1//===- Transform/Utils/BasicBlockUtils.h - BasicBlock Utils -----*- C++ -*-===//
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 family of functions perform manipulations on basic blocks, and
10// instructions contained within basic blocks.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H
15#define LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H
16
17// FIXME: Move to this file: BasicBlock::removePredecessor, BB::splitBasicBlock
18
19#include "llvm/ADT/ArrayRef.h"
20#include "llvm/ADT/SetVector.h"
21#include "llvm/Analysis/DomTreeUpdater.h"
22#include "llvm/Analysis/LoopInfo.h"
23#include "llvm/IR/BasicBlock.h"
24#include "llvm/IR/CFG.h"
25#include "llvm/IR/InstrTypes.h"
26#include <cassert>
27
28namespace llvm {
29
30class BlockFrequencyInfo;
31class BranchProbabilityInfo;
32class DominatorTree;
33class DomTreeUpdater;
34class Function;
35class Instruction;
36class LoopInfo;
37class MDNode;
38class MemoryDependenceResults;
39class MemorySSAUpdater;
40class PostDominatorTree;
41class ReturnInst;
42class TargetLibraryInfo;
43class Value;
44
45/// Replace contents of every block in \p BBs with single unreachable
46/// instruction. If \p Updates is specified, collect all necessary DT updates
47/// into this vector. If \p KeepOneInputPHIs is true, one-input Phis in
48/// successors of blocks being deleted will be preserved.
49void DetatchDeadBlocks(ArrayRef <BasicBlock *> BBs,
50 SmallVectorImpl<DominatorTree::UpdateType> *Updates,
51 bool KeepOneInputPHIs = false);
52
53/// Delete the specified block, which must have no predecessors.
54void DeleteDeadBlock(BasicBlock *BB, DomTreeUpdater *DTU = nullptr,
55 bool KeepOneInputPHIs = false);
56
57/// Delete the specified blocks from \p BB. The set of deleted blocks must have
58/// no predecessors that are not being deleted themselves. \p BBs must have no
59/// duplicating blocks. If there are loops among this set of blocks, all
60/// relevant loop info updates should be done before this function is called.
61/// If \p KeepOneInputPHIs is true, one-input Phis in successors of blocks
62/// being deleted will be preserved.
63void DeleteDeadBlocks(ArrayRef <BasicBlock *> BBs,
64 DomTreeUpdater *DTU = nullptr,
65 bool KeepOneInputPHIs = false);
66
67/// Delete all basic blocks from \p F that are not reachable from its entry
68/// node. If \p KeepOneInputPHIs is true, one-input Phis in successors of
69/// blocks being deleted will be preserved.
70bool EliminateUnreachableBlocks(Function &F, DomTreeUpdater *DTU = nullptr,
71 bool KeepOneInputPHIs = false);
72
73/// We know that BB has one predecessor. If there are any single-entry PHI nodes
74/// in it, fold them away. This handles the case when all entries to the PHI
75/// nodes in a block are guaranteed equal, such as when the block has exactly
76/// one predecessor.
77bool FoldSingleEntryPHINodes(BasicBlock *BB,
78 MemoryDependenceResults *MemDep = nullptr);
79
80/// Examine each PHI in the given block and delete it if it is dead. Also
81/// recursively delete any operands that become dead as a result. This includes
82/// tracing the def-use list from the PHI to see if it is ultimately unused or
83/// if it reaches an unused cycle. Return true if any PHIs were deleted.
84bool DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI = nullptr,
85 MemorySSAUpdater *MSSAU = nullptr);
86
87/// Attempts to merge a block into its predecessor, if possible. The return
88/// value indicates success or failure.
89/// By default do not merge blocks if BB's predecessor has multiple successors.
90/// If PredecessorWithTwoSuccessors = true, the blocks can only be merged
91/// if BB's Pred has a branch to BB and to AnotherBB, and BB has a single
92/// successor Sing. In this case the branch will be updated with Sing instead of
93/// BB, and BB will still be merged into its predecessor and removed.
