1 //===- StackLifetime.cpp - Alloca Lifetime Analysis -----------------------===//
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 #include "llvm/Analysis/StackLifetime.h"
10 #include "llvm/ADT/DepthFirstIterator.h"
11 #include "llvm/ADT/STLExtras.h"
12 #include "llvm/ADT/SmallVector.h"
13 #include "llvm/ADT/StringExtras.h"
14 #include "llvm/Config/llvm-config.h"
15 #include "llvm/IR/AssemblyAnnotationWriter.h"
16 #include "llvm/IR/BasicBlock.h"
17 #include "llvm/IR/CFG.h"
18 #include "llvm/IR/InstIterator.h"
19 #include "llvm/IR/Instructions.h"
20 #include "llvm/IR/IntrinsicInst.h"
21 #include "llvm/IR/Intrinsics.h"
22 #include "llvm/IR/User.h"
23 #include "llvm/IR/Value.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Support/Casting.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Compiler.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/FormattedStream.h"
30 #include <memory>
31 #include <tuple>
32 
33 using namespace llvm;
34 
35 #define DEBUG_TYPE "stack-lifetime"
36 
37 const StackLifetime::LiveRange &
38 StackLifetime::getLiveRange(const AllocaInst *AI) const {
39   const auto IT = AllocaNumbering.find(AI);
40   assert(IT != AllocaNumbering.end());
41   return LiveRanges[IT->second];
42 }
43 
44 static bool readMarker(const Instruction *I, bool *IsStart) {
45   if (!I->isLifetimeStartOrEnd())
46     return false;
47 
48   auto *II = cast<IntrinsicInst>(I);
49   *IsStart = II->getIntrinsicID() == Intrinsic::lifetime_start;
50   return true;
51 }
52 
53 std::vector<const IntrinsicInst *> StackLifetime::getMarkers() const {
54   std::vector<const IntrinsicInst *> Markers;
55   for (auto &M : InstructionNumbering)
56     if (M.getFirst()->isLifetimeStartOrEnd())
57       Markers.push_back(M.getFirst());
58   return Markers;
59 }
60 
61 void StackLifetime::collectMarkers() {
62   InterestingAllocas.resize(NumAllocas);
63   DenseMap<const BasicBlock *, SmallDenseMap<const IntrinsicInst *, Marker>>
64       BBMarkerSet;
65 
66   // Compute the set of start/end markers per basic block.
67   for (unsigned AllocaNo = 0; AllocaNo < NumAllocas; ++AllocaNo) {
68     const AllocaInst *AI = Allocas[AllocaNo];
69     SmallVector<const Instruction *, 8> WorkList;
70     WorkList.push_back(AI);
71     while (!WorkList.empty()) {
72       const Instruction *I = WorkList.pop_back_val();
73       for (const User *U : I->users()) {
74         if (auto *BI = dyn_cast<BitCastInst>(U)) {
75           WorkList.push_back(BI);
76           continue;
77         }
78         auto *UI = dyn_cast<IntrinsicInst>(U);
79         if (!UI)
80           continue;
81         bool IsStart;
82         if (!readMarker(UI, &IsStart))
83           continue;
84         if (IsStart)
85           InterestingAllocas.set(AllocaNo);
86         BBMarkerSet[UI->getParent()][UI] = {AllocaNo, IsStart};
87       }
88     }
89   }
90 
91   // Compute instruction numbering. Only the following instructions are
92   // considered:
93   // * Basic block entries
94   // * Lifetime markers
95   // For each basic block, compute
96   // * the list of markers in the instruction order
97   // * the sets of allocas whose lifetime starts or ends in this BB
98   LLVM_DEBUG(dbgs() << "Instructions:\n");
99   unsigned InstNo = 0;
100   for (const BasicBlock *BB : depth_first(&F)) {
101     LLVM_DEBUG(dbgs() << "  " << InstNo << ": BB " << BB->getName() << "\n");
102     unsigned BBStart = InstNo++;
103 
104     BlockLifetimeInfo &BlockInfo =
105         BlockLiveness.try_emplace(BB, NumAllocas).first->getSecond();
106 
107     auto &BlockMarkerSet = BBMarkerSet[BB];
108     if (BlockMarkerSet.empty()) {
109       unsigned BBEnd = InstNo;
110       BlockInstRange[BB] = std::make_pair(BBStart, BBEnd);
111       continue;
112     }
113 
114     auto ProcessMarker = [&](const IntrinsicInst *I, const Marker &M) {
115       LLVM_DEBUG(dbgs() << "  " << InstNo << ":  "
116                         << (M.IsStart ? "start " : "end   ") << M.AllocaNo
117                         << ", " << *I << "\n");
118 
119       BBMarkers[BB].push_back({InstNo, M});
120 
121       InstructionNumbering[I] = InstNo++;
122 
123       if (M.IsStart) {
124         BlockInfo.End.reset(M.AllocaNo);
125         BlockInfo.Begin.set(M.AllocaNo);
126       } else {
127         BlockInfo.Begin.reset(M.AllocaNo);
128         BlockInfo.End.set(M.AllocaNo);
129       }
130     };
131 
132     if (BlockMarkerSet.size() == 1) {
133       ProcessMarker(BlockMarkerSet.begin()->getFirst(),
134                     BlockMarkerSet.begin()->getSecond());
135     } else {
136       // Scan the BB to determine the marker order.
