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