94bool MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU = nullptr,
95 LoopInfo *LI = nullptr,
96 MemorySSAUpdater *MSSAU = nullptr,
97 MemoryDependenceResults *MemDep = nullptr,
98 bool PredecessorWithTwoSuccessors = false);
99
100/// Merge block(s) sucessors, if possible. Return true if at least two
101/// of the blocks were merged together.
102/// In order to merge, each block must be terminated by an unconditional
103/// branch. If L is provided, then the blocks merged into their predecessors
104/// must be in L. In addition, This utility calls on another utility:
105/// MergeBlockIntoPredecessor. Blocks are successfully merged when the call to
106/// MergeBlockIntoPredecessor returns true.
107bool MergeBlockSuccessorsIntoGivenBlocks(
108 SmallPtrSetImpl<BasicBlock *> &MergeBlocks, Loop *L = nullptr,
109 DomTreeUpdater *DTU = nullptr, LoopInfo *LI = nullptr);
110
111/// Try to remove redundant dbg.value instructions from given basic block.
112/// Returns true if at least one instruction was removed. Remove redundant
113/// pseudo ops when RemovePseudoOp is true.
114bool RemoveRedundantDbgInstrs(BasicBlock *BB, bool RemovePseudoOp = false);
115
116/// Replace all uses of an instruction (specified by BI) with a value, then
117/// remove and delete the original instruction.
118void ReplaceInstWithValue(BasicBlock::InstListType &BIL,
119 BasicBlock::iterator &BI, Value *V);
120
121/// Replace the instruction specified by BI with the instruction specified by I.
122/// Copies DebugLoc from BI to I, if I doesn't already have a DebugLoc. The
123/// original instruction is deleted and BI is updated to point to the new
124/// instruction.
125void ReplaceInstWithInst(BasicBlock::InstListType &BIL,
126 BasicBlock::iterator &BI, Instruction *I);
127
128/// Replace the instruction specified by From with the instruction specified by
129/// To. Copies DebugLoc from BI to I, if I doesn't already have a DebugLoc.
130void ReplaceInstWithInst(Instruction *From, Instruction *To);
131
132/// Option class for critical edge splitting.
133///
134/// This provides a builder interface for overriding the default options used
135/// during critical edge splitting.
136struct CriticalEdgeSplittingOptions {
137 DominatorTree *DT;
138 PostDominatorTree *PDT;
139 LoopInfo *LI;
140 MemorySSAUpdater *MSSAU;
141 bool MergeIdenticalEdges = false;
142 bool KeepOneInputPHIs = false;
143 bool PreserveLCSSA = false;
144 bool IgnoreUnreachableDests = false;
145 /// SplitCriticalEdge is guaranteed to preserve loop-simplify form if LI is
146 /// provided. If it cannot be preserved, no splitting will take place. If it
147 /// is not set, preserve loop-simplify form if possible.
148 bool PreserveLoopSimplify = true;
149
150 CriticalEdgeSplittingOptions(DominatorTree *DT = nullptr,
151 LoopInfo *LI = nullptr,
152 MemorySSAUpdater *MSSAU = nullptr,
153 PostDominatorTree *PDT = nullptr)
154 : DT(DT), PDT(PDT), LI(LI), MSSAU(MSSAU) {}
155
156 CriticalEdgeSplittingOptions &setMergeIdenticalEdges() {
157 MergeIdenticalEdges = true;
158 return *this;
159 }
160
161 CriticalEdgeSplittingOptions &setKeepOneInputPHIs() {
162 KeepOneInputPHIs = true;
163 return *this;
164 }
165
166 CriticalEdgeSplittingOptions &setPreserveLCSSA() {
167 PreserveLCSSA = true;
168 return *this;
169 }
170
171 CriticalEdgeSplittingOptions &setIgnoreUnreachableDests() {
172 IgnoreUnreachableDests = true;
173 return *this;
174 }
175
176 CriticalEdgeSplittingOptions &unsetPreserveLoopSimplify() {
177 PreserveLoopSimplify = false;
178 return *this;
179 }
180};
181
182/// When a loop exit edge is split, LCSSA form may require new PHIs in the new
183/// exit block. This function inserts the new PHIs, as needed. Preds is a list
184/// of preds inside the loop, SplitBB is the new loop exit block, and DestBB is
185/// the old loop exit, now the successor of SplitBB.