137       for (const Instruction &I : *BB) {
138         const IntrinsicInst *II = dyn_cast<IntrinsicInst>(&I);
139         if (!II)
140           continue;
141         auto It = BlockMarkerSet.find(II);
142         if (It == BlockMarkerSet.end())
143           continue;
144         ProcessMarker(II, It->getSecond());
145       }
146     }
147 
148     unsigned BBEnd = InstNo;
149     BlockInstRange[BB] = std::make_pair(BBStart, BBEnd);
150   }
151   NumInst = InstNo;
152 }
153 
154 void StackLifetime::calculateLocalLiveness() {
155   bool Changed = true;
156   while (Changed) {
157     Changed = false;
158 
159     for (const BasicBlock *BB : depth_first(&F)) {
160       BlockLifetimeInfo &BlockInfo = BlockLiveness.find(BB)->getSecond();
161 
162       // Compute LiveIn by unioning together the LiveOut sets of all preds.
163       BitVector LocalLiveIn;
164       for (auto *PredBB : predecessors(BB)) {
165         LivenessMap::const_iterator I = BlockLiveness.find(PredBB);
166         // If a predecessor is unreachable, ignore it.
167         if (I == BlockLiveness.end())
168           continue;
169         LocalLiveIn |= I->second.LiveOut;
170       }
171 
172       // Compute LiveOut by subtracting out lifetimes that end in this
173       // block, then adding in lifetimes that begin in this block.  If
174       // we have both BEGIN and END markers in the same basic block
175       // then we know that the BEGIN marker comes after the END,
176       // because we already handle the case where the BEGIN comes
177       // before the END when collecting the markers (and building the
178       // BEGIN/END vectors).
179       BitVector LocalLiveOut = LocalLiveIn;
180       LocalLiveOut.reset(BlockInfo.End);
181       LocalLiveOut |= BlockInfo.Begin;
182 
183       // Update block LiveIn set, noting whether it has changed.
184       if (LocalLiveIn.test(BlockInfo.LiveIn)) {
185         Changed = true;
186         BlockInfo.LiveIn |= LocalLiveIn;
187       }
188 
189       // Update block LiveOut set, noting whether it has changed.
190       if (LocalLiveOut.test(BlockInfo.LiveOut)) {
191         Changed = true;
192         BlockInfo.LiveOut |= LocalLiveOut;
193       }
194     }
195   } // while changed.
196 }
197 
198 void StackLifetime::calculateLiveIntervals() {
199   for (auto IT : BlockLiveness) {
200     const BasicBlock *BB = IT.getFirst();
201     BlockLifetimeInfo &BlockInfo = IT.getSecond();
202     unsigned BBStart, BBEnd;
203     std::tie(BBStart, BBEnd) = BlockInstRange[BB];
204 
205     BitVector Started, Ended;
206     Started.resize(NumAllocas);
207     Ended.resize(NumAllocas);
208     SmallVector<unsigned, 8> Start;
209     Start.resize(NumAllocas);
210 
211     // LiveIn ranges start at the first instruction.
212     for (unsigned AllocaNo = 0; AllocaNo < NumAllocas; ++AllocaNo) {
213       if (BlockInfo.LiveIn.test(AllocaNo)) {
214         Started.set(AllocaNo);
215         Start[AllocaNo] = BBStart;
216       }
217     }
218 
219     for (auto &It : BBMarkers[BB]) {
220       unsigned InstNo = It.first;
221       bool IsStart = It.second.IsStart;
222       unsigned AllocaNo = It.second.AllocaNo;
223 
224       if (IsStart) {
225         assert(!Started.test(AllocaNo) || Start[AllocaNo] == BBStart);
226         if (!Started.test(AllocaNo)) {
227           Started.set(AllocaNo);
228           Ended.reset(AllocaNo);
229           Start[AllocaNo] = InstNo;
230         }
231       } else {
232         assert(!Ended.test(AllocaNo));
233         if (Started.test(AllocaNo)) {
234           LiveRanges[AllocaNo].addRange(Start[AllocaNo], InstNo);
235           Started.reset(AllocaNo);
236         }
237         Ended.set(AllocaNo);
238       }
239     }
240 
241     for (unsigned AllocaNo = 0; AllocaNo < NumAllocas; ++AllocaNo)
242       if (Started.test(AllocaNo))
243         LiveRanges[AllocaNo].addRange(Start[AllocaNo], BBEnd);
244   }
245 }
246 
247 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
248 LLVM_DUMP_METHOD void StackLifetime::dumpAllocas() const {
249   dbgs() << "Allocas:\n";
