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/Analysis/MallocHelper.h"
21 #include "llvm/Instructions.h"
22 #include "llvm/Value.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.
33 bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures) {
34 assert(isa<PointerType>(V->getType()) && "Capture is for pointers only!");
35 SmallVector<Use*, 16> Worklist;
36 SmallSet<Use*, 16> Visited;
38 for (Value::use_const_iterator UI = V->use_begin(), UE = V->use_end();
40 Use *U = &UI.getUse();
42 Worklist.push_back(U);
45 while (!Worklist.empty()) {
46 Use *U = Worklist.pop_back_val();
47 Instruction *I = cast<Instruction>(U->getUser());
50 switch (I->getOpcode()) {
51 case Instruction::Call:
53 // Freeing a pointer does not cause it to be captured.
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() &&
61 I->getType() == Type::getVoidTy(V->getContext()))
64 // Not captured if only passed via 'nocapture' arguments. Note that
65 // calling a function pointer does not in itself cause the pointer to
66 // be captured. This is a subtle point considering that (for example)
67 // the callee might return its own address. It is analogous to saying
68 // that loading a value from a pointer does not cause the pointer to be
69 // captured, even though the loaded value might be the pointer itself
70 // (think of self-referential objects).
71 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
72 for (CallSite::arg_iterator A = B; A != E; ++A)
73 if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
74 // The parameter is not marked 'nocapture' - captured.
76 // Only passed via 'nocapture' arguments, or is the called function - not
80 case Instruction::Load:
81 // Loading from a pointer does not cause it to be captured.
83 case Instruction::Ret:
87 case Instruction::Store:
88 if (V == I->getOperand(0))
89 // Stored the pointer - it may be captured.
91 // Storing to the pointee does not cause the pointer to be captured.
93 case Instruction::BitCast:
94 case Instruction::GetElementPtr:
95 case Instruction::PHI:
96 case Instruction::Select:
97 // The original value is not captured via this if the new value isn't.
98 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
100 Use *U = &UI.getUse();
101 if (Visited.insert(U))
102 Worklist.push_back(U);
106 // Something else - be conservative and say it is captured.
111 // All uses examined - not captured.