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"
20 #include "llvm/Instructions.h"
21 #include "llvm/Value.h"
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/ADT/SmallSet.h"
24 #include "llvm/ADT/SmallVector.h"
25 #include "llvm/Support/CallSite.h"
28 /// PointerMayBeCaptured - Return true if this pointer value may be captured
29 /// by the enclosing function (which is required to exist). This routine can
30 /// be expensive, so consider caching the results. The boolean ReturnCaptures
31 /// specifies whether returning the value (or part of it) from the function
32 /// counts as capturing it or not. The boolean StoreCaptures specified whether
33 /// storing the value (or part of it) into memory anywhere automatically
34 /// counts as capturing it or not.
35 bool llvm::PointerMayBeCaptured(const Value *V,
36 bool ReturnCaptures, bool StoreCaptures) {
37 assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
38 SmallVector<Use*, 16> Worklist;
39 SmallSet<Use*, 16> Visited;
41 for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
43 Use *U = &UI.getUse();
45 Worklist.push_back(U);
48 while (!Worklist.empty()) {
49 Use *U = Worklist.pop_back_val();
50 Instruction *I = cast<Instruction>(U->getUser());
53 switch (I->getOpcode()) {
54 case Instruction::Call:
55 case Instruction::Invoke: {
56 CallSite CS = CallSite::get(I);
57 // Not captured if the callee is readonly, doesn't return a copy through
58 // its return value and doesn't unwind (a readonly function can leak bits
59 // by throwing an exception or not depending on the input value).
60 if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy())
63 // Not captured if only passed via 'nocapture' arguments. Note that
64 // calling a function pointer does not in itself cause the pointer to
65 // be captured. This is a subtle point considering that (for example)
66 // the callee might return its own address. It is analogous to saying
67 // that loading a value from a pointer does not cause the pointer to be
68 // captured, even though the loaded value might be the pointer itself
69 // (think of self-referential objects).
70 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
71 for (CallSite::arg_iterator A = B; A != E; ++A)
72 if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
73 // The parameter is not marked 'nocapture' - captured.
75 // Only passed via 'nocapture' arguments, or is the called function - not
79 case Instruction::Load:
80 // Loading from a pointer does not cause it to be captured.
82 case Instruction::Ret:
86 case Instruction::Store:
87 if (V == I->getOperand(0))
88 // Stored the pointer - conservatively assume it may be captured.
89 // TODO: If StoreCaptures is not true, we could do Fancy analysis
90 // to determine whether this store is not actually an escape point.
91 // In that case, BasicAliasAnalysis should be updated as well to
92 // take advantage of this.
94 // Storing to the pointee does not cause the pointer to be captured.
96 case Instruction::BitCast:
97 case Instruction::GetElementPtr:
98 case Instruction::PHI:
99 case Instruction::Select:
100 // The original value is not captured via this if the new value isn't.
101 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
103 Use *U = &UI.getUse();
104 if (Visited.insert(U))
105 Worklist.push_back(U);
108 case Instruction::ICmp:
109 // Don't count comparisons of a no-alias return value against null as
110 // captures. This allows us to ignore comparisons of malloc results
111 // with null, for example.
112 if (isNoAliasCall(V->stripPointerCasts()))
113 if (ConstantPointerNull *CPN =
114 dyn_cast<ConstantPointerNull>(I->getOperand(1)))
115 if (CPN->getType()->getAddressSpace() == 0)
117 // Otherwise, be conservative. There are crazy ways to capture pointers
118 // using comparisons.
121 // Something else - be conservative and say it is captured.
126 // All uses examined - not captured.