NDEBUG
2748	using FunctionBlockSetType = SmallPtrSet<MachineBasicBlock *, 16>;
2749	#endif
2750	LLVM_DEBUG({
2751	// Crash at the end so we get all of the debugging output first.
2752	bool BadFunc = false;
2753	FunctionBlockSetType FunctionBlockSet;
2754	for (MachineBasicBlock &MBB : *F)
2755	FunctionBlockSet.insert(&MBB);
2756
2757	for (MachineBasicBlock *ChainBB : FunctionChain)
2758	if (!FunctionBlockSet.erase(ChainBB)) {
2759	BadFunc = true;
2760	dbgs() << "Function chain contains a block not in the function!\n"
2761	<< " Bad block: " << getBlockName(ChainBB) << "\n";
2762	}
2763
2764	if (!FunctionBlockSet.empty()) {
2765	BadFunc = true;
2766	for (MachineBasicBlock *RemainingBB : FunctionBlockSet)
2767	dbgs() << "Function contains blocks never placed into a chain!\n"
2768	<< " Bad block: " << getBlockName(RemainingBB) << "\n";
2769	}
2770	assert(!BadFunc && "Detected problems with the block placement.");
2771	});
2772
2773	// Remember original layout ordering, so we can update terminators after
2774	// reordering to point to the original layout successor.
2775	SmallVector<MachineBasicBlock *, 4> OriginalLayoutSuccessors(
2776	F->getNumBlockIDs());
2777	{
2778	MachineBasicBlock *LastMBB = nullptr;
2779	for (auto &MBB : *F) {
2780	if (LastMBB != nullptr)
2781	OriginalLayoutSuccessors[LastMBB->getNumber()] = &MBB;
2782	LastMBB = &MBB;
2783	}
2784	OriginalLayoutSuccessors[F->back().getNumber()] = nullptr;
2785	}
2786
2787	// Splice the blocks into place.
2788	MachineFunction::iterator InsertPos = F->begin();
2789	LLVM_DEBUG(dbgs() << "[MBP] Function: " << F->getName() << "\n");
2790	for (MachineBasicBlock *ChainBB : FunctionChain) {
2791	LLVM_DEBUG(dbgs() << (ChainBB == *FunctionChain.begin() ? "Placing chain "
2792	: " ... ")
2793	<< getBlockName(ChainBB) << "\n");
2794	if (InsertPos != MachineFunction::iterator(ChainBB))
2795	F->splice(InsertPt: InsertPos, MBB: ChainBB);
2796	else
2797	++InsertPos;
2798
2799	// Update the terminator of the previous block.
2800	if (ChainBB == *FunctionChain.begin())
2801	continue;
2802	MachineBasicBlock *PrevBB = &*std::prev(x: MachineFunction::iterator(ChainBB));
2803
2804	// FIXME: It would be awesome of updateTerminator would just return rather
2805	// than assert when the branch cannot be analyzed in order to remove this
2806	// boiler plate.
2807	Cond.clear();
2808	MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
2809
2810	#ifndef NDEBUG
2811	if (!BlocksWithUnanalyzableExits.count(Ptr: PrevBB)) {
2812	// Given the exact block placement we chose, we may actually not _need_ to
2813	// be able to edit PrevBB's terminator sequence, but not being _able_ to
2814	// do that at this point is a bug.
2815	assert((!TII->analyzeBranch(*PrevBB, TBB, FBB, Cond) ||
2816	!PrevBB->canFallThrough()) &&
2817	"Unexpected block with un-analyzable fallthrough!");
2818	Cond.clear();
2819	TBB = FBB = nullptr;
2820	}
2821	#endif
2822
2823	// The "PrevBB" is not yet updated to reflect current code layout, so,
2824	// o. it may fall-through to a block without explicit "goto" instruction
2825	// before layout, and no longer fall-through it after layout; or
2826	// o. just opposite.
2827	//
2828	// analyzeBranch() may return erroneous value for FBB when these two
2829	// situations take place. For the first scenario FBB is mistakenly set NULL;
2830	// for the 2nd scenario, the FBB, which is expected to be NULL, is
2831	// mistakenly pointing to "*BI".
2832	// Thus, if the future change needs to use FBB before the layout is set, it
2833	// has to correct FBB first by using the code similar to the following:
2834	//
2835	// if (!Cond.empty() && (!FBB || FBB == ChainBB)) {
2836	// PrevBB->updateTerminator();
2837	// Cond.clear();
2838	// TBB = FBB = nullptr;
2839	// if (TII->analyzeBranch(*PrevBB, TBB, FBB, Cond)) {
2840	// // FIXME: This should never take place.
2841	// TBB = FBB = nullptr;
2842	// }
2843	// }
2844	if (!TII->analyzeBranch(MBB&: *PrevBB, TBB, FBB, Cond)) {
2845	PrevBB->updateTerminator(PreviousLayoutSuccessor: OriginalLayoutSuccessors[PrevBB->getNumber()]);
2846	}
2847	}
2848
2849	// Fixup the last block.
2850	Cond.clear();
2851	MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
2852	if (!TII->analyzeBranch(MBB&: F->back(), TBB, FBB, Cond)) {
2853	MachineBasicBlock *PrevBB = &F->back();
2854	PrevBB->updateTerminator(PreviousLayoutSuccessor: OriginalLayoutSuccessors[PrevBB->getNumber()]);
2855	}
2856
2857	BlockWorkList.clear();
2858	EHPadWorkList.clear();
2859	}
2860
2861	void MachineBlockPlacement::optimizeBranches() {
2862	BlockChain &FunctionChain = *BlockToChain[&F->front()];
2863	SmallVector<MachineOperand, 4> Cond; // For analyzeBranch.
2864
2865	// Now that all the basic blocks in the chain have the proper layout,
2866	// make a final call to analyzeBranch with AllowModify set.
2867	// Indeed, the target may be able to optimize the branches in a way we
2868	// cannot because all branches may not be analyzable.
2869	// E.g., the target may be able to remove an unconditional branch to
2870	// a fallthrough when it occurs after predicated terminators.
