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" 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" 26 using namespace llvm; 27 28 /// As its comment mentions, PointerMayBeCaptured can be expensive. 29 /// However, it's not easy for BasicAA to cache the result, because 30 /// it's an ImmutablePass. To work around this, bound queries at a 31 /// fixed number of uses. 32 /// 33 /// TODO: Write a new FunctionPass AliasAnalysis so that it can keep 34 /// a cache. Then we can move the code from BasicAliasAnalysis into 35 /// that path, and remove this threshold. 36 static int const Threshold = 20; 37 38 /// PointerMayBeCaptured - Return true if this pointer value may be captured 39 /// by the enclosing function (which is required to exist). This routine can 40 /// be expensive, so consider caching the results. The boolean ReturnCaptures 41 /// specifies whether returning the value (or part of it) from the function 42 /// counts as capturing it or not. The boolean StoreCaptures specified whether 43 /// storing the value (or part of it) into memory anywhere automatically 44 /// counts as capturing it or not. 45 bool llvm::PointerMayBeCaptured(const Value *V, 46 bool ReturnCaptures, bool StoreCaptures) { 47 assert(V->getType()->isPointerTy() && "Capture is for pointers only!"); 48 SmallVector<Use*, Threshold> Worklist; 49 SmallSet<Use*, Threshold> Visited; 50 int Count = 0; 51 52 for (Value::const_use_iterator UI = V->use_begin(), UE = V->use_end(); 53 UI != UE; ++UI) { 54 // If there are lots of uses, conservatively say that the value 55 // is captured to avoid taking too much compile time. 56 if (Count++ >= Threshold) 57 return true; 58 59 Use *U = &UI.getUse(); 60 Visited.insert(U); 61 Worklist.push_back(U); 62 } 63 64 while (!Worklist.empty()) { 65 Use *U = Worklist.pop_back_val(); 66 Instruction *I = cast<Instruction>(U->getUser()); 67 V = U->get(); 68 69 switch (I->getOpcode()) { 70 case Instruction::Call: 71 case Instruction::Invoke: { 72 CallSite CS = CallSite::get(I); 73 // Not captured if the callee is readonly, doesn't return a copy through 74 // its return value and doesn't unwind (a readonly function can leak bits 75 // by throwing an exception or not depending on the input value). 76 if (CS.onlyReadsMemory() && CS.doesNotThrow() && I->getType()->isVoidTy()) 77 break; 78 79 // Not captured if only passed via 'nocapture' arguments. Note that 80 // calling a function pointer does not in itself cause the pointer to 81 // be captured. This is a subtle point considering that (for example) 82 // the callee might return its own address. It is analogous to saying 83 // that loading a value from a pointer does not cause the pointer to be 84 // captured, even though the loaded value might be the pointer itself 85 // (think of self-referential objects). 86 CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end(); 87 for (CallSite::arg_iterator A = B; A != E; ++A) 88 if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture)) 89 // The parameter is not marked 'nocapture' - captured. 90 return true; 91 // Only passed via 'nocapture' arguments, or is the called function - not 92 // captured. 93 break; 94 } 95 case Instruction::Load: 96 // Loading from a pointer does not cause it to be captured. 97 break; 98 case Instruction::Ret: 99 if (ReturnCaptures) 100 return true; 101 break; 102 case Instruction::Store: 103 if (V == I->getOperand(0)) 104 // Stored the pointer - conservatively assume it may be captured. 105 // TODO: If StoreCaptures is not true, we could do Fancy analysis 106 // to determine whether this store is not actually an escape point. 107 // In that case, BasicAliasAnalysis should be updated as well to 108 // take advantage of this. 109 return true; 110 // Storing to the pointee does not cause the pointer to be captured. 111 break; 112 case Instruction::BitCast: 113 case Instruction::GetElementPtr: 114 case Instruction::PHI: 115 case Instruction::Select: 116 // The original value is not captured via this if the new value isn't. 117 for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end(); 118 UI != UE; ++UI) { 119 Use *U = &UI.getUse(); 120 if (Visited.insert(U)) 121 Worklist.push_back(U); 122 } 123 break; 124 case Instruction::ICmp: 125 // Don't count comparisons of a no-alias return value against null as 126 // captures. This allows us to ignore comparisons of malloc results 127 // with null, for example. 128 if (isNoAliasCall(V->stripPointerCasts())) 129 if (ConstantPointerNull *CPN = 130 dyn_cast<ConstantPointerNull>(I->getOperand(1))) 131 if (CPN->getType()->getAddressSpace() == 0) 132 break; 133 // Otherwise, be conservative. There are crazy ways to capture pointers 134 // using comparisons. 135 return true; 136 default: 137 // Something else - be conservative and say it is captured. 138 return true; 139 } 140 } 141 142 // All uses examined - not captured. 143 return false; 144 } 145