LLVM  13.0.0git
CaptureTracking.cpp
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1 //===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
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 contains routines that help determine which pointers are captured.
10 // A pointer value is captured if the function makes a copy of any part of the
11 // pointer that outlives the call. Not being captured means, more or less, that
12 // the pointer is only dereferenced and not stored in a global. Returning part
13 // of the pointer as the function return value may or may not count as capturing
14 // the pointer, depending on the context.
15 //
16 //===----------------------------------------------------------------------===//
17 
19 #include "llvm/ADT/SmallSet.h"
20 #include "llvm/ADT/SmallVector.h"
21 #include "llvm/ADT/Statistic.h"
23 #include "llvm/Analysis/CFG.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/IntrinsicInst.h"
30 
31 using namespace llvm;
32 
33 #define DEBUG_TYPE "capture-tracking"
34 
35 STATISTIC(NumCaptured, "Number of pointers maybe captured");
36 STATISTIC(NumNotCaptured, "Number of pointers not captured");
37 STATISTIC(NumCapturedBefore, "Number of pointers maybe captured before");
38 STATISTIC(NumNotCapturedBefore, "Number of pointers not captured before");
39 
40 /// The default value for MaxUsesToExplore argument. It's relatively small to
41 /// keep the cost of analysis reasonable for clients like BasicAliasAnalysis,
42 /// where the results can't be cached.
43 /// TODO: we should probably introduce a caching CaptureTracking analysis and
44 /// use it where possible. The caching version can use much higher limit or
45 /// don't have this cap at all.
46 static cl::opt<unsigned>
47 DefaultMaxUsesToExplore("capture-tracking-max-uses-to-explore", cl::Hidden,
48  cl::desc("Maximal number of uses to explore."),
49  cl::init(20));
50 
53 }
54 
56 
57 bool CaptureTracker::shouldExplore(const Use *U) { return true; }
58 
60  // An inbounds GEP can either be a valid pointer (pointing into
61  // or to the end of an allocation), or be null in the default
62  // address space. So for an inbounds GEP there is no way to let
63  // the pointer escape using clever GEP hacking because doing so
64  // would make the pointer point outside of the allocated object
65  // and thus make the GEP result a poison value. Similarly, other
66  // dereferenceable pointers cannot be manipulated without producing
67  // poison.
68  if (auto *GEP = dyn_cast<GetElementPtrInst>(O))
69  if (GEP->isInBounds())
70  return true;
71  bool CanBeNull, CanBeFreed;
72  return O->getPointerDereferenceableBytes(DL, CanBeNull, CanBeFreed);
73 }
74 
75 namespace {
76  struct SimpleCaptureTracker : public CaptureTracker {
77  explicit SimpleCaptureTracker(bool ReturnCaptures)
78  : ReturnCaptures(ReturnCaptures), Captured(false) {}
79 
80  void tooManyUses() override { Captured = true; }
81 
82  bool captured(const Use *U) override {
83  if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
84  return false;
85 
86  Captured = true;
87  return true;
88  }
89 
90  bool ReturnCaptures;
91 
92  bool Captured;
93  };
94 
95  /// Only find pointer captures which happen before the given instruction. Uses
96  /// the dominator tree to determine whether one instruction is before another.
97  /// Only support the case where the Value is defined in the same basic block
98  /// as the given instruction and the use.
99  struct CapturesBefore : public CaptureTracker {
100 
101  CapturesBefore(bool ReturnCaptures, const Instruction *I, const DominatorTree *DT,
102  bool IncludeI)
103  : BeforeHere(I), DT(DT),
104  ReturnCaptures(ReturnCaptures), IncludeI(IncludeI), Captured(false) {}
105 
106  void tooManyUses() override { Captured = true; }
107 
108  bool isSafeToPrune(Instruction *I) {
109  if (BeforeHere == I)
110  return !IncludeI;
111 
112  // We explore this usage only if the usage can reach "BeforeHere".
113  // If use is not reachable from entry, there is no need to explore.
114  if (!DT->isReachableFromEntry(I->getParent()))
115  return true;
116 
117  // Check whether there is a path from I to BeforeHere.
118  return !isPotentiallyReachable(I, BeforeHere, nullptr, DT);
119  }
120 
121  bool captured(const Use *U) override {
122  Instruction *I = cast<Instruction>(U->getUser());
123  if (isa<ReturnInst>(I) && !ReturnCaptures)
124  return false;
125 
126  // Check isSafeToPrune() here rather than in shouldExplore() to avoid
127  // an expensive reachability query for every instruction we look at.
128  // Instead we only do one for actual capturing candidates.
