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 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;
76 Uses.push_back(std::make_pair(&U, &F));
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
88 // For now, return nullptr without creating a wrapper if the wrapper cannot
89 // be generated due to incompatible types.
90 static Function *CreateWrapper(Function *F, FunctionType *Ty) {
91 Module *M = F->getParent();
94 Function::Create(Ty, Function::PrivateLinkage, "bitcast", M);
95 BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
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();
107 Args.push_back(&*AI);
109 for (; PI != PE; ++PI)
110 Args.push_back(UndefValue::get(*PI));
112 CallInst *Call = CallInst::Create(F, Args, "", BB);
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()),
120 else if (F->getFunctionType()->getReturnType() == Ty->getReturnType())
121 ReturnInst::Create(M->getContext(), Call, BB);
123 Wrapper->eraseFromParent();
130 bool FixFunctionBitcasts::runOnModule(Module &M) {
131 SmallVector<std::pair<Use *, Function *>, 0> Uses;
132 SmallPtrSet<Constant *, 2> ConstantBCs;
134 // Collect all the places that need wrappers.
135 for (Function &F : M) FindUses(&F, F, Uses, ConstantBCs);
137 DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
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());
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.
151 // Wasm varargs are not ABI-compatible with non-varargs. Just ignore
152 // such casts for now.
153 if (Ty->isVarArg() || F->isVarArg())
156 auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
158 Pair.first->second = CreateWrapper(F, Ty);
160 Function *Wrapper = Pair.first->second;
164 if (isa<Constant>(U->get()))
165 U->get()->replaceAllUsesWith(Wrapper);