2871	for (MachineBasicBlock *ChainBB : FunctionChain) {
2872	Cond.clear();
2873	MachineBasicBlock *TBB = nullptr, *FBB = nullptr; // For analyzeBranch.
2874	if (!TII->analyzeBranch(MBB&: *ChainBB, TBB, FBB, Cond, /*AllowModify*/ true)) {
2875	// If PrevBB has a two-way branch, try to re-order the branches
2876	// such that we branch to the successor with higher probability first.
2877	if (TBB && !Cond.empty() && FBB &&
2878	MBPI->getEdgeProbability(Src: ChainBB, Dst: FBB) >
2879	MBPI->getEdgeProbability(Src: ChainBB, Dst: TBB) &&
2880	!TII->reverseBranchCondition(Cond)) {
2881	LLVM_DEBUG(dbgs() << "Reverse order of the two branches: "
2882	<< getBlockName(ChainBB) << "\n");
2883	LLVM_DEBUG(dbgs() << " Edge probability: "
2884	<< MBPI->getEdgeProbability(ChainBB, FBB) << " vs "
2885	<< MBPI->getEdgeProbability(ChainBB, TBB) << "\n");
2886	DebugLoc dl; // FIXME: this is nowhere
2887	TII->removeBranch(MBB&: *ChainBB);
2888	TII->insertBranch(MBB&: *ChainBB, TBB: FBB, FBB: TBB, Cond, DL: dl);
2889	}
2890	}
2891	}
2892	}
2893
2894	void MachineBlockPlacement::alignBlocks() {
2895	// Walk through the backedges of the function now that we have fully laid out
2896	// the basic blocks and align the destination of each backedge. We don't rely
2897	// exclusively on the loop info here so that we can align backedges in
2898	// unnatural CFGs and backedges that were introduced purely because of the
2899	// loop rotations done during this layout pass.
2900	if (F->getFunction().hasMinSize() ||
2901	(F->getFunction().hasOptSize() && !TLI->alignLoopsWithOptSize()))
2902	return;
2903	BlockChain &FunctionChain = *BlockToChain[&F->front()];
2904	if (FunctionChain.begin() == FunctionChain.end())
2905	return; // Empty chain.
2906
2907	const BranchProbability ColdProb(1, 5); // 20%
2908	BlockFrequency EntryFreq = MBFI->getBlockFreq(MBB: &F->front());
2909	BlockFrequency WeightedEntryFreq = EntryFreq * ColdProb;
2910	for (MachineBasicBlock *ChainBB : FunctionChain) {
2911	if (ChainBB == *FunctionChain.begin())
2912	continue;
2913
2914	// Don't align non-looping basic blocks. These are unlikely to execute
2915	// enough times to matter in practice. Note that we'll still handle
2916	// unnatural CFGs inside of a natural outer loop (the common case) and
2917	// rotated loops.
2918	MachineLoop *L = MLI->getLoopFor(BB: ChainBB);
2919	if (!L)
2920	continue;
2921
2922	const Align TLIAlign = TLI->getPrefLoopAlignment(ML: L);
2923	unsigned MDAlign = 1;
2924	MDNode *LoopID = L->getLoopID();
2925	if (LoopID) {
2926	for (const MDOperand &MDO : llvm::drop_begin(RangeOrContainer: LoopID->operands())) {
2927	MDNode *MD = dyn_cast<MDNode>(Val: MDO);
2928	if (MD == nullptr)
2929	continue;
2930	MDString *S = dyn_cast<MDString>(Val: MD->getOperand(I: 0));
2931	if (S == nullptr)
2932	continue;
2933	if (S->getString() == "llvm.loop.align") {
2934	assert(MD->getNumOperands() == 2 &&
2935	"per-loop align metadata should have two operands.");
2936	MDAlign =
2937	mdconst::extract<ConstantInt>(MD: MD->getOperand(I: 1))->getZExtValue();
2938	assert(MDAlign >= 1 && "per-loop align value must be positive.");
2939	}
2940	}
2941	}
2942
2943	// Use max of the TLIAlign and MDAlign
2944	const Align LoopAlign = std::max(a: TLIAlign, b: Align(MDAlign));
2945	if (LoopAlign == 1)
2946	continue; // Don't care about loop alignment.
2947
2948	// If the block is cold relative to the function entry don't waste space
2949	// aligning it.
2950	BlockFrequency Freq = MBFI->getBlockFreq(MBB: ChainBB);
2951	if (Freq < WeightedEntryFreq)
2952	continue;
2953
2954	// If the block is cold relative to its loop header, don't align it
2955	// regardless of what edges into the block exist.
2956	MachineBasicBlock *LoopHeader = L->getHeader();
2957	BlockFrequency LoopHeaderFreq = MBFI->getBlockFreq(MBB: LoopHeader);
2958	if (Freq < (LoopHeaderFreq * ColdProb))
2959	continue;
2960
2961	// If the global profiles indicates so, don't align it.
2962	if (llvm::shouldOptimizeForSize(MBB: ChainBB, PSI, MBFIWrapper: MBFI.get()) &&
2963	!TLI->alignLoopsWithOptSize())
2964	continue;
2965
2966	// Check for the existence of a non-layout predecessor which would benefit
2967	// from aligning this block.
2968	MachineBasicBlock *LayoutPred =
2969	&*std::prev(x: MachineFunction::iterator(ChainBB));
2970
2971	auto DetermineMaxAlignmentPadding = [&]() {
2972	// Set the maximum bytes allowed to be emitted for alignment.
2973	unsigned MaxBytes;
2974	if (MaxBytesForAlignmentOverride.getNumOccurrences() > 0)
2975	MaxBytes = MaxBytesForAlignmentOverride;
2976	else
2977	MaxBytes = TLI->getMaxPermittedBytesForAlignment(MBB: ChainBB);
2978	ChainBB->setMaxBytesForAlignment(MaxBytes);
2979	};
2980
2981	// Force alignment if all the predecessors are jumps. We already checked
2982	// that the block isn't cold above.