129  if (isSafeToPrune(I))
130  return false;
131 
132  Captured = true;
133  return true;
134  }
135 
136  const Instruction *BeforeHere;
137  const DominatorTree *DT;
138 
139  bool ReturnCaptures;
140  bool IncludeI;
141 
142  bool Captured;
143  };
144 }
145 
146 /// PointerMayBeCaptured - Return true if this pointer value may be captured
147 /// by the enclosing function (which is required to exist). This routine can
148 /// be expensive, so consider caching the results. The boolean ReturnCaptures
149 /// specifies whether returning the value (or part of it) from the function
150 /// counts as capturing it or not. The boolean StoreCaptures specified whether
151 /// storing the value (or part of it) into memory anywhere automatically
152 /// counts as capturing it or not.
154  bool ReturnCaptures, bool StoreCaptures,
155  unsigned MaxUsesToExplore) {
156  assert(!isa<GlobalValue>(V) &&
157  "It doesn't make sense to ask whether a global is captured.");
158 
159  // TODO: If StoreCaptures is not true, we could do Fancy analysis
160  // to determine whether this store is not actually an escape point.
161  // In that case, BasicAliasAnalysis should be updated as well to
162  // take advantage of this.
163  (void)StoreCaptures;
164 
165  SimpleCaptureTracker SCT(ReturnCaptures);
166  PointerMayBeCaptured(V, &SCT, MaxUsesToExplore);
167  if (SCT.Captured)
168  ++NumCaptured;
169  else
170  ++NumNotCaptured;
171  return SCT.Captured;
172 }
173 
174 /// PointerMayBeCapturedBefore - Return true if this pointer value may be
175 /// captured by the enclosing function (which is required to exist). If a
176 /// DominatorTree is provided, only captures which happen before the given
177 /// instruction are considered. This routine can be expensive, so consider
178 /// caching the results. The boolean ReturnCaptures specifies whether
179 /// returning the value (or part of it) from the function counts as capturing
180 /// it or not. The boolean StoreCaptures specified whether storing the value
181 /// (or part of it) into memory anywhere automatically counts as capturing it
182 /// or not.
183 bool llvm::PointerMayBeCapturedBefore(const Value *V, bool ReturnCaptures,
184  bool StoreCaptures, const Instruction *I,
185  const DominatorTree *DT, bool IncludeI,
186  unsigned MaxUsesToExplore) {
187  assert(!isa<GlobalValue>(V) &&
188  "It doesn't make sense to ask whether a global is captured.");
189 
190  if (!DT)
191  return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures,
192  MaxUsesToExplore);
193 
194  // TODO: See comment in PointerMayBeCaptured regarding what could be done
195  // with StoreCaptures.
196 
197  CapturesBefore CB(ReturnCaptures, I, DT, IncludeI);
198  PointerMayBeCaptured(V, &CB, MaxUsesToExplore);
199  if (CB.Captured)
200  ++NumCapturedBefore;
201  else
202  ++NumNotCapturedBefore;
203  return CB.Captured;
204 }
205 
207  unsigned MaxUsesToExplore) {
208  assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
209  if (MaxUsesToExplore == 0)
210  MaxUsesToExplore = DefaultMaxUsesToExplore;
211 
215 
216  auto AddUses = [&](const Value *V) {
217  unsigned Count = 0;
218  for (const Use &U : V->uses()) {
219  // If there are lots of uses, conservatively say that the value
220  // is captured to avoid taking too much compile time.
221  if (Count++ >= MaxUsesToExplore) {
222  Tracker->tooManyUses();
223  return false;
224  }
225  if (!Visited.insert(&U).second)
226  continue;
227  if (!Tracker->shouldExplore(&U))
228  continue;
229  Worklist.push_back(&U);
230  }
231  return true;
232  };
233  if (!AddUses(V))
234  return;
235 
236  while (!Worklist.empty()) {
237  const Use *U = Worklist.pop_back_val();
238  Instruction *I = cast<Instruction>(U->getUser());
239 
240  switch (I->getOpcode()) {
241  case Instruction::Call:
242  case Instruction::Invoke: {
243  auto *Call = cast<CallBase>(I);
244  // Not captured if the callee is readonly, doesn't return a copy through
245  // its return value and doesn't unwind (a readonly function can leak bits
246  // by throwing an exception or not depending on the input value).
247  if (Call->onlyReadsMemory() && Call->doesNotThrow() &&
248  Call->getType()->isVoidTy())
249  break;
250 
251  // The pointer is not captured if returned pointer is not captured.
252  // NOTE: CaptureTracking users should not assume that only functions
253  // marked with nocapture do not capture. This means that places like
254  // getUnderlyingObject in ValueTracking or DecomposeGEPExpression
255  // in BasicAA also need to know about this property.
257  true)) {
258  if (!AddUses(Call))
259  return;
260  break;
261  }
262 
263  // Volatile operations effectively capture the memory location that they
264  // load and store to.
265  if (auto *MI = dyn_cast<MemIntrinsic>(Call))
266  if (MI->isVolatile())
267  if (Tracker->captured(U))
268  return;
269 
270  // Not captured if only passed via 'nocapture' arguments. Note that
271  // calling a function pointer does not in itself cause the pointer to
272  // be captured. This is a subtle point considering that (for example)
273  // the callee might return its own address. It is analogous to saying
274  // that loading a value from a pointer does not cause the pointer to be
275  // captured, even though the loaded value might be the pointer itself
276  // (think of self-referential objects).
