1//===-- CFGMST.h - Minimum Spanning Tree for CFG ----------------*- 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 file implements a Union-find algorithm to compute Minimum Spanning Tree
10// for a given CFG.
11//
12//===----------------------------------------------------------------------===//
13
14#ifndef LLVM_TRANSFORMS_INSTRUMENTATION_CFGMST_H
15#define LLVM_TRANSFORMS_INSTRUMENTATION_CFGMST_H
16
17#include "llvm/ADT/DenseMap.h"
18#include "llvm/ADT/STLExtras.h"
19#include "llvm/Analysis/BlockFrequencyInfo.h"
20#include "llvm/Analysis/BranchProbabilityInfo.h"
21#include "llvm/Analysis/CFG.h"
22#include "llvm/IR/Instructions.h"
23#include "llvm/IR/IntrinsicInst.h"
24#include "llvm/Support/BranchProbability.h"
25#include "llvm/Support/Debug.h"
26#include "llvm/Support/raw_ostream.h"
27#include "llvm/Transforms/Utils/BasicBlockUtils.h"
28#include <utility>
29#include <vector>
30
31#define DEBUG_TYPE "cfgmst"
32
33namespace llvm {
34
35/// An union-find based Minimum Spanning Tree for CFG
36///
37/// Implements a Union-find algorithm to compute Minimum Spanning Tree
38/// for a given CFG.
39template <class Edge, class BBInfo> class CFGMST {
40 Function &F;
41
42 // Store all the edges in CFG. It may contain some stale edges
43 // when Removed is set.
44 std::vector<std::unique_ptr<Edge>> AllEdges;
45
46 // This map records the auxiliary information for each BB.
47 DenseMap<const BasicBlock *, std::unique_ptr<BBInfo>> BBInfos;
48
49 // Whehter the function has an exit block with no successors.
50 // (For function with an infinite loop, this block may be absent)
51 bool ExitBlockFound = false;
52
53 BranchProbabilityInfo *const BPI;
54 BlockFrequencyInfo *const BFI;
55
56 // If function entry will be always instrumented.
57 const bool InstrumentFuncEntry;
58
59 // Find the root group of the G and compress the path from G to the root.
60 BBInfo *findAndCompressGroup(BBInfo *G) {
61 if (G->Group != G)
62 G->Group = findAndCompressGroup(G: static_cast<BBInfo *>(G->Group));
63 return static_cast<BBInfo *>(G->Group);
64 }
65
66 // Union BB1 and BB2 into the same group and return true.
67 // Returns false if BB1 and BB2 are already in the same group.
68 bool unionGroups(const BasicBlock *BB1, const BasicBlock *BB2) {
69 BBInfo *BB1G = findAndCompressGroup(G: &getBBInfo(BB: BB1));
70 BBInfo *BB2G = findAndCompressGroup(G: &getBBInfo(BB: BB2));
71
72 if (BB1G == BB2G)
73 return false;
74
75 // Make the smaller rank tree a direct child or the root of high rank tree.
76 if (BB1G->Rank < BB2G->Rank)
77 BB1G->Group = BB2G;
78 else {
79 BB2G->Group = BB1G;
80 // If the ranks are the same, increment root of one tree by one.
81 if (BB1G->Rank == BB2G->Rank)
82 BB1G->Rank++;
83 }
84 return true;
85 }
86
87 void handleCoroSuspendEdge(Edge *E) {
88 // We must not add instrumentation to the BB representing the
89 // "suspend" path, else CoroSplit won't be able to lower
90 // llvm.coro.suspend to a tail call. We do want profiling info for
91 // the other branches (resume/destroy). So we do 2 things:
92 // 1. we prefer instrumenting those other edges by setting the weight
93 // of the "suspend" edge to max, and
94 // 2. we mark the edge as "Removed" to guarantee it is not considered
95 // for instrumentation. That could technically happen:
96 // (from test/Transforms/Coroutines/coro-split-musttail.ll)
97 //
98 // %suspend = call i8 @llvm.coro.suspend(token %save, i1 false)
99 // switch i8 %suspend, label %exit [
100 // i8 0, label %await.ready
101 // i8 1, label %exit
102 // ]
103 if (!E->DestBB)
104 return;
105 assert(E->SrcBB);
106 if (llvm::isPresplitCoroSuspendExitEdge(Src: *E->SrcBB, Dest: *E->DestBB))
107 E->Removed = true;
108 }
109
110 // Traverse the CFG using a stack. Find all the edges and assign the weight.
111 // Edges with large weight will be put into MST first so they are less likely
112 // to be instrumented.
113 void buildEdges() {
114 LLVM_DEBUG(dbgs() << "Build Edge on " << F.getName() << "\n");
115
116 BasicBlock *Entry = &(F.getEntryBlock());
117 uint64_t EntryWeight =
118 (BFI != nullptr ? BFI->getEntryFreq().getFrequency() : 2);
119 // If we want to instrument the entry count, lower the weight to 0.