2983	if (!LayoutPred->isSuccessor(MBB: ChainBB)) {
2984	ChainBB->setAlignment(LoopAlign);
2985	DetermineMaxAlignmentPadding();
2986	continue;
2987	}
2988
2989	// Align this block if the layout predecessor's edge into this block is
2990	// cold relative to the block. When this is true, other predecessors make up
2991	// all of the hot entries into the block and thus alignment is likely to be
2992	// important.
2993	BranchProbability LayoutProb =
2994	MBPI->getEdgeProbability(Src: LayoutPred, Dst: ChainBB);
2995	BlockFrequency LayoutEdgeFreq = MBFI->getBlockFreq(MBB: LayoutPred) * LayoutProb;
2996	if (LayoutEdgeFreq <= (Freq * ColdProb)) {
2997	ChainBB->setAlignment(LoopAlign);
2998	DetermineMaxAlignmentPadding();
2999	}
3000	}
3001	}
3002
3003	/// Tail duplicate \p BB into (some) predecessors if profitable, repeating if
3004	/// it was duplicated into its chain predecessor and removed.
3005	/// \p BB - Basic block that may be duplicated.
3006	///
3007	/// \p LPred - Chosen layout predecessor of \p BB.
3008	/// Updated to be the chain end if LPred is removed.
3009	/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
3010	/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
3011	/// Used to identify which blocks to update predecessor
3012	/// counts.
3013	/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
3014	/// chosen in the given order due to unnatural CFG
3015	/// only needed if \p BB is removed and
3016	/// \p PrevUnplacedBlockIt pointed to \p BB.
3017	/// @return true if \p BB was removed.
3018	bool MachineBlockPlacement::repeatedlyTailDuplicateBlock(
3019	MachineBasicBlock *BB, MachineBasicBlock *&LPred,
3020	const MachineBasicBlock *LoopHeaderBB,
3021	BlockChain &Chain, BlockFilterSet *BlockFilter,
3022	MachineFunction::iterator &PrevUnplacedBlockIt) {
3023	bool Removed, DuplicatedToLPred;
3024	bool DuplicatedToOriginalLPred;
3025	Removed = maybeTailDuplicateBlock(BB, LPred, Chain, BlockFilter,
3026	PrevUnplacedBlockIt,
3027	DuplicatedToLPred);
3028	if (!Removed)
3029	return false;
3030	DuplicatedToOriginalLPred = DuplicatedToLPred;
3031	// Iteratively try to duplicate again. It can happen that a block that is
3032	// duplicated into is still small enough to be duplicated again.
3033	// No need to call markBlockSuccessors in this case, as the blocks being
3034	// duplicated from here on are already scheduled.
3035	while (DuplicatedToLPred && Removed) {
3036	MachineBasicBlock *DupBB, *DupPred;
3037	// The removal callback causes Chain.end() to be updated when a block is
3038	// removed. On the first pass through the loop, the chain end should be the
3039	// same as it was on function entry. On subsequent passes, because we are
3040	// duplicating the block at the end of the chain, if it is removed the
3041	// chain will have shrunk by one block.
3042	BlockChain::iterator ChainEnd = Chain.end();
3043	DupBB = *(--ChainEnd);
3044	// Now try to duplicate again.
3045	if (ChainEnd == Chain.begin())
3046	break;
3047	DupPred = *std::prev(x: ChainEnd);
3048	Removed = maybeTailDuplicateBlock(BB: DupBB, LPred: DupPred, Chain, BlockFilter,
3049	PrevUnplacedBlockIt,
3050	DuplicatedToLPred);
3051	}
3052	// If BB was duplicated into LPred, it is now scheduled. But because it was
3053	// removed, markChainSuccessors won't be called for its chain. Instead we
3054	// call markBlockSuccessors for LPred to achieve the same effect. This must go
3055	// at the end because repeating the tail duplication can increase the number
3056	// of unscheduled predecessors.
3057	LPred = *std::prev(x: Chain.end());
3058	if (DuplicatedToOriginalLPred)
3059	markBlockSuccessors(Chain, MBB: LPred, LoopHeaderBB, BlockFilter);
3060	return true;
3061	}
3062
3063	/// Tail duplicate \p BB into (some) predecessors if profitable.
3064	/// \p BB - Basic block that may be duplicated
3065	/// \p LPred - Chosen layout predecessor of \p BB
3066	/// \p Chain - Chain to which \p LPred belongs, and \p BB will belong.
3067	/// \p BlockFilter - Set of blocks that belong to the loop being laid out.
3068	/// Used to identify which blocks to update predecessor
3069	/// counts.
3070	/// \p PrevUnplacedBlockIt - Iterator pointing to the last block that was
3071	/// chosen in the given order due to unnatural CFG
3072	/// only needed if \p BB is removed and
3073	/// \p PrevUnplacedBlockIt pointed to \p BB.
3074	/// \p DuplicatedToLPred - True if the block was duplicated into LPred.
3075	/// \return - True if the block was duplicated into all preds and removed.
3076	bool MachineBlockPlacement::maybeTailDuplicateBlock(
3077	MachineBasicBlock *BB, MachineBasicBlock *LPred,
3078	BlockChain &Chain, BlockFilterSet *BlockFilter,
3079	MachineFunction::iterator &PrevUnplacedBlockIt,
3080	bool &DuplicatedToLPred) {
3081	DuplicatedToLPred = false;
3082	if (!shouldTailDuplicate(BB))
3083	return false;
3084
3085	LLVM_DEBUG(dbgs() << "Redoing tail duplication for Succ#" << BB->getNumber()
3086	<< "\n");
3087
3088	// This has to be a callback because none of it can be done after
3089	// BB is deleted.
3090	bool Removed = false;
3091	auto RemovalCallback =
3092	[&](MachineBasicBlock *RemBB) {
3093	// Signal to outer function
3094	Removed = true;
3095
3096	// Conservative default.
