| //===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===// |
| // |
| // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| // See https://llvm.org/LICENSE.txt for license information. |
| // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| // |
| //===----------------------------------------------------------------------===// |
| /// |
| /// \file |
| /// Fix bitcasted functions. |
| /// |
| /// WebAssembly requires caller and callee signatures to match, however in LLVM, |
| /// some amount of slop is vaguely permitted. Detect mismatch by looking for |
| /// bitcasts of functions and rewrite them to use wrapper functions instead. |
| /// |
| /// This doesn't catch all cases, such as when a function's address is taken in |
| /// one place and casted in another, but it works for many common cases. |
| /// |
| /// Note that LLVM already optimizes away function bitcasts in common cases by |
| /// dropping arguments as needed, so this pass only ends up getting used in less |
| /// common cases. |
| /// |
| //===----------------------------------------------------------------------===// |
| |
| #include "WebAssembly.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/Module.h" |
| #include "llvm/IR/Operator.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| using namespace llvm; |
| |
| #define DEBUG_TYPE "wasm-fix-function-bitcasts" |
| |
| namespace { |
| class FixFunctionBitcasts final : public ModulePass { |
| StringRef getPassName() const override { |
| return "WebAssembly Fix Function Bitcasts"; |
| } |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.setPreservesCFG(); |
| ModulePass::getAnalysisUsage(AU); |
| } |
| |
| bool runOnModule(Module &M) override; |
| |
| public: |
| static char ID; |
| FixFunctionBitcasts() : ModulePass(ID) {} |
| }; |
| } // End anonymous namespace |
| |
| char FixFunctionBitcasts::ID = 0; |
| INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE, |
| "Fix mismatching bitcasts for WebAssembly", false, false) |
| |
| ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() { |
| return new FixFunctionBitcasts(); |
| } |
| |
| // Recursively descend the def-use lists from V to find non-bitcast users of |
| // bitcasts of V. |
| static void findUses(Value *V, Function &F, |
| SmallVectorImpl<std::pair<CallBase *, Function *>> &Uses) { |
| for (User *U : V->users()) { |
| if (auto *BC = dyn_cast<BitCastOperator>(U)) |
| findUses(BC, F, Uses); |
| else if (auto *A = dyn_cast<GlobalAlias>(U)) |
| findUses(A, F, Uses); |
| else if (auto *CB = dyn_cast<CallBase>(U)) { |
| Value *Callee = CB->getCalledOperand(); |
| if (Callee != V) |
| // Skip calls where the function isn't the callee |
| continue; |
| if (CB->getFunctionType() == F.getValueType()) |
| // Skip uses that are immediately called |
| continue; |
| Uses.push_back(std::make_pair(CB, &F)); |
| } |
| } |
| } |
| |
| // Create a wrapper function with type Ty that calls F (which may have a |
| // different type). Attempt to support common bitcasted function idioms: |
| // - Call with more arguments than needed: arguments are dropped |
| // - Call with fewer arguments than needed: arguments are filled in with undef |
| // - Return value is not needed: drop it |
| // - Return value needed but not present: supply an undef |
| // |
| // If the all the argument types of trivially castable to one another (i.e. |
| // I32 vs pointer type) then we don't create a wrapper at all (return nullptr |
| // instead). |
| // |
| // If there is a type mismatch that we know would result in an invalid wasm |
| // module then generate wrapper that contains unreachable (i.e. abort at |
| // runtime). Such programs are deep into undefined behaviour territory, |
| // but we choose to fail at runtime rather than generate and invalid module |
| // or fail at compiler time. The reason we delay the error is that we want |
| // to support the CMake which expects to be able to compile and link programs |
| // that refer to functions with entirely incorrect signatures (this is how |
| // CMake detects the existence of a function in a toolchain). |
| // |
| // For bitcasts that involve struct types we don't know at this stage if they |
| // would be equivalent at the wasm level and so we can't know if we need to |
| // generate a wrapper. |
| static Function *createWrapper(Function *F, FunctionType *Ty) { |
| Module *M = F->getParent(); |
| |
| Function *Wrapper = Function::Create(Ty, Function::PrivateLinkage, |
| F->getName() + "_bitcast", M); |
| BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper); |
| const DataLayout &DL = BB->getModule()->getDataLayout(); |
| |
| // Determine what arguments to pass. |
| SmallVector<Value *, 4> Args; |
| Function::arg_iterator AI = Wrapper->arg_begin(); |
| Function::arg_iterator AE = Wrapper->arg_end(); |
| FunctionType::param_iterator PI = F->getFunctionType()->param_begin(); |
| FunctionType::param_iterator PE = F->getFunctionType()->param_end(); |
| bool TypeMismatch = false; |
| bool WrapperNeeded = false; |
| |
| Type *ExpectedRtnType = F->getFunctionType()->getReturnType(); |
| Type *RtnType = Ty->getReturnType(); |
| |
| if ((F->getFunctionType()->getNumParams() != Ty->getNumParams()) || |
| (F->getFunctionType()->isVarArg() != Ty->isVarArg()) || |
| (ExpectedRtnType != RtnType)) |
| WrapperNeeded = true; |
| |