186void createPHIsForSplitLoopExit(ArrayRef<BasicBlock *> Preds,
187 BasicBlock *SplitBB, BasicBlock *DestBB);
188
189/// If this edge is a critical edge, insert a new node to split the critical
190/// edge. This will update the analyses passed in through the option struct.
191/// This returns the new block if the edge was split, null otherwise.
192///
193/// If MergeIdenticalEdges in the options struct is true (not the default),
194/// *all* edges from TI to the specified successor will be merged into the same
195/// critical edge block. This is most commonly interesting with switch
196/// instructions, which may have many edges to any one destination. This
197/// ensures that all edges to that dest go to one block instead of each going
198/// to a different block, but isn't the standard definition of a "critical
199/// edge".
200///
201/// It is invalid to call this function on a critical edge that starts at an
202/// IndirectBrInst. Splitting these edges will almost always create an invalid
203/// program because the address of the new block won't be the one that is jumped
204/// to.
205BasicBlock *SplitCriticalEdge(Instruction *TI, unsigned SuccNum,
206 const CriticalEdgeSplittingOptions &Options =
207 CriticalEdgeSplittingOptions(),
208 const Twine &BBName = "");
209
210/// If it is known that an edge is critical, SplitKnownCriticalEdge can be
211/// called directly, rather than calling SplitCriticalEdge first.
212BasicBlock *SplitKnownCriticalEdge(Instruction *TI, unsigned SuccNum,
213 const CriticalEdgeSplittingOptions &Options =
214 CriticalEdgeSplittingOptions(),
215 const Twine &BBName = "");
216
217inline BasicBlock *
218SplitCriticalEdge(BasicBlock *BB, succ_iterator SI,
219 const CriticalEdgeSplittingOptions &Options =
220 CriticalEdgeSplittingOptions()) {
221 return SplitCriticalEdge(BB->getTerminator(), SI.getSuccessorIndex(),
222 Options);
223}
224
225/// If the edge from *PI to BB is not critical, return false. Otherwise, split
226/// all edges between the two blocks and return true. This updates all of the
227/// same analyses as the other SplitCriticalEdge function. If P is specified, it
228/// updates the analyses described above.
229inline bool SplitCriticalEdge(BasicBlock *Succ, pred_iterator PI,
230 const CriticalEdgeSplittingOptions &Options =
231 CriticalEdgeSplittingOptions()) {
232 bool MadeChange = false;
233 Instruction *TI = (*PI)->getTerminator();
234 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
235 if (TI->getSuccessor(i) == Succ)
236 MadeChange |= !!SplitCriticalEdge(TI, i, Options);
237 return MadeChange;
238}
239
240/// If an edge from Src to Dst is critical, split the edge and return true,
241/// otherwise return false. This method requires that there be an edge between
242/// the two blocks. It updates the analyses passed in the options struct
243inline BasicBlock *
244SplitCriticalEdge(BasicBlock *Src, BasicBlock *Dst,
245 const CriticalEdgeSplittingOptions &Options =
246 CriticalEdgeSplittingOptions()) {
247 Instruction *TI = Src->getTerminator();
248 unsigned i = 0;
249 while (true) {
250 assert(i != TI->getNumSuccessors() && "Edge doesn't exist!");
251 if (TI->getSuccessor(i) == Dst)
252 return SplitCriticalEdge(TI, i, Options);
253 ++i;
254 }
255}
256
257/// Loop over all of the edges in the CFG, breaking critical edges as they are
258/// found. Returns the number of broken edges.
