1 //===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
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 //===----------------------------------------------------------------------===//
11 /// \brief Fix bitcasted functions.
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.
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.
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
24 //===----------------------------------------------------------------------===//
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"
36 #define DEBUG_TYPE "wasm-fix-function-bitcasts"
39 class FixFunctionBitcasts final : public ModulePass {
40 StringRef getPassName() const override {
41 return "WebAssembly Fix Function Bitcasts";
44 void getAnalysisUsage(AnalysisUsage &AU) const override {
46 ModulePass::getAnalysisUsage(AU);
49 bool runOnModule(Module &M) override;
53 FixFunctionBitcasts() : ModulePass(ID) {}
55 } // End anonymous namespace
57 char FixFunctionBitcasts::ID = 0;
58 ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
59 return new FixFunctionBitcasts();
62 // Recursively descend the def-use lists from V to find non-bitcast users of
64 static void FindUses(Value *V, Function &F,
65 SmallVectorImpl<std::pair<Use *, Function *>> &Uses) {
66 for (Use &U : V->uses()) {
67 if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser()))
68 FindUses(BC, F, Uses);
69 else if (U.get()->getType() != F.getType())
70 Uses.push_back(std::make_pair(&U, &F));
74 // Create a wrapper function with type Ty that calls F (which may have a
75 // different type). Attempt to support common bitcasted function idioms:
76 // - Call with more arguments than needed: arguments are dropped
77 // - Call with fewer arguments than needed: arguments are filled in with undef
78 // - Return value is not needed: drop it
79 // - Return value needed but not present: supply an undef
81 // For now, return nullptr without creating a wrapper if the wrapper cannot
82 // be generated due to incompatible types.
83 static Function *CreateWrapper(Function *F, FunctionType *Ty) {
84 Module *M = F->getParent();
87 Function::Create(Ty, Function::PrivateLinkage, "bitcast", M);
88 BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
90 // Determine what arguments to pass.
91 SmallVector<Value *, 4> Args;
92 Function::arg_iterator AI = Wrapper->arg_begin();
93 FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
94 FunctionType::param_iterator PE = F->getFunctionType()->param_end();
95 for (; AI != Wrapper->arg_end() && PI != PE; ++AI, ++PI) {
96 if (AI->getType() != *PI) {
97 Wrapper->eraseFromParent();
100 Args.push_back(&*AI);
102 for (; PI != PE; ++PI)
103 Args.push_back(UndefValue::get(*PI));
105 CallInst *Call = CallInst::Create(F, Args, "", BB);
107 // Determine what value to return.
108 if (Ty->getReturnType()->isVoidTy())
109 ReturnInst::Create(M->getContext(), BB);
110 else if (F->getFunctionType()->getReturnType()->isVoidTy())
111 ReturnInst::Create(M->getContext(), UndefValue::get(Ty->getReturnType()),
113 else if (F->getFunctionType()->getReturnType() == Ty->getReturnType())
114 ReturnInst::Create(M->getContext(), Call, BB);
116 Wrapper->eraseFromParent();
123 bool FixFunctionBitcasts::runOnModule(Module &M) {
124 SmallVector<std::pair<Use *, Function *>, 0> Uses;
126 // Collect all the places that need wrappers.
127 for (Function &F : M)
128 FindUses(&F, F, Uses);
130 DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
132 for (auto &UseFunc : Uses) {
133 Use *U = UseFunc.first;
134 Function *F = UseFunc.second;
135 PointerType *PTy = cast<PointerType>(U->get()->getType());
136 FunctionType *Ty = dyn_cast<FunctionType>(PTy->getElementType());
138 // If the function is casted to something like i8* as a "generic pointer"
139 // to be later casted to something else, we can't generate a wrapper for it.
140 // Just ignore such casts for now.
144 auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
146 Pair.first->second = CreateWrapper(F, Ty);
148 Function *Wrapper = Pair.first->second;
152 if (isa<Constant>(U->get()))
153 U->get()->replaceAllUsesWith(Wrapper);