3097	bool InWorkList = true;
3098	// Remove from the Chain and Chain Map
3099	if (BlockToChain.count(Val: RemBB)) {
3100	BlockChain *Chain = BlockToChain[RemBB];
3101	InWorkList = Chain->UnscheduledPredecessors == 0;
3102	Chain->remove(BB: RemBB);
3103	BlockToChain.erase(Val: RemBB);
3104	}
3105
3106	// Handle the unplaced block iterator
3107	if (&(*PrevUnplacedBlockIt) == RemBB) {
3108	PrevUnplacedBlockIt++;
3109	}
3110
3111	// Handle the Work Lists
3112	if (InWorkList) {
3113	SmallVectorImpl<MachineBasicBlock *> &RemoveList = BlockWorkList;
3114	if (RemBB->isEHPad())
3115	RemoveList = EHPadWorkList;
3116	llvm::erase(C&: RemoveList, V: RemBB);
3117	}
3118
3119	// Handle the filter set
3120	if (BlockFilter) {
3121	BlockFilter->remove(X: RemBB);
3122	}
3123
3124	// Remove the block from loop info.
3125	MLI->removeBlock(BB: RemBB);
3126	if (RemBB == PreferredLoopExit)
3127	PreferredLoopExit = nullptr;
3128
3129	LLVM_DEBUG(dbgs() << "TailDuplicator deleted block: "
3130	<< getBlockName(RemBB) << "\n");
3131	};
3132	auto RemovalCallbackRef =
3133	function_ref<void(MachineBasicBlock*)>(RemovalCallback);
3134
3135	SmallVector<MachineBasicBlock *, 8> DuplicatedPreds;
3136	bool IsSimple = TailDup.isSimpleBB(TailBB: BB);
3137	SmallVector<MachineBasicBlock *, 8> CandidatePreds;
3138	SmallVectorImpl<MachineBasicBlock *> *CandidatePtr = nullptr;
3139	if (F->getFunction().hasProfileData()) {
3140	// We can do partial duplication with precise profile information.
3141	findDuplicateCandidates(Candidates&: CandidatePreds, BB, BlockFilter);
3142	if (CandidatePreds.size() == 0)
3143	return false;
3144	if (CandidatePreds.size() < BB->pred_size())
3145	CandidatePtr = &CandidatePreds;
3146	}
3147	TailDup.tailDuplicateAndUpdate(IsSimple, MBB: BB, ForcedLayoutPred: LPred, DuplicatedPreds: &DuplicatedPreds,
3148	RemovalCallback: &RemovalCallbackRef, CandidatePtr);
3149
3150	// Update UnscheduledPredecessors to reflect tail-duplication.
3151	DuplicatedToLPred = false;
3152	for (MachineBasicBlock *Pred : DuplicatedPreds) {
3153	// We're only looking for unscheduled predecessors that match the filter.
3154	BlockChain* PredChain = BlockToChain[Pred];
3155	if (Pred == LPred)
3156	DuplicatedToLPred = true;
3157	if (Pred == LPred || (BlockFilter && !BlockFilter->count(key: Pred))
3158	|| PredChain == &Chain)
3159	continue;
3160	for (MachineBasicBlock *NewSucc : Pred->successors()) {
3161	if (BlockFilter && !BlockFilter->count(key: NewSucc))
3162	continue;
3163	BlockChain *NewChain = BlockToChain[NewSucc];
3164	if (NewChain != &Chain && NewChain != PredChain)
3165	NewChain->UnscheduledPredecessors++;
3166	}
3167	}
3168	return Removed;
3169	}
3170
3171	// Count the number of actual machine instructions.
3172	static uint64_t countMBBInstruction(MachineBasicBlock *MBB) {
3173	uint64_t InstrCount = 0;
3174	for (MachineInstr &MI : *MBB) {
3175	if (!MI.isPHI() && !MI.isMetaInstruction())
3176	InstrCount += 1;
3177	}
3178	return InstrCount;
3179	}
3180
3181	// The size cost of duplication is the instruction size of the duplicated block.
3182	// So we should scale the threshold accordingly. But the instruction size is not
3183	// available on all targets, so we use the number of instructions instead.
3184	BlockFrequency MachineBlockPlacement::scaleThreshold(MachineBasicBlock *BB) {
3185	return BlockFrequency(DupThreshold.getFrequency() * countMBBInstruction(MBB: BB));
3186	}
3187
3188	// Returns true if BB is Pred's best successor.
3189	bool MachineBlockPlacement::isBestSuccessor(MachineBasicBlock *BB,
3190	MachineBasicBlock *Pred,
3191	BlockFilterSet *BlockFilter) {
3192	if (BB == Pred)
3193	return false;
3194	if (BlockFilter && !BlockFilter->count(key: Pred))
3195	return false;
3196	BlockChain *PredChain = BlockToChain[Pred];
3197	if (PredChain && (Pred != *std::prev(x: PredChain->end())))
3198	return false;
3199
3200	// Find the successor with largest probability excluding BB.
3201	BranchProbability BestProb = BranchProbability::getZero();
3202	for (MachineBasicBlock *Succ : Pred->successors())
3203	if (Succ != BB) {
3204	if (BlockFilter && !BlockFilter->count(key: Succ))
3205	continue;
3206	BlockChain *SuccChain = BlockToChain[Succ];
3207	if (SuccChain && (Succ != *SuccChain->begin()))
3208	continue;
3209	BranchProbability SuccProb = MBPI->getEdgeProbability(Src: Pred, Dst: Succ);
3210	if (SuccProb > BestProb)
3211	BestProb = SuccProb;
3212	}
3213
3214	BranchProbability BBProb = MBPI->getEdgeProbability(Src: Pred, Dst: BB);
3215	if (BBProb <= BestProb)
3216	return false;
3217
3218	// Compute the number of reduced taken branches if Pred falls through to BB
3219	// instead of another successor. Then compare it with threshold.
3220	BlockFrequency PredFreq = getBlockCountOrFrequency(BB: Pred);
3221	BlockFrequency Gain = PredFreq * (BBProb - BestProb);
3222	return Gain > scaleThreshold(BB);
3223	}
3224
3225	// Find out the predecessors of BB and BB can be beneficially duplicated into
3226	// them.