259unsigned SplitAllCriticalEdges(Function &F,
260 const CriticalEdgeSplittingOptions &Options =
261 CriticalEdgeSplittingOptions());
262
263/// Split the edge connecting the specified blocks, and return the newly created
264/// basic block between \p From and \p To.
265BasicBlock *SplitEdge(BasicBlock *From, BasicBlock *To,
266 DominatorTree *DT = nullptr, LoopInfo *LI = nullptr,
267 MemorySSAUpdater *MSSAU = nullptr,
268 const Twine &BBName = "");
269
270/// Sets the unwind edge of an instruction to a particular successor.
271void setUnwindEdgeTo(Instruction *TI, BasicBlock *Succ);
272
273/// Replaces all uses of OldPred with the NewPred block in all PHINodes in a
274/// block.
275void updatePhiNodes(BasicBlock *DestBB, BasicBlock *OldPred,
276 BasicBlock *NewPred, PHINode *Until = nullptr);
277
278/// Split the edge connect the specficed blocks in the case that \p Succ is an
279/// Exception Handling Block
280BasicBlock *ehAwareSplitEdge(BasicBlock *BB, BasicBlock *Succ,
281 LandingPadInst *OriginalPad = nullptr,
282 PHINode *LandingPadReplacement = nullptr,
283 const CriticalEdgeSplittingOptions &Options =
284 CriticalEdgeSplittingOptions(),
285 const Twine &BBName = "");
286
287/// Split the specified block at the specified instruction.
288///
289/// If \p Before is true, splitBlockBefore handles the block
290/// splitting. Otherwise, execution proceeds as described below.
291///
292/// Everything before \p SplitPt stays in \p Old and everything starting with \p
293/// SplitPt moves to a new block. The two blocks are joined by an unconditional
294/// branch. The new block with name \p BBName is returned.
295///
296/// FIXME: deprecated, switch to the DomTreeUpdater-based one.
297BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt, DominatorTree *DT,
298 LoopInfo *LI = nullptr,
299 MemorySSAUpdater *MSSAU = nullptr,
300 const Twine &BBName = "", bool Before = false);
301
302/// Split the specified block at the specified instruction.
303///
304/// If \p Before is true, splitBlockBefore handles the block
305/// splitting. Otherwise, execution proceeds as described below.
306///
307/// Everything before \p SplitPt stays in \p Old and everything starting with \p
308/// SplitPt moves to a new block. The two blocks are joined by an unconditional
309/// branch. The new block with name \p BBName is returned.
310BasicBlock *SplitBlock(BasicBlock *Old, Instruction *SplitPt,
311 DomTreeUpdater *DTU = nullptr, LoopInfo *LI = nullptr,
312 MemorySSAUpdater *MSSAU = nullptr,
313 const Twine &BBName = "", bool Before = false);
314
315/// Split the specified block at the specified instruction \p SplitPt.
316/// All instructions before \p SplitPt are moved to a new block and all
317/// instructions after \p SplitPt stay in the old block. The new block and the
318/// old block are joined by inserting an unconditional branch to the end of the
319/// new block. The new block with name \p BBName is returned.
320BasicBlock *splitBlockBefore(BasicBlock *Old, Instruction *SplitPt,
321 DomTreeUpdater *DTU, LoopInfo *LI,
322 MemorySSAUpdater *MSSAU, const Twine &BBName = "");
323
324/// This method introduces at least one new basic block into the function and
325/// moves some of the predecessors of BB to be predecessors of the new block.
326/// The new predecessors are indicated by the Preds array. The new block is
327/// given a suffix of 'Suffix'. Returns new basic block to which predecessors
328/// from Preds are now pointing.
329///
330/// If BB is a landingpad block then additional basicblock might be introduced.
331/// It will have Suffix+".split_lp". See SplitLandingPadPredecessors for more
332/// details on this case.
