1 //===------------- EscapeAnalysis.h - Pointer escape analysis -------------===//
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 provides the implementation of the pointer escape analysis.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "escape-analysis"
15 #include "llvm/Analysis/EscapeAnalysis.h"
16 #include "llvm/Module.h"
17 #include "llvm/Support/InstIterator.h"
18 #include "llvm/ADT/SmallPtrSet.h"
21 char EscapeAnalysis::ID = 0;
22 static RegisterPass<EscapeAnalysis> X("escape-analysis",
23 "Pointer Escape Analysis", true, true);
26 /// runOnFunction - Precomputation for escape analysis. This collects all know
27 /// "escape points" in the def-use graph of the function. These are
28 /// instructions which allow their inputs to escape from the current function.
29 bool EscapeAnalysis::runOnFunction(Function& F) {
32 TargetData& TD = getAnalysis<TargetData>();
33 AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
34 Module* M = F.getParent();
36 // Walk through all instructions in the function, identifying those that
37 // may allow their inputs to escape.
38 for(inst_iterator II = inst_begin(F), IE = inst_end(F); II != IE; ++II) {
39 Instruction* I = &*II;
41 // The most obvious case is stores. Any store that may write to global
42 // memory or to a function argument potentially allows its input to escape.
43 if (StoreInst* S = dyn_cast<StoreInst>(I)) {
44 const Type* StoreType = S->getOperand(0)->getType();
45 unsigned StoreSize = TD.getTypeStoreSize(StoreType);
46 Value* Pointer = S->getPointerOperand();
48 bool inserted = false;
49 for (Function::arg_iterator AI = F.arg_begin(), AE = F.arg_end();
51 if (!isa<PointerType>(AI->getType())) continue;
52 AliasAnalysis::AliasResult R = AA.alias(Pointer, StoreSize, AI, ~0UL);
53 if (R != AliasAnalysis::NoAlias) {
54 EscapePoints.insert(S);
63 for (Module::global_iterator GI = M->global_begin(), GE = M->global_end();
65 AliasAnalysis::AliasResult R = AA.alias(Pointer, StoreSize, GI, ~0UL);
66 if (R != AliasAnalysis::NoAlias) {
67 EscapePoints.insert(S);
72 // Calls and invokes potentially allow their parameters to escape.
73 // FIXME: This can and should be refined. Intrinsics have known escape
74 // behavior, and alias analysis may be able to tell us more about callees.
75 } else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
76 EscapePoints.insert(I);
78 // Returns allow the return value to escape. This is mostly important
79 // for malloc to alloca promotion.
80 } else if (isa<ReturnInst>(I)) {
81 EscapePoints.insert(I);
83 // Branching on the value of a pointer may allow the value to escape through
84 // methods not discoverable via def-use chaining.
85 } else if(isa<BranchInst>(I) || isa<SwitchInst>(I)) {
86 EscapePoints.insert(I);
89 // FIXME: Are there any other possible escape points?
95 /// escapes - Determines whether the passed allocation can escape from the
96 /// current function. It does this by using a simple worklist algorithm to
97 /// search for a path in the def-use graph from the allocation to an
99 /// FIXME: Once we've discovered a path, it would be a good idea to memoize it,
100 /// and all of its subpaths, to amortize the cost of future queries.
101 bool EscapeAnalysis::escapes(Value* A) {
102 assert(isa<PointerType>(A->getType()) &&
103 "Can't do escape analysis on non-pointer types!");
105 std::vector<Value*> worklist;
106 worklist.push_back(A);
108 SmallPtrSet<Value*, 8> visited;
110 while (!worklist.empty()) {
111 Value* curr = worklist.back();
114 if (Instruction* I = dyn_cast<Instruction>(curr))
115 if (EscapePoints.count(I))
118 if (StoreInst* S = dyn_cast<StoreInst>(curr)) {
119 // We know this must be an instruction, because constant gep's would
120 // have been found to alias a global, so stores to them would have
121 // been in EscapePoints.
122 if (visited.insert(cast<Instruction>(S->getPointerOperand())))
123 worklist.push_back(cast<Instruction>(S->getPointerOperand()));
125 for (Instruction::use_iterator UI = curr->use_begin(),
126 UE = curr->use_end(); UI != UE; ++UI)
127 if (Instruction* U = dyn_cast<Instruction>(UI))
128 if (visited.insert(U))
129 worklist.push_back(U);