3227	void MachineBlockPlacement::findDuplicateCandidates(
3228	SmallVectorImpl<MachineBasicBlock *> &Candidates,
3229	MachineBasicBlock *BB,
3230	BlockFilterSet *BlockFilter) {
3231	MachineBasicBlock *Fallthrough = nullptr;
3232	BranchProbability DefaultBranchProb = BranchProbability::getZero();
3233	BlockFrequency BBDupThreshold(scaleThreshold(BB));
3234	SmallVector<MachineBasicBlock *, 8> Preds(BB->predecessors());
3235	SmallVector<MachineBasicBlock *, 8> Succs(BB->successors());
3236
3237	// Sort for highest frequency.
3238	auto CmpSucc = [&](MachineBasicBlock *A, MachineBasicBlock *B) {
3239	return MBPI->getEdgeProbability(Src: BB, Dst: A) > MBPI->getEdgeProbability(Src: BB, Dst: B);
3240	};
3241	auto CmpPred = [&](MachineBasicBlock *A, MachineBasicBlock *B) {
3242	return MBFI->getBlockFreq(MBB: A) > MBFI->getBlockFreq(MBB: B);
3243	};
3244	llvm::stable_sort(Range&: Succs, C: CmpSucc);
3245	llvm::stable_sort(Range&: Preds, C: CmpPred);
3246
3247	auto SuccIt = Succs.begin();
3248	if (SuccIt != Succs.end()) {
3249	DefaultBranchProb = MBPI->getEdgeProbability(Src: BB, Dst: *SuccIt).getCompl();
3250	}
3251
3252	// For each predecessors of BB, compute the benefit of duplicating BB,
3253	// if it is larger than the threshold, add it into Candidates.
3254	//
3255	// If we have following control flow.
3256	//
3257	// PB1 PB2 PB3 PB4
3258	// \ | / /\
3259	// \ | / / \
3260	// \ |/ / \
3261	// BB----/ OB
3262	// /\
3263	// / \
3264	// SB1 SB2
3265	//
3266	// And it can be partially duplicated as
3267	//
3268	// PB2+BB
3269	// | PB1 PB3 PB4
3270	// | | / /\
3271	// | | / / \
3272	// | |/ / \
3273	// | BB----/ OB
3274	// |\ /|
3275	// | X |
3276	// |/ \|
3277	// SB2 SB1
3278	//
3279	// The benefit of duplicating into a predecessor is defined as
3280	// Orig_taken_branch - Duplicated_taken_branch
3281	//
3282	// The Orig_taken_branch is computed with the assumption that predecessor
3283	// jumps to BB and the most possible successor is laid out after BB.
3284	//
3285	// The Duplicated_taken_branch is computed with the assumption that BB is
3286	// duplicated into PB, and one successor is layout after it (SB1 for PB1 and
3287	// SB2 for PB2 in our case). If there is no available successor, the combined
3288	// block jumps to all BB's successor, like PB3 in this example.
3289	//
3290	// If a predecessor has multiple successors, so BB can't be duplicated into
3291	// it. But it can beneficially fall through to BB, and duplicate BB into other
3292	// predecessors.
3293	for (MachineBasicBlock *Pred : Preds) {
3294	BlockFrequency PredFreq = getBlockCountOrFrequency(BB: Pred);
3295
3296	if (!TailDup.canTailDuplicate(TailBB: BB, PredBB: Pred)) {
3297	// BB can't be duplicated into Pred, but it is possible to be layout
3298	// below Pred.
3299	if (!Fallthrough && isBestSuccessor(BB, Pred, BlockFilter)) {
3300	Fallthrough = Pred;
3301	if (SuccIt != Succs.end())
3302	SuccIt++;
3303	}
3304	continue;
3305	}
3306
3307	BlockFrequency OrigCost = PredFreq + PredFreq * DefaultBranchProb;
3308	BlockFrequency DupCost;
3309	if (SuccIt == Succs.end()) {
3310	// Jump to all successors;
3311	if (Succs.size() > 0)
3312	DupCost += PredFreq;
3313	} else {
3314	// Fallthrough to *SuccIt, jump to all other successors;
3315	DupCost += PredFreq;
3316	DupCost -= PredFreq * MBPI->getEdgeProbability(Src: BB, Dst: *SuccIt);
3317	}
3318
3319	assert(OrigCost >= DupCost);
3320	OrigCost -= DupCost;
3321	if (OrigCost > BBDupThreshold) {
3322	Candidates.push_back(Elt: Pred);
3323	if (SuccIt != Succs.end())
3324	SuccIt++;
3325	}
3326	}
3327
3328	// No predecessors can optimally fallthrough to BB.
3329	// So we can change one duplication into fallthrough.
3330	if (!Fallthrough) {
3331	if ((Candidates.size() < Preds.size()) && (Candidates.size() > 0)) {
3332	Candidates[0] = Candidates.back();
3333	Candidates.pop_back();
3334	}
3335	}
3336	}
3337
3338	void MachineBlockPlacement::initDupThreshold() {
3339	DupThreshold = BlockFrequency(0);
3340	if (!F->getFunction().hasProfileData())
3341	return;
3342
3343	// We prefer to use prifile count.
3344	uint64_t HotThreshold = PSI->getOrCompHotCountThreshold();
3345	if (HotThreshold != UINT64_MAX) {
3346	UseProfileCount = true;
3347	DupThreshold =
3348	BlockFrequency(HotThreshold * TailDupProfilePercentThreshold / 100);
3349	return;
3350	}
3351
3352	// Profile count is not available, we can use block frequency instead.