333///
334/// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but
335/// no other analyses. In particular, it does not preserve LoopSimplify
336/// (because it's complicated to handle the case where one of the edges being
337/// split is an exit of a loop with other exits).
338///
339/// FIXME: deprecated, switch to the DomTreeUpdater-based one.
340BasicBlock *SplitBlockPredecessors(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
341 const char *Suffix, DominatorTree *DT,
342 LoopInfo *LI = nullptr,
343 MemorySSAUpdater *MSSAU = nullptr,
344 bool PreserveLCSSA = false);
345
346/// This method introduces at least one new basic block into the function and
347/// moves some of the predecessors of BB to be predecessors of the new block.
348/// The new predecessors are indicated by the Preds array. The new block is
349/// given a suffix of 'Suffix'. Returns new basic block to which predecessors
350/// from Preds are now pointing.
351///
352/// If BB is a landingpad block then additional basicblock might be introduced.
353/// It will have Suffix+".split_lp". See SplitLandingPadPredecessors for more
354/// details on this case.
355///
356/// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but
357/// no other analyses. In particular, it does not preserve LoopSimplify
358/// (because it's complicated to handle the case where one of the edges being
359/// split is an exit of a loop with other exits).
360BasicBlock *SplitBlockPredecessors(BasicBlock *BB, ArrayRef<BasicBlock *> Preds,
361 const char *Suffix,
362 DomTreeUpdater *DTU = nullptr,
363 LoopInfo *LI = nullptr,
364 MemorySSAUpdater *MSSAU = nullptr,
365 bool PreserveLCSSA = false);
366
367/// This method transforms the landing pad, OrigBB, by introducing two new basic
368/// blocks into the function. One of those new basic blocks gets the
369/// predecessors listed in Preds. The other basic block gets the remaining
370/// predecessors of OrigBB. The landingpad instruction OrigBB is clone into both
371/// of the new basic blocks. The new blocks are given the suffixes 'Suffix1' and
372/// 'Suffix2', and are returned in the NewBBs vector.
373///
374/// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but
375/// no other analyses. In particular, it does not preserve LoopSimplify
376/// (because it's complicated to handle the case where one of the edges being
377/// split is an exit of a loop with other exits).
378///
379/// FIXME: deprecated, switch to the DomTreeUpdater-based one.
380void SplitLandingPadPredecessors(BasicBlock *OrigBB,
381 ArrayRef<BasicBlock *> Preds,
382 const char *Suffix, const char *Suffix2,
383 SmallVectorImpl<BasicBlock *> &NewBBs,
384 DominatorTree *DT, LoopInfo *LI = nullptr,
385 MemorySSAUpdater *MSSAU = nullptr,
386 bool PreserveLCSSA = false);
387
388/// This method transforms the landing pad, OrigBB, by introducing two new basic
389/// blocks into the function. One of those new basic blocks gets the
390/// predecessors listed in Preds. The other basic block gets the remaining
391/// predecessors of OrigBB. The landingpad instruction OrigBB is clone into both
392/// of the new basic blocks. The new blocks are given the suffixes 'Suffix1' and
393/// 'Suffix2', and are returned in the NewBBs vector.
394///
395/// This currently updates the LLVM IR, DominatorTree, LoopInfo, and LCCSA but
396/// no other analyses. In particular, it does not preserve LoopSimplify
397/// (because it's complicated to handle the case where one of the edges being
398/// split is an exit of a loop with other exits).
399void SplitLandingPadPredecessors(
400 BasicBlock *OrigBB, ArrayRef<BasicBlock *> Preds, const char *Suffix,
401 const char *Suffix2, SmallVectorImpl<BasicBlock *> &NewBBs,
402 DomTreeUpdater *DTU = nullptr, LoopInfo *LI = nullptr,
403 MemorySSAUpdater *MSSAU = nullptr, bool PreserveLCSSA = false);
404
405/// This method duplicates the specified return instruction into a predecessor
406/// which ends in an unconditional branch. If the return instruction returns a
407/// value defined by a PHI, propagate the right value into the return. It
408/// returns the new return instruction in the predecessor.