120 if (InstrumentFuncEntry)
121 EntryWeight = 0;
122 Edge *EntryIncoming = nullptr, *EntryOutgoing = nullptr,
123 *ExitOutgoing = nullptr, *ExitIncoming = nullptr;
124 uint64_t MaxEntryOutWeight = 0, MaxExitOutWeight = 0, MaxExitInWeight = 0;
125
126 // Add a fake edge to the entry.
127 EntryIncoming = &addEdge(Src: nullptr, Dest: Entry, W: EntryWeight);
128 LLVM_DEBUG(dbgs() << " Edge: from fake node to " << Entry->getName()
129 << " w = " << EntryWeight << "\n");
130
131 // Special handling for single BB functions.
132 if (succ_empty(BB: Entry)) {
133 addEdge(Src: Entry, Dest: nullptr, W: EntryWeight);
134 return;
135 }
136
137 static const uint32_t CriticalEdgeMultiplier = 1000;
138
139 for (BasicBlock &BB : F) {
140 Instruction *TI = BB.getTerminator();
141 uint64_t BBWeight =
142 (BFI != nullptr ? BFI->getBlockFreq(BB: &BB).getFrequency() : 2);
143 uint64_t Weight = 2;
144 if (int successors = TI->getNumSuccessors()) {
145 for (int i = 0; i != successors; ++i) {
146 BasicBlock *TargetBB = TI->getSuccessor(Idx: i);
147 bool Critical = isCriticalEdge(TI, SuccNum: i);
148 uint64_t scaleFactor = BBWeight;
149 if (Critical) {
150 if (scaleFactor < UINT64_MAX / CriticalEdgeMultiplier)
151 scaleFactor *= CriticalEdgeMultiplier;
152 else
153 scaleFactor = UINT64_MAX;
154 }
155 if (BPI != nullptr)
156 Weight = BPI->getEdgeProbability(Src: &BB, Dst: TargetBB).scale(Num: scaleFactor);
157 if (Weight == 0)
158 Weight++;
159 auto *E = &addEdge(Src: &BB, Dest: TargetBB, W: Weight);
160 E->IsCritical = Critical;
161 handleCoroSuspendEdge(E);
162 LLVM_DEBUG(dbgs() << " Edge: from " << BB.getName() << " to "
163 << TargetBB->getName() << " w=" << Weight << "\n");
164
165 // Keep track of entry/exit edges:
166 if (&BB == Entry) {
167 if (Weight > MaxEntryOutWeight) {
168 MaxEntryOutWeight = Weight;
169 EntryOutgoing = E;
170 }
171 }
172
173 auto *TargetTI = TargetBB->getTerminator();
174 if (TargetTI && !TargetTI->getNumSuccessors()) {
175 if (Weight > MaxExitInWeight) {
176 MaxExitInWeight = Weight;
177 ExitIncoming = E;
178 }
179 }
180 }
181 } else {
182 ExitBlockFound = true;
183 Edge *ExitO = &addEdge(Src: &BB, Dest: nullptr, W: BBWeight);
184 if (BBWeight > MaxExitOutWeight) {
185 MaxExitOutWeight = BBWeight;
186 ExitOutgoing = ExitO;
187 }
188 LLVM_DEBUG(dbgs() << " Edge: from " << BB.getName() << " to fake exit"
189 << " w = " << BBWeight << "\n");
190 }
191 }
192
193 // Entry/exit edge adjustment heurisitic:
194 // prefer instrumenting entry edge over exit edge
195 // if possible. Those exit edges may never have a chance to be
196 // executed (for instance the program is an event handling loop)
197 // before the profile is asynchronously dumped.
198 //
199 // If EntryIncoming and ExitOutgoing has similar weight, make sure
200 // ExitOutging is selected as the min-edge. Similarly, if EntryOutgoing
201 // and ExitIncoming has similar weight, make sure ExitIncoming becomes
202 // the min-edge.
203 uint64_t EntryInWeight = EntryWeight;
204
205 if (EntryInWeight >= MaxExitOutWeight &&
206 EntryInWeight * 2 < MaxExitOutWeight * 3) {
207 EntryIncoming->Weight = MaxExitOutWeight;
208 ExitOutgoing->Weight = EntryInWeight + 1;
209 }
210
211 if (MaxEntryOutWeight >= MaxExitInWeight &&
212 MaxEntryOutWeight * 2 < MaxExitInWeight * 3) {
213 EntryOutgoing->Weight = MaxExitInWeight;
214 ExitIncoming->Weight = MaxEntryOutWeight + 1;
215 }
216 }
217
218 // Sort CFG edges based on its weight.