3353	BlockFrequency MaxFreq = BlockFrequency(0);
3354	for (MachineBasicBlock &MBB : *F) {
3355	BlockFrequency Freq = MBFI->getBlockFreq(MBB: &MBB);
3356	if (Freq > MaxFreq)
3357	MaxFreq = Freq;
3358	}
3359
3360	BranchProbability ThresholdProb(TailDupPlacementPenalty, 100);
3361	DupThreshold = BlockFrequency(MaxFreq * ThresholdProb);
3362	UseProfileCount = false;
3363	}
3364
3365	bool MachineBlockPlacement::runOnMachineFunction(MachineFunction &MF) {
3366	if (skipFunction(F: MF.getFunction()))
3367	return false;
3368
3369	// Check for single-block functions and skip them.
3370	if (std::next(x: MF.begin()) == MF.end())
3371	return false;
3372
3373	F = &MF;
3374	MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
3375	MBFI = std::make_unique<MBFIWrapper>(
3376	args&: getAnalysis<MachineBlockFrequencyInfo>());
3377	MLI = &getAnalysis<MachineLoopInfo>();
3378	TII = MF.getSubtarget().getInstrInfo();
3379	TLI = MF.getSubtarget().getTargetLowering();
3380	MPDT = nullptr;
3381	PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
3382
3383	initDupThreshold();
3384
3385	// Initialize PreferredLoopExit to nullptr here since it may never be set if
3386	// there are no MachineLoops.
3387	PreferredLoopExit = nullptr;
3388
3389	assert(BlockToChain.empty() &&
3390	"BlockToChain map should be empty before starting placement.");
3391	assert(ComputedEdges.empty() &&
3392	"Computed Edge map should be empty before starting placement.");
3393
3394	unsigned TailDupSize = TailDupPlacementThreshold;
3395	// If only the aggressive threshold is explicitly set, use it.
3396	if (TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0 &&
3397	TailDupPlacementThreshold.getNumOccurrences() == 0)
3398	TailDupSize = TailDupPlacementAggressiveThreshold;
3399
3400	TargetPassConfig *PassConfig = &getAnalysis<TargetPassConfig>();
3401	// For aggressive optimization, we can adjust some thresholds to be less
3402	// conservative.
3403	if (PassConfig->getOptLevel() >= CodeGenOptLevel::Aggressive) {
3404	// At O3 we should be more willing to copy blocks for tail duplication. This
3405	// increases size pressure, so we only do it at O3
3406	// Do this unless only the regular threshold is explicitly set.
3407	if (TailDupPlacementThreshold.getNumOccurrences() == 0 ||
3408	TailDupPlacementAggressiveThreshold.getNumOccurrences() != 0)
3409	TailDupSize = TailDupPlacementAggressiveThreshold;
3410	}
3411
3412	// If there's no threshold provided through options, query the target
3413	// information for a threshold instead.
3414	if (TailDupPlacementThreshold.getNumOccurrences() == 0 &&
3415	(PassConfig->getOptLevel() < CodeGenOptLevel::Aggressive ||
3416	TailDupPlacementAggressiveThreshold.getNumOccurrences() == 0))
3417	TailDupSize = TII->getTailDuplicateSize(OptLevel: PassConfig->getOptLevel());
3418
3419	if (allowTailDupPlacement()) {
3420	MPDT = &getAnalysis<MachinePostDominatorTree>();
3421	bool OptForSize = MF.getFunction().hasOptSize() ||
3422	llvm::shouldOptimizeForSize(MF: &MF, PSI, BFI: &MBFI->getMBFI());
3423	if (OptForSize)
3424	TailDupSize = 1;
3425	bool PreRegAlloc = false;
3426	TailDup.initMF(MF, PreRegAlloc, MBPI, MBFI: MBFI.get(), PSI,
3427	/* LayoutMode */ true, TailDupSize);
3428	precomputeTriangleChains();
3429	}
3430
3431	buildCFGChains();
3432
3433	// Changing the layout can create new tail merging opportunities.
3434	// TailMerge can create jump into if branches that make CFG irreducible for
3435	// HW that requires structured CFG.
3436	bool EnableTailMerge = !MF.getTarget().requiresStructuredCFG() &&
3437	PassConfig->getEnableTailMerge() &&
3438	BranchFoldPlacement;
3439	// No tail merging opportunities if the block number is less than four.
3440	if (MF.size() > 3 && EnableTailMerge) {
3441	unsigned TailMergeSize = TailDupSize + 1;
3442	BranchFolder BF(/*DefaultEnableTailMerge=*/true, /*CommonHoist=*/false,
3443	*MBFI, *MBPI, PSI, TailMergeSize);
3444
3445	if (BF.OptimizeFunction(MF, tii: TII, tri: MF.getSubtarget().getRegisterInfo(), mli: MLI,
3446	/*AfterPlacement=*/true)) {
3447	// Redo the layout if tail merging creates/removes/moves blocks.
3448	BlockToChain.clear();
3449	ComputedEdges.clear();
3450	// Must redo the post-dominator tree if blocks were changed.
3451	if (MPDT)
3452	MPDT->runOnMachineFunction(MF);
3453	ChainAllocator.DestroyAll();
3454	buildCFGChains();
3455	}
3456	}
3457
3458	// Apply a post-processing optimizing block placement.
3459	if (MF.size() >= 3 && EnableExtTspBlockPlacement &&
3460	(ApplyExtTspWithoutProfile || MF.getFunction().hasProfileData())) {
3461	// Find a new placement and modify the layout of the blocks in the function.
3462	applyExtTsp();
3463
3464	// Re-create CFG chain so that we can optimizeBranches and alignBlocks.
3465	createCFGChainExtTsp();
3466	}
3467
3468	optimizeBranches();
3469	alignBlocks();
3470
3471	BlockToChain.clear();
3472	ComputedEdges.clear();
3473	ChainAllocator.DestroyAll();
3474
3475	bool HasMaxBytesOverride =
3476	MaxBytesForAlignmentOverride.getNumOccurrences() > 0;
3477
3478	if (AlignAllBlock)
3479	// Align all of the blocks in the function to a specific alignment.