409ReturnInst *FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
410 BasicBlock *Pred,
411 DomTreeUpdater *DTU = nullptr);
412
413/// Split the containing block at the specified instruction - everything before
414/// SplitBefore stays in the old basic block, and the rest of the instructions
415/// in the BB are moved to a new block. The two blocks are connected by a
416/// conditional branch (with value of Cmp being the condition).
417/// Before:
418/// Head
419/// SplitBefore
420/// Tail
421/// After:
422/// Head
423/// if (Cond)
424/// ThenBlock
425/// SplitBefore
426/// Tail
427///
428/// If \p ThenBlock is not specified, a new block will be created for it.
429/// If \p Unreachable is true, the newly created block will end with
430/// UnreachableInst, otherwise it branches to Tail.
431/// Returns the NewBasicBlock's terminator.
432///
433/// Updates DT and LI if given.
434///
435/// FIXME: deprecated, switch to the DomTreeUpdater-based one.
436Instruction *SplitBlockAndInsertIfThen(Value *Cond, Instruction *SplitBefore,
437 bool Unreachable, MDNode *BranchWeights,
438 DominatorTree *DT,
439 LoopInfo *LI = nullptr,
440 BasicBlock *ThenBlock = nullptr);
441
442/// Split the containing block at the specified instruction - everything before
443/// SplitBefore stays in the old basic block, and the rest of the instructions
444/// in the BB are moved to a new block. The two blocks are connected by a
445/// conditional branch (with value of Cmp being the condition).
446/// Before:
447/// Head
448/// SplitBefore
449/// Tail
450/// After:
451/// Head
452/// if (Cond)
453/// ThenBlock
454/// SplitBefore
455/// Tail
456///
457/// If \p ThenBlock is not specified, a new block will be created for it.
458/// If \p Unreachable is true, the newly created block will end with
459/// UnreachableInst, otherwise it branches to Tail.
460/// Returns the NewBasicBlock's terminator.
461///
462/// Updates DT and LI if given.
463Instruction *SplitBlockAndInsertIfThen(Value *Cond, Instruction *SplitBefore,
464 bool Unreachable,
465 MDNode *BranchWeights = nullptr,
466 DomTreeUpdater *DTU = nullptr,
467 LoopInfo *LI = nullptr,
468 BasicBlock *ThenBlock = nullptr);
469
470/// SplitBlockAndInsertIfThenElse is similar to SplitBlockAndInsertIfThen,
471/// but also creates the ElseBlock.
472/// Before:
473/// Head
474/// SplitBefore
475/// Tail
476/// After:
477/// Head
478/// if (Cond)
479/// ThenBlock
480/// else
481/// ElseBlock
482/// SplitBefore
483/// Tail
484void SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
485 Instruction **ThenTerm,
486 Instruction **ElseTerm,
487 MDNode *BranchWeights = nullptr);
488
489/// Check whether BB is the merge point of a if-region.
490/// If so, return the boolean condition that determines which entry into
491/// BB will be taken. Also, return by references the block that will be
492/// entered from if the condition is true, and the block that will be
493/// entered if the condition is false.
494///
495/// This does no checking to see if the true/false blocks have large or unsavory
496/// instructions in them.
497Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
498 BasicBlock *&IfFalse);
499
500// Split critical edges where the source of the edge is an indirectbr
501// instruction. This isn't always possible, but we can handle some easy cases.
502// This is useful because MI is unable to split such critical edges,
503// which means it will not be able to sink instructions along those edges.
504// This is especially painful for indirect branches with many successors, where
505// we end up having to prepare all outgoing values in the origin block.