277  if (Call->isDataOperand(U) &&
278  !Call->doesNotCapture(Call->getDataOperandNo(U))) {
279  // The parameter is not marked 'nocapture' - captured.
280  if (Tracker->captured(U))
281  return;
282  }
283  break;
284  }
285  case Instruction::Load:
286  // Volatile loads make the address observable.
287  if (cast<LoadInst>(I)->isVolatile())
288  if (Tracker->captured(U))
289  return;
290  break;
291  case Instruction::VAArg:
292  // "va-arg" from a pointer does not cause it to be captured.
293  break;
294  case Instruction::Store:
295  // Stored the pointer - conservatively assume it may be captured.
296  // Volatile stores make the address observable.
297  if (U->getOperandNo() == 0 || cast<StoreInst>(I)->isVolatile())
298  if (Tracker->captured(U))
299  return;
300  break;
301  case Instruction::AtomicRMW: {
302  // atomicrmw conceptually includes both a load and store from
303  // the same location.
304  // As with a store, the location being accessed is not captured,
305  // but the value being stored is.
306  // Volatile stores make the address observable.
307  auto *ARMWI = cast<AtomicRMWInst>(I);
308  if (U->getOperandNo() == 1 || ARMWI->isVolatile())
309  if (Tracker->captured(U))
310  return;
311  break;
312  }
313  case Instruction::AtomicCmpXchg: {
314  // cmpxchg conceptually includes both a load and store from
315  // the same location.
316  // As with a store, the location being accessed is not captured,
317  // but the value being stored is.
318  // Volatile stores make the address observable.
319  auto *ACXI = cast<AtomicCmpXchgInst>(I);
320  if (U->getOperandNo() == 1 || U->getOperandNo() == 2 ||
321  ACXI->isVolatile())
322  if (Tracker->captured(U))
323  return;
324  break;
325  }
326  case Instruction::BitCast:
327  case Instruction::GetElementPtr:
328  case Instruction::PHI:
329  case Instruction::Select:
330  case Instruction::AddrSpaceCast:
331  // The original value is not captured via this if the new value isn't.
332  if (!AddUses(I))
333  return;
334  break;
335  case Instruction::ICmp: {
336  unsigned Idx = U->getOperandNo();
337  unsigned OtherIdx = 1 - Idx;
338  if (auto *CPN = dyn_cast<ConstantPointerNull>(I->getOperand(OtherIdx))) {
339  // Don't count comparisons of a no-alias return value against null as
340  // captures. This allows us to ignore comparisons of malloc results
341  // with null, for example.
342  if (CPN->getType()->getAddressSpace() == 0)
343  if (isNoAliasCall(U->get()->stripPointerCasts()))
344  break;
345  if (!I->getFunction()->nullPointerIsDefined()) {
346  auto *O = I->getOperand(Idx)->stripPointerCastsSameRepresentation();
347  // Comparing a dereferenceable_or_null pointer against null cannot
348  // lead to pointer escapes, because if it is not null it must be a
349  // valid (in-bounds) pointer.
350  if (Tracker->isDereferenceableOrNull(O, I->getModule()->getDataLayout()))
351  break;
352  }
353  }
354  // Comparison against value stored in global variable. Given the pointer
355  // does not escape, its value cannot be guessed and stored separately in a
356  // global variable.
357  auto *LI = dyn_cast<LoadInst>(I->getOperand(OtherIdx));
358  if (LI && isa<GlobalVariable>(LI->getPointerOperand()))
359  break;
360  // Otherwise, be conservative. There are crazy ways to capture pointers
361  // using comparisons.
362  if (Tracker->captured(U))
363  return;
364  break;
365  }
366  default:
367  // Something else - be conservative and say it is captured.
368  if (Tracker->captured(U))
369  return;
370  break;
371  }
372  }
373 
374  // All uses examined.
375 }
376 
378  const Value *V, SmallDenseMap<const Value *, bool, 8> *IsCapturedCache) {
380  if (IsCapturedCache) {
381  bool Inserted;
382  std::tie(CacheIt, Inserted) = IsCapturedCache->insert({V, false});
383  if (!Inserted)
384  // Found cached result, return it!
385  return CacheIt->second;
386  }
387 
388  // If this is an identified function-local object, check to see if it escapes.
389  if (isIdentifiedFunctionLocal(V)) {
390  // Set StoreCaptures to True so that we can assume in our callers that the
391  // pointer is not the result of a load instruction. Currently
392  // PointerMayBeCaptured doesn't have any special analysis for the
393  // StoreCaptures=false case; if it did, our callers could be refined to be
394  // more precise.
395  auto Ret = !PointerMayBeCaptured(V, false, /*StoreCaptures=*/true);
396  if (IsCapturedCache)
397  CacheIt->second = Ret;
398  return Ret;
399  }
400 
401  return false;
402 }
MI
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