3480	for (MachineBasicBlock &MBB : MF) {
3481	if (HasMaxBytesOverride)
3482	MBB.setAlignment(A: Align(1ULL << AlignAllBlock),
3483	MaxBytes: MaxBytesForAlignmentOverride);
3484	else
3485	MBB.setAlignment(Align(1ULL << AlignAllBlock));
3486	}
3487	else if (AlignAllNonFallThruBlocks) {
3488	// Align all of the blocks that have no fall-through predecessors to a
3489	// specific alignment.
3490	for (auto MBI = std::next(x: MF.begin()), MBE = MF.end(); MBI != MBE; ++MBI) {
3491	auto LayoutPred = std::prev(x: MBI);
3492	if (!LayoutPred->isSuccessor(MBB: &*MBI)) {
3493	if (HasMaxBytesOverride)
3494	MBI->setAlignment(A: Align(1ULL << AlignAllNonFallThruBlocks),
3495	MaxBytes: MaxBytesForAlignmentOverride);
3496	else
3497	MBI->setAlignment(Align(1ULL << AlignAllNonFallThruBlocks));
3498	}
3499	}
3500	}
3501	if (ViewBlockLayoutWithBFI != GVDT_None &&
3502	(ViewBlockFreqFuncName.empty() ||
3503	F->getFunction().getName().equals(RHS: ViewBlockFreqFuncName))) {
3504	if (RenumberBlocksBeforeView)
3505	MF.RenumberBlocks();
3506	MBFI->view(Name: "MBP." + MF.getName(), isSimple: false);
3507	}
3508
3509	// We always return true as we have no way to track whether the final order
3510	// differs from the original order.
3511	return true;
3512	}
3513
3514	void MachineBlockPlacement::applyExtTsp() {
3515	// Prepare data; blocks are indexed by their index in the current ordering.
3516	DenseMap<const MachineBasicBlock *, uint64_t> BlockIndex;
3517	BlockIndex.reserve(NumEntries: F->size());
3518	std::vector<const MachineBasicBlock *> CurrentBlockOrder;
3519	CurrentBlockOrder.reserve(n: F->size());
3520	size_t NumBlocks = 0;
3521	for (const MachineBasicBlock &MBB : *F) {
3522	BlockIndex[&MBB] = NumBlocks++;
3523	CurrentBlockOrder.push_back(x: &MBB);
3524	}
3525
3526	auto BlockSizes = std::vector<uint64_t>(F->size());
3527	auto BlockCounts = std::vector<uint64_t>(F->size());
3528	std::vector<codelayout::EdgeCount> JumpCounts;
3529	for (MachineBasicBlock &MBB : *F) {
3530	// Getting the block frequency.
3531	BlockFrequency BlockFreq = MBFI->getBlockFreq(MBB: &MBB);
3532	BlockCounts[BlockIndex[&MBB]] = BlockFreq.getFrequency();
3533	// Getting the block size:
3534	// - approximate the size of an instruction by 4 bytes, and
3535	// - ignore debug instructions.
3536	// Note: getting the exact size of each block is target-dependent and can be
3537	// done by extending the interface of MCCodeEmitter. Experimentally we do
3538	// not see a perf improvement with the exact block sizes.
3539	auto NonDbgInsts =
3540	instructionsWithoutDebug(It: MBB.instr_begin(), End: MBB.instr_end());
3541	int NumInsts = std::distance(first: NonDbgInsts.begin(), last: NonDbgInsts.end());
3542	BlockSizes[BlockIndex[&MBB]] = 4 * NumInsts;
3543	// Getting jump frequencies.
3544	for (MachineBasicBlock *Succ : MBB.successors()) {
3545	auto EP = MBPI->getEdgeProbability(Src: &MBB, Dst: Succ);
3546	BlockFrequency JumpFreq = BlockFreq * EP;
3547	JumpCounts.push_back(
3548	x: {.src: BlockIndex[&MBB], .dst: BlockIndex[Succ], .count: JumpFreq.getFrequency()});
3549	}
3550	}
3551
3552	LLVM_DEBUG(dbgs() << "Applying ext-tsp layout for |V| = " << F->size()
3553	<< " with profile = " << F->getFunction().hasProfileData()
3554	<< " (" << F->getName().str() << ")"
3555	<< "\n");
3556	LLVM_DEBUG(
3557	dbgs() << format(" original layout score: %0.2f\n",
3558	calcExtTspScore(BlockSizes, BlockCounts, JumpCounts)));
3559
3560	// Run the layout algorithm.
3561	auto NewOrder = computeExtTspLayout(NodeSizes: BlockSizes, NodeCounts: BlockCounts, EdgeCounts: JumpCounts);
3562	std::vector<const MachineBasicBlock *> NewBlockOrder;
3563	NewBlockOrder.reserve(n: F->size());
3564	for (uint64_t Node : NewOrder) {
3565	NewBlockOrder.push_back(x: CurrentBlockOrder[Node]);
3566	}
3567	LLVM_DEBUG(dbgs() << format(" optimized layout score: %0.2f\n",
3568	calcExtTspScore(NewOrder, BlockSizes, BlockCounts,
3569	JumpCounts)));
3570
3571	// Assign new block order.
3572	assignBlockOrder(NewOrder: NewBlockOrder);
3573	}
3574
3575	void MachineBlockPlacement::assignBlockOrder(
3576	const std::vector<const MachineBasicBlock *> &NewBlockOrder) {
3577	assert(F->size() == NewBlockOrder.size() && "Incorrect size of block order");
3578	F->RenumberBlocks();
3579
3580	bool HasChanges = false;
3581	for (size_t I = 0; I < NewBlockOrder.size(); I++) {
3582	if (NewBlockOrder[I] != F->getBlockNumbered(N: I)) {
3583	HasChanges = true;
3584	break;
3585	}
3586	}
3587	// Stop early if the new block order is identical to the existing one.
3588	if (!HasChanges)
3589	return;
3590
3591	SmallVector<MachineBasicBlock *, 4> PrevFallThroughs(F->getNumBlockIDs());
3592	for (auto &MBB : *F) {
3593	PrevFallThroughs[MBB.getNumber()] = MBB.getFallThrough();
3594	}
3595
3596	// Sort basic blocks in the function according to the computed order.