506//
507// Our normal algorithm for splitting critical edges requires us to update
508// the outgoing edges of the edge origin block, but for an indirectbr this
509// is hard, since it would require finding and updating the block addresses
510// the indirect branch uses. But if a block only has a single indirectbr
511// predecessor, with the others being regular branches, we can do it in a
512// different way.
513// Say we have A -> D, B -> D, I -> D where only I -> D is an indirectbr.
514// We can split D into D0 and D1, where D0 contains only the PHIs from D,
515// and D1 is the D block body. We can then duplicate D0 as D0A and D0B, and
516// create the following structure:
517// A -> D0A, B -> D0A, I -> D0B, D0A -> D1, D0B -> D1
518// If BPI and BFI aren't non-null, BPI/BFI will be updated accordingly.
519bool SplitIndirectBrCriticalEdges(Function &F,
520 BranchProbabilityInfo *BPI = nullptr,
521 BlockFrequencyInfo *BFI = nullptr);
522
523/// Given a set of incoming and outgoing blocks, create a "hub" such that every
524/// edge from an incoming block InBB to an outgoing block OutBB is now split
525/// into two edges, one from InBB to the hub and another from the hub to
526/// OutBB. The hub consists of a series of guard blocks, one for each outgoing
527/// block. Each guard block conditionally branches to the corresponding outgoing
528/// block, or the next guard block in the chain. These guard blocks are returned
529/// in the argument vector.
530///
531/// Since the control flow edges from InBB to OutBB have now been replaced, the
532/// function also updates any PHINodes in OutBB. For each such PHINode, the
533/// operands corresponding to incoming blocks are moved to a new PHINode in the
534/// hub, and the hub is made an operand of the original PHINode.
535///
536/// Input CFG:
537/// ----------
538///
539/// Def
540/// |
541/// v
542/// In1 In2
543/// | |
544/// | |
545/// v v
546/// Foo ---> Out1 Out2
547/// |
548/// v
549/// Use
550///
551///
552/// Create hub: Incoming = {In1, In2}, Outgoing = {Out1, Out2}
553/// ----------------------------------------------------------
554///
555/// Def
556/// |
557/// v
558/// In1 In2 Foo
559/// | Hub | |
560/// | + - - | - - + |
561/// | ' v ' V
562/// +------> Guard1 -----> Out1
563/// ' | '
564/// ' v '
565/// ' Guard2 -----> Out2
566/// ' ' |
567/// + - - - - - + |
568/// v
569/// Use
570///
571/// Limitations:
572/// -----------
573/// 1. This assumes that all terminators in the CFG are direct branches (the
574/// "br" instruction). The presence of any other control flow such as
575/// indirectbr, switch or callbr will cause an assert.
576///
577/// 2. The updates to the PHINodes are not sufficient to restore SSA
578/// form. Consider a definition Def, its use Use, incoming block In2 and
579/// outgoing block Out2, such that:
580/// a. In2 is reachable from D or contains D.
581/// b. U is reachable from Out2 or is contained in Out2.
582/// c. U is not a PHINode if U is contained in Out2.
583///
584/// Clearly, Def dominates Out2 since the program is valid SSA. But when the
585/// hub is introduced, there is a new path through the hub along which Use is
586/// reachable from entry without passing through Def, and SSA is no longer
587/// valid. To fix this, we need to look at all the blocks post-dominated by
588/// the hub on the one hand, and dominated by Out2 on the other. This is left
589/// for the caller to accomplish, since each specific use of this function
590/// may have additional information which simplifies this fixup. For example,
591/// see restoreSSA() in the UnifyLoopExits pass.
592BasicBlock *CreateControlFlowHub(DomTreeUpdater *DTU,
593 SmallVectorImpl<BasicBlock *> &GuardBlocks,
594 const SetVector<BasicBlock *> &Predecessors,
595 const SetVector<BasicBlock *> &Successors,
596 const StringRef Prefix);
597
598} // end namespace llvm
599
600#endif // LLVM_TRANSFORMS_UTILS_BASICBLOCKUTILS_H
601