| for (; AI != AE && PI != PE; ++AI, ++PI) { |
| Type *ArgType = AI->getType(); |
| Type *ParamType = *PI; |
| |
| if (ArgType == ParamType) { |
| Args.push_back(&*AI); |
| } else { |
| if (CastInst::isBitOrNoopPointerCastable(ArgType, ParamType, DL)) { |
| Instruction *PtrCast = |
| CastInst::CreateBitOrPointerCast(AI, ParamType, "cast"); |
| BB->getInstList().push_back(PtrCast); |
| Args.push_back(PtrCast); |
| } else if (ArgType->isStructTy() || ParamType->isStructTy()) { |
| LLVM_DEBUG(dbgs() << "createWrapper: struct param type in bitcast: " |
| << F->getName() << "\n"); |
| WrapperNeeded = false; |
| } else { |
| LLVM_DEBUG(dbgs() << "createWrapper: arg type mismatch calling: " |
| << F->getName() << "\n"); |
| LLVM_DEBUG(dbgs() << "Arg[" << Args.size() << "] Expected: " |
| << *ParamType << " Got: " << *ArgType << "\n"); |
| TypeMismatch = true; |
| break; |
| } |
| } |
| } |
| |
| if (WrapperNeeded && !TypeMismatch) { |
| for (; PI != PE; ++PI) |
| Args.push_back(UndefValue::get(*PI)); |
| if (F->isVarArg()) |
| for (; AI != AE; ++AI) |
| Args.push_back(&*AI); |
| |
| CallInst *Call = CallInst::Create(F, Args, "", BB); |
| |
| Type *ExpectedRtnType = F->getFunctionType()->getReturnType(); |
| Type *RtnType = Ty->getReturnType(); |
| // Determine what value to return. |
| if (RtnType->isVoidTy()) { |
| ReturnInst::Create(M->getContext(), BB); |
| } else if (ExpectedRtnType->isVoidTy()) { |
| LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType << "\n"); |
| ReturnInst::Create(M->getContext(), UndefValue::get(RtnType), BB); |
| } else if (RtnType == ExpectedRtnType) { |
| ReturnInst::Create(M->getContext(), Call, BB); |
| } else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType, RtnType, |
| DL)) { |
| Instruction *Cast = |
| CastInst::CreateBitOrPointerCast(Call, RtnType, "cast"); |
| BB->getInstList().push_back(Cast); |
| ReturnInst::Create(M->getContext(), Cast, BB); |
| } else if (RtnType->isStructTy() || ExpectedRtnType->isStructTy()) { |
| LLVM_DEBUG(dbgs() << "createWrapper: struct return type in bitcast: " |
| << F->getName() << "\n"); |
| WrapperNeeded = false; |
| } else { |
| LLVM_DEBUG(dbgs() << "createWrapper: return type mismatch calling: " |
| << F->getName() << "\n"); |
| LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType |
| << " Got: " << *RtnType << "\n"); |
| TypeMismatch = true; |
| } |
| } |
| |
| if (TypeMismatch) { |
| // Create a new wrapper that simply contains `unreachable`. |
| Wrapper->eraseFromParent(); |
| Wrapper = Function::Create(Ty, Function::PrivateLinkage, |
| F->getName() + "_bitcast_invalid", M); |
| BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper); |
| new UnreachableInst(M->getContext(), BB); |
| Wrapper->setName(F->getName() + "_bitcast_invalid"); |
| } else if (!WrapperNeeded) { |
| LLVM_DEBUG(dbgs() << "createWrapper: no wrapper needed: " << F->getName() |
| << "\n"); |
| Wrapper->eraseFromParent(); |
| return nullptr; |
| } |
| LLVM_DEBUG(dbgs() << "createWrapper: " << F->getName() << "\n"); |
| return Wrapper; |
| } |
| |
| // Test whether a main function with type FuncTy should be rewritten to have |
| // type MainTy. |
| static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy) { |
| // Only fix the main function if it's the standard zero-arg form. That way, |
| // the standard cases will work as expected, and users will see signature |
| // mismatches from the linker for non-standard cases. |
| return FuncTy->getReturnType() == MainTy->getReturnType() && |
| FuncTy->getNumParams() == 0 && |
| !FuncTy->isVarArg(); |
| } |
| |
| bool FixFunctionBitcasts::runOnModule(Module &M) { |
| LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n"); |
| |
| Function *Main = nullptr; |
| CallInst *CallMain = nullptr; |
| SmallVector<std::pair<CallBase *, Function *>, 0> Uses; |
| |
| // Collect all the places that need wrappers. |
| for (Function &F : M) { |
| // Skip to fix when the function is swiftcc because swiftcc allows |
| // bitcast type difference for swiftself and swifterror. |
| if (F.getCallingConv() == CallingConv::Swift) |
| continue; |
| findUses(&F, F, Uses); |
| |
| // If we have a "main" function, and its type isn't |
| // "int main(int argc, char *argv[])", create an artificial call with it |
| // bitcasted to that type so that we generate a wrapper for it, so that |
| // the C runtime can call it. |
| if (F.getName() == "main") { |
| Main = &F; |
| LLVMContext &C = M.getContext(); |
| Type *MainArgTys[] = {Type::getInt32Ty(C), |
| PointerType::get(Type::getInt8PtrTy(C), 0)}; |
| FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys, |
| /*isVarArg=*/false); |
| if (shouldFixMainFunction(F.getFunctionType(), MainTy)) { |
| LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: " |
| << *F.getFunctionType() << "\n"); |
| Value *Args[] = {UndefValue::get(MainArgTys[0]), |
| UndefValue::get(MainArgTys[1])}; |
| Value *Casted = |
| ConstantExpr::getBitCast(Main, PointerType::get(MainTy, 0)); |
| CallMain = CallInst::Create(MainTy, Casted, Args, "call_main"); |
| Uses.push_back(std::make_pair(CallMain, &F)); |
| } |
| } |
| } |
| |
| DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers; |
| |
| for (auto &UseFunc : Uses) { |
| CallBase *CB = UseFunc.first; |
| Function *F = UseFunc.second; |
| FunctionType *Ty = CB->getFunctionType(); |