250   for (unsigned AllocaNo = 0; AllocaNo < NumAllocas; ++AllocaNo)
251     dbgs() << "  " << AllocaNo << ": " << *Allocas[AllocaNo] << "\n";
252 }
253 
254 LLVM_DUMP_METHOD void StackLifetime::dumpBlockLiveness() const {
255   dbgs() << "Block liveness:\n";
256   for (auto IT : BlockLiveness) {
257     const BasicBlock *BB = IT.getFirst();
258     const BlockLifetimeInfo &BlockInfo = BlockLiveness.find(BB)->getSecond();
259     auto BlockRange = BlockInstRange.find(BB)->getSecond();
260     dbgs() << "  BB [" << BlockRange.first << ", " << BlockRange.second
261            << "): begin " << BlockInfo.Begin << ", end " << BlockInfo.End
262            << ", livein " << BlockInfo.LiveIn << ", liveout "
263            << BlockInfo.LiveOut << "\n";
264   }
265 }
266 
267 LLVM_DUMP_METHOD void StackLifetime::dumpLiveRanges() const {
268   dbgs() << "Alloca liveness:\n";
269   for (unsigned AllocaNo = 0; AllocaNo < NumAllocas; ++AllocaNo)
270     dbgs() << "  " << AllocaNo << ": " << LiveRanges[AllocaNo] << "\n";
271 }
272 #endif
273 
274 StackLifetime::StackLifetime(const Function &F,
275                              ArrayRef<const AllocaInst *> Allocas)
276     : F(F), Allocas(Allocas), NumAllocas(Allocas.size()) {
277   LLVM_DEBUG(dumpAllocas());
278 
279   for (unsigned I = 0; I < NumAllocas; ++I)
280     AllocaNumbering[Allocas[I]] = I;
281 
282   collectMarkers();
283 }
284 
285 void StackLifetime::run() {
286   LiveRanges.resize(NumAllocas, LiveRange(NumInst));
287   for (unsigned I = 0; I < NumAllocas; ++I)
288     if (!InterestingAllocas.test(I))
289       LiveRanges[I] = getFullLiveRange();
290 
291   calculateLocalLiveness();
292   LLVM_DEBUG(dumpBlockLiveness());
293   calculateLiveIntervals();
294   LLVM_DEBUG(dumpLiveRanges());
295 }
296 
297 class StackLifetime::LifetimeAnnotationWriter
298     : public AssemblyAnnotationWriter {
299   const StackLifetime &SL;
300   SmallVector<StringRef, 16> Names;
301 
302   void printInstrAlive(unsigned InstrNo, formatted_raw_ostream &OS) {
303     Names.clear();
304     for (const auto &KV : SL.AllocaNumbering) {
305       if (SL.LiveRanges[KV.getSecond()].test(InstrNo))
306         Names.push_back(KV.getFirst()->getName());
307     }
308     llvm::sort(Names);
309     OS << "  ; Alive: <" << llvm::join(Names, " ") << ">\n";
310   }
311 
312   void printBBAlive(const BasicBlock *BB, bool Start,
313                     formatted_raw_ostream &OS) {
314     auto ItBB = SL.BlockInstRange.find(BB);
315     if (ItBB == SL.BlockInstRange.end())
316       return; // Unreachable.
317     unsigned InstrNo =
318         Start ? ItBB->getSecond().first : (ItBB->getSecond().second - 1);
319     printInstrAlive(InstrNo, OS);
320   }
321 
322   void emitBasicBlockStartAnnot(const BasicBlock *BB,
323                                 formatted_raw_ostream &OS) override {
324     printBBAlive(BB, true, OS);
325   }
326   void emitBasicBlockEndAnnot(const BasicBlock *BB,
327                               formatted_raw_ostream &OS) override {
328     printBBAlive(BB, false, OS);
329   }
330 
331   void printInfoComment(const Value &V, formatted_raw_ostream &OS) override {
332     auto It = SL.InstructionNumbering.find(dyn_cast<IntrinsicInst>(&V));
333     if (It == SL.InstructionNumbering.end())
334       return; // Unintresting.
335     OS << "\n";
336     printInstrAlive(It->getSecond(), OS);
337   }
338 
339 public:
340   LifetimeAnnotationWriter(const StackLifetime &SL) : SL(SL) {}
341 };
342 
343 void StackLifetime::print(raw_ostream &OS) {
344   LifetimeAnnotationWriter AAW(*this);
345   F.print(OS, &AAW);
346 }
347 
348 PreservedAnalyses StackLifetimePrinterPass::run(Function &F,
349                                                 FunctionAnalysisManager &AM) {
350   SmallVector<const AllocaInst *, 8> Allocas;
351   for (auto &I : instructions(F))
352     if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I))
353       Allocas.push_back(AI);
354   StackLifetime SL(F, Allocas);
355   SL.run();
356   SL.print(OS);
357   return PreservedAnalyses::all();
358 }
359