CaptureTracking.cpp revision 7912ef97ffde3ab3334143ddfb4cafdf04e2ebfc 
1//===--- CaptureTracking.cpp - Determine whether a pointer is captured ----===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file contains routines that help determine which pointers are captured.
11// A pointer value is captured if the function makes a copy of any part of the
12// pointer that outlives the call.  Not being captured means, more or less, that
13// the pointer is only dereferenced and not stored in a global.  Returning part
14// of the pointer as the function return value may or may not count as capturing
15// the pointer, depending on the context.
16//
17//===----------------------------------------------------------------------===//
18
19#include "llvm/Analysis/CaptureTracking.h"
20using namespace llvm;
21
22namespace {
23  struct SimpleCaptureTracker : public CaptureTracker {
24    explicit SimpleCaptureTracker(bool ReturnCaptures)
25      : ReturnCaptures(ReturnCaptures), Captured(false) {}
26
27    void tooManyUses() { Captured = true; }
28
29    bool shouldExplore(Use *U) { return true; }
30
31    bool captured(Instruction *I) {
32      if (isa<ReturnInst>(I) && !ReturnCaptures)
33	return false;
34
35      Captured = true;
36      return true;
37    }
38
39    bool ReturnCaptures;
40
41    bool Captured;
42  };
43}
44
45/// PointerMayBeCaptured - Return true if this pointer value may be captured
46/// by the enclosing function (which is required to exist).  This routine can
47/// be expensive, so consider caching the results.  The boolean ReturnCaptures
48/// specifies whether returning the value (or part of it) from the function
49/// counts as capturing it or not.  The boolean StoreCaptures specified whether
50/// storing the value (or part of it) into memory anywhere automatically
51/// counts as capturing it or not.
52bool llvm::PointerMayBeCaptured(const Value *V,
53                                bool ReturnCaptures, bool StoreCaptures) {
54  // TODO: If StoreCaptures is not true, we could do Fancy analysis
55  // to determine whether this store is not actually an escape point.
56  // In that case, BasicAliasAnalysis should be updated as well to
57  // take advantage of this.
58  (void)StoreCaptures;
59
60  SimpleCaptureTracker SCT(ReturnCaptures);
61  PointerMayBeCaptured(V, &SCT);
62  return SCT.Captured;
63}
64
65/// TODO: Write a new FunctionPass AliasAnalysis so that it can keep
66/// a cache. Then we can move the code from BasicAliasAnalysis into
67/// that path, and remove this threshold.
68static int const Threshold = 20;
69
70void llvm::PointerMayBeCaptured(const Value *V, CaptureTracker *Tracker) {
71  assert(V->getType()->isPointerTy() && "Capture is for pointers only!");
72  SmallVector<Use*, Threshold> Worklist;
73  SmallSet<Use*, Threshold> Visited;
74  int Count = 0;
75
76  for (Value::const_use_iterator UI = V->use_begin(), UE = V->use_end();
77       UI != UE; ++UI) {
78    // If there are lots of uses, conservatively say that the value
79    // is captured to avoid taking too much compile time.
80    if (Count++ >= Threshold)
81      return Tracker->tooManyUses();
82
83    Use *U = &UI.getUse();
84    if (!Tracker->shouldExplore(U)) continue;
85    Visited.insert(U);
86    Worklist.push_back(U);
87  }
88
89  while (!Worklist.empty()) {
90    Use *U = Worklist.pop_back_val();
91    Instruction *I = cast<Instruction>(U->getUser());
92    V = U->get();
93
94    switch (I->getOpcode()) {
95    case Instruction::Call:
96    case Instruction::Invoke: {
97      CallSite CS(I);
98      // Not captured if the callee is readonly, doesn't return a copy through
99      // its return value and doesn't unwind (a readonly function can leak bits
100      // by throwing an exception or not depending on the input value).
101      if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
102        break;
103
104      // Not captured if only passed via 'nocapture' arguments.  Note that
105      // calling a function pointer does not in itself cause the pointer to
106      // be captured.  This is a subtle point considering that (for example)
107      // the callee might return its own address.  It is analogous to saying
108      // that loading a value from a pointer does not cause the pointer to be
109      // captured, even though the loaded value might be the pointer itself
110      // (think of self-referential objects).
111      CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
112      for (CallSite::arg_iterator A = B; A != E; ++A)
113        if (A->get() == V && !CS.doesNotCapture(A - B))
114          // The parameter is not marked 'nocapture' - captured.
115          if (Tracker->captured(I))
116            return;
117      break;
118    }
119    case Instruction::Load:
120      // Loading from a pointer does not cause it to be captured.
121      break;
122    case Instruction::VAArg:
123      // "va-arg" from a pointer does not cause it to be captured.
124      break;
125    case Instruction::Store:
126      if (V == I->getOperand(0))
127        // Stored the pointer - conservatively assume it may be captured.
128        if (Tracker->captured(I))
129          return;
130      // Storing to the pointee does not cause the pointer to be captured.
131      break;
132    case Instruction::BitCast:
133    case Instruction::GetElementPtr:
134    case Instruction::PHI:
135    case Instruction::Select:
136      // The original value is not captured via this if the new value isn't.
137      for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
138           UI != UE; ++UI) {
139        Use *U = &UI.getUse();
140        if (Visited.insert(U))
141          if (Tracker->shouldExplore(U))
142            Worklist.push_back(U);
143      }
144      break;
145    case Instruction::ICmp:
146      // Don't count comparisons of a no-alias return value against null as
147      // captures. This allows us to ignore comparisons of malloc results
148      // with null, for example.
149      if (isNoAliasCall(V->stripPointerCasts()))
150        if (ConstantPointerNull *CPN =
151              dyn_cast<ConstantPointerNull>(I->getOperand(1)))
152          if (CPN->getType()->getAddressSpace() == 0)
153            break;
154      // Otherwise, be conservative. There are crazy ways to capture pointers
155      // using comparisons.
156      if (Tracker->captured(I))
157        return;
158      break;
159    default:
160      // Something else - be conservative and say it is captured.
161      if (Tracker->captured(I))
162        return;
163      break;
164    }
165  }
166
167  // All uses examined.
168}
169