3597	DenseMap<const MachineBasicBlock *, size_t> NewIndex;
3598	for (const MachineBasicBlock *MBB : NewBlockOrder) {
3599	NewIndex[MBB] = NewIndex.size();
3600	}
3601	F->sort(comp: [&](MachineBasicBlock &L, MachineBasicBlock &R) {
3602	return NewIndex[&L] < NewIndex[&R];
3603	});
3604
3605	// Update basic block branches by inserting explicit fallthrough branches
3606	// when required and re-optimize branches when possible.
3607	const TargetInstrInfo *TII = F->getSubtarget().getInstrInfo();
3608	SmallVector<MachineOperand, 4> Cond;
3609	for (auto &MBB : *F) {
3610	MachineFunction::iterator NextMBB = std::next(x: MBB.getIterator());
3611	MachineFunction::iterator EndIt = MBB.getParent()->end();
3612	auto *FTMBB = PrevFallThroughs[MBB.getNumber()];
3613	// If this block had a fallthrough before we need an explicit unconditional
3614	// branch to that block if the fallthrough block is not adjacent to the
3615	// block in the new order.
3616	if (FTMBB && (NextMBB == EndIt || &*NextMBB != FTMBB)) {
3617	TII->insertUnconditionalBranch(MBB, DestBB: FTMBB, DL: MBB.findBranchDebugLoc());
3618	}
3619
3620	// It might be possible to optimize branches by flipping the condition.
3621	Cond.clear();
3622	MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
3623	if (TII->analyzeBranch(MBB, TBB, FBB, Cond))
3624	continue;
3625	MBB.updateTerminator(PreviousLayoutSuccessor: FTMBB);
3626	}
3627
3628	#ifndef NDEBUG
3629	// Make sure we correctly constructed all branches.
3630	F->verify(p: this, Banner: "After optimized block reordering");
3631	#endif
3632	}
3633
3634	void MachineBlockPlacement::createCFGChainExtTsp() {
3635	BlockToChain.clear();
3636	ComputedEdges.clear();
3637	ChainAllocator.DestroyAll();
3638
3639	MachineBasicBlock *HeadBB = &F->front();
3640	BlockChain *FunctionChain =
3641	new (ChainAllocator.Allocate()) BlockChain(BlockToChain, HeadBB);
3642
3643	for (MachineBasicBlock &MBB : *F) {
3644	if (HeadBB == &MBB)
3645	continue; // Ignore head of the chain
3646	FunctionChain->merge(BB: &MBB, Chain: nullptr);
3647	}
3648	}
3649
3650	namespace {
3651
3652	/// A pass to compute block placement statistics.
3653	///
3654	/// A separate pass to compute interesting statistics for evaluating block
3655	/// placement. This is separate from the actual placement pass so that they can
3656	/// be computed in the absence of any placement transformations or when using
3657	/// alternative placement strategies.
3658	class MachineBlockPlacementStats : public MachineFunctionPass {
3659	/// A handle to the branch probability pass.
3660	const MachineBranchProbabilityInfo *MBPI;
3661
3662	/// A handle to the function-wide block frequency pass.
3663	const MachineBlockFrequencyInfo *MBFI;
3664
3665	public:
3666	static char ID; // Pass identification, replacement for typeid
3667
3668	MachineBlockPlacementStats() : MachineFunctionPass(ID) {
3669	initializeMachineBlockPlacementStatsPass(*PassRegistry::getPassRegistry());
3670	}
3671
3672	bool runOnMachineFunction(MachineFunction &F) override;
3673
3674	void getAnalysisUsage(AnalysisUsage &AU) const override {
3675	AU.addRequired<MachineBranchProbabilityInfo>();
3676	AU.addRequired<MachineBlockFrequencyInfo>();
3677	AU.setPreservesAll();
3678	MachineFunctionPass::getAnalysisUsage(AU);
3679	}
3680	};
3681
3682	} // end anonymous namespace
3683
3684	char MachineBlockPlacementStats::ID = 0;
3685
3686	char &llvm::MachineBlockPlacementStatsID = MachineBlockPlacementStats::ID;
3687
3688	INITIALIZE_PASS_BEGIN(MachineBlockPlacementStats, "block-placement-stats",
3689	"Basic Block Placement Stats", false, false)
3690	INITIALIZE_PASS_DEPENDENCY(MachineBranchProbabilityInfo)
3691	INITIALIZE_PASS_DEPENDENCY(MachineBlockFrequencyInfo)
3692	INITIALIZE_PASS_END(MachineBlockPlacementStats, "block-placement-stats",
3693	"Basic Block Placement Stats", false, false)
3694
3695	bool MachineBlockPlacementStats::runOnMachineFunction(MachineFunction &F) {
3696	// Check for single-block functions and skip them.
3697	if (std::next(x: F.begin()) == F.end())
3698	return false;
3699
3700	if (!isFunctionInPrintList(FunctionName: F.getName()))
3701	return false;
3702
3703	MBPI = &getAnalysis<MachineBranchProbabilityInfo>();
3704	MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
3705
3706	for (MachineBasicBlock &MBB : F) {
3707	BlockFrequency BlockFreq = MBFI->getBlockFreq(MBB: &MBB);
3708	Statistic &NumBranches =
3709	(MBB.succ_size() > 1) ? NumCondBranches : NumUncondBranches;
3710	Statistic &BranchTakenFreq =
3711	(MBB.succ_size() > 1) ? CondBranchTakenFreq : UncondBranchTakenFreq;
3712	for (MachineBasicBlock *Succ : MBB.successors()) {
3713	// Skip if this successor is a fallthrough.
3714	if (MBB.isLayoutSuccessor(MBB: Succ))
3715	continue;
3716
3717	BlockFrequency EdgeFreq =
3718	BlockFreq * MBPI->getEdgeProbability(Src: &MBB, Dst: Succ);
3719	++NumBranches;
3720	BranchTakenFreq += EdgeFreq.getFrequency();
3721	}
3722	}
3723
3724	return false;
3725	}
3726