219 void sortEdgesByWeight() {
220 llvm::stable_sort(AllEdges, [](const std::unique_ptr<Edge> &Edge1,
221 const std::unique_ptr<Edge> &Edge2) {
222 return Edge1->Weight > Edge2->Weight;
223 });
224 }
225
226 // Traverse all the edges and compute the Minimum Weight Spanning Tree
227 // using union-find algorithm.
228 void computeMinimumSpanningTree() {
229 // First, put all the critical edge with landing-pad as the Dest to MST.
230 // This works around the insufficient support of critical edges split
231 // when destination BB is a landing pad.
232 for (auto &Ei : AllEdges) {
233 if (Ei->Removed)
234 continue;
235 if (Ei->IsCritical) {
236 if (Ei->DestBB && Ei->DestBB->isLandingPad()) {
237 if (unionGroups(BB1: Ei->SrcBB, BB2: Ei->DestBB))
238 Ei->InMST = true;
239 }
240 }
241 }
242
243 for (auto &Ei : AllEdges) {
244 if (Ei->Removed)
245 continue;
246 // If we detect infinite loops, force
247 // instrumenting the entry edge:
248 if (!ExitBlockFound && Ei->SrcBB == nullptr)
249 continue;
250 if (unionGroups(BB1: Ei->SrcBB, BB2: Ei->DestBB))
251 Ei->InMST = true;
252 }
253 }
254
255public:
256 // Dump the Debug information about the instrumentation.
257 void dumpEdges(raw_ostream &OS, const Twine &Message) const {
258 if (!Message.str().empty())
259 OS << Message << "\n";
260 OS << " Number of Basic Blocks: " << BBInfos.size() << "\n";
261 for (auto &BI : BBInfos) {
262 const BasicBlock *BB = BI.first;
263 OS << " BB: " << (BB == nullptr ? "FakeNode" : BB->getName()) << " "
264 << BI.second->infoString() << "\n";
265 }
266
267 OS << " Number of Edges: " << AllEdges.size()
268 << " (*: Instrument, C: CriticalEdge, -: Removed)\n";
269 uint32_t Count = 0;
270 for (auto &EI : AllEdges)
271 OS << " Edge " << Count++ << ": " << getBBInfo(BB: EI->SrcBB).Index << "-->"
272 << getBBInfo(BB: EI->DestBB).Index << EI->infoString() << "\n";
273 }
274
275 // Add an edge to AllEdges with weight W.
276 Edge &addEdge(BasicBlock *Src, BasicBlock *Dest, uint64_t W) {
277 uint32_t Index = BBInfos.size();
278 auto Iter = BBInfos.end();
279 bool Inserted;
280 std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(x&: Src, y: nullptr));
281 if (Inserted) {
282 // Newly inserted, update the real info.
283 Iter->second = std::move(std::make_unique<BBInfo>(Index));
284 Index++;
285 }
286 std::tie(Iter, Inserted) = BBInfos.insert(std::make_pair(x&: Dest, y: nullptr));
287 if (Inserted)
288 // Newly inserted, update the real info.
289 Iter->second = std::move(std::make_unique<BBInfo>(Index));
290 AllEdges.emplace_back(new Edge(Src, Dest, W));
291 return *AllEdges.back();
292 }
293
294 CFGMST(Function &Func, bool InstrumentFuncEntry,
295 BranchProbabilityInfo *BPI = nullptr,
296 BlockFrequencyInfo *BFI = nullptr)
297 : F(Func), BPI(BPI), BFI(BFI), InstrumentFuncEntry(InstrumentFuncEntry) {
298 buildEdges();
299 sortEdgesByWeight();
300 computeMinimumSpanningTree();
301 if (AllEdges.size() > 1 && InstrumentFuncEntry)
302 std::iter_swap(std::move(AllEdges.begin()),
303 std::move(AllEdges.begin() + AllEdges.size() - 1));
304 }
305
306 const std::vector<std::unique_ptr<Edge>> &allEdges() const {
307 return AllEdges;
308 }
309
310 std::vector<std::unique_ptr<Edge>> &allEdges() { return AllEdges; }
311
312 size_t numEdges() const { return AllEdges.size(); }
313
314 size_t bbInfoSize() const { return BBInfos.size(); }
315
316 // Give BB, return the auxiliary information.
317 BBInfo &getBBInfo(const BasicBlock *BB) const {
318 auto It = BBInfos.find(BB);
319 assert(It->second.get() != nullptr);
320 return *It->second.get();
321 }
322
323 // Give BB, return the auxiliary information if it's available.
324 BBInfo *findBBInfo(const BasicBlock *BB) const {
325 auto It = BBInfos.find(BB);
326 if (It == BBInfos.end())
327 return nullptr;
328 return It->second.get();
329 }
330};
331
332} // end namespace llvm
333
334#undef DEBUG_TYPE // "cfgmst"
335
336#endif // LLVM_TRANSFORMS_INSTRUMENTATION_CFGMST_H
337

source code of llvm/include/llvm/Transforms/Instrumentation/CFGMST.h