WebAssemblyFixFunctionBitcasts.cpp revision 321369
1//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===// 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/// \file 11/// \brief Fix bitcasted functions. 12/// 13/// WebAssembly requires caller and callee signatures to match, however in LLVM, 14/// some amount of slop is vaguely permitted. Detect mismatch by looking for 15/// bitcasts of functions and rewrite them to use wrapper functions instead. 16/// 17/// This doesn't catch all cases, such as when a function's address is taken in 18/// one place and casted in another, but it works for many common cases. 19/// 20/// Note that LLVM already optimizes away function bitcasts in common cases by 21/// dropping arguments as needed, so this pass only ends up getting used in less 22/// common cases. 23/// 24//===----------------------------------------------------------------------===// 25 26#include "WebAssembly.h" 27#include "llvm/IR/Constants.h" 28#include "llvm/IR/Instructions.h" 29#include "llvm/IR/Module.h" 30#include "llvm/IR/Operator.h" 31#include "llvm/Pass.h" 32#include "llvm/Support/Debug.h" 33#include "llvm/Support/raw_ostream.h" 34using namespace llvm; 35 36#define DEBUG_TYPE "wasm-fix-function-bitcasts" 37 38namespace { 39class FixFunctionBitcasts final : public ModulePass { 40 StringRef getPassName() const override { 41 return "WebAssembly Fix Function Bitcasts"; 42 } 43 44 void getAnalysisUsage(AnalysisUsage &AU) const override { 45 AU.setPreservesCFG(); 46 ModulePass::getAnalysisUsage(AU); 47 } 48 49 bool runOnModule(Module &M) override; 50 51public: 52 static char ID; 53 FixFunctionBitcasts() : ModulePass(ID) {} 54}; 55} // End anonymous namespace 56 57char FixFunctionBitcasts::ID = 0; 58ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() { 59 return new FixFunctionBitcasts(); 60} 61 62// Recursively descend the def-use lists from V to find non-bitcast users of 63// bitcasts of V. 64static void FindUses(Value *V, Function &F, 65 SmallVectorImpl<std::pair<Use *, Function *>> &Uses, 66 SmallPtrSetImpl<Constant *> &ConstantBCs) { 67 for (Use &U : V->uses()) { 68 if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser())) 69 FindUses(BC, F, Uses, ConstantBCs); 70 else if (U.get()->getType() != F.getType()) { 71 if (isa<Constant>(U.get())) { 72 // Only add constant bitcasts to the list once; they get RAUW'd 73 auto c = ConstantBCs.insert(cast<Constant>(U.get())); 74 if (!c.second) continue; 75 } 76 Uses.push_back(std::make_pair(&U, &F)); 77 } 78 } 79} 80 81// Create a wrapper function with type Ty that calls F (which may have a 82// different type). Attempt to support common bitcasted function idioms: 83// - Call with more arguments than needed: arguments are dropped 84// - Call with fewer arguments than needed: arguments are filled in with undef 85// - Return value is not needed: drop it 86// - Return value needed but not present: supply an undef 87// 88// For now, return nullptr without creating a wrapper if the wrapper cannot 89// be generated due to incompatible types. 90static Function *CreateWrapper(Function *F, FunctionType *Ty) { 91 Module *M = F->getParent(); 92 93 Function *Wrapper = 94 Function::Create(Ty, Function::PrivateLinkage, "bitcast", M); 95 BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper); 96 97 // Determine what arguments to pass. 98 SmallVector<Value *, 4> Args; 99 Function::arg_iterator AI = Wrapper->arg_begin(); 100 FunctionType::param_iterator PI = F->getFunctionType()->param_begin(); 101 FunctionType::param_iterator PE = F->getFunctionType()->param_end(); 102 for (; AI != Wrapper->arg_end() && PI != PE; ++AI, ++PI) { 103 if (AI->getType() != *PI) { 104 Wrapper->eraseFromParent(); 105 return nullptr; 106 } 107 Args.push_back(&*AI); 108 } 109 for (; PI != PE; ++PI) 110 Args.push_back(UndefValue::get(*PI)); 111 112 CallInst *Call = CallInst::Create(F, Args, "", BB); 113 114 // Determine what value to return. 115 if (Ty->getReturnType()->isVoidTy()) 116 ReturnInst::Create(M->getContext(), BB); 117 else if (F->getFunctionType()->getReturnType()->isVoidTy()) 118 ReturnInst::Create(M->getContext(), UndefValue::get(Ty->getReturnType()), 119 BB); 120 else if (F->getFunctionType()->getReturnType() == Ty->getReturnType()) 121 ReturnInst::Create(M->getContext(), Call, BB); 122 else { 123 Wrapper->eraseFromParent(); 124 return nullptr; 125 } 126 127 return Wrapper; 128} 129 130bool FixFunctionBitcasts::runOnModule(Module &M) { 131 SmallVector<std::pair<Use *, Function *>, 0> Uses; 132 SmallPtrSet<Constant *, 2> ConstantBCs; 133 134 // Collect all the places that need wrappers. 135 for (Function &F : M) FindUses(&F, F, Uses, ConstantBCs); 136 137 DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers; 138 139 for (auto &UseFunc : Uses) { 140 Use *U = UseFunc.first; 141 Function *F = UseFunc.second; 142 PointerType *PTy = cast<PointerType>(U->get()->getType()); 143 FunctionType *Ty = dyn_cast<FunctionType>(PTy->getElementType()); 144 145 // If the function is casted to something like i8* as a "generic pointer" 146 // to be later casted to something else, we can't generate a wrapper for it. 147 // Just ignore such casts for now. 148 if (!Ty) 149 continue; 150 151 // Wasm varargs are not ABI-compatible with non-varargs. Just ignore 152 // such casts for now. 153 if (Ty->isVarArg() || F->isVarArg()) 154 continue; 155 156 auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr)); 157 if (Pair.second) 158 Pair.first->second = CreateWrapper(F, Ty); 159 160 Function *Wrapper = Pair.first->second; 161 if (!Wrapper) 162 continue; 163 164 if (isa<Constant>(U->get())) 165 U->get()->replaceAllUsesWith(Wrapper); 166 else 167 U->set(Wrapper); 168 } 169 170 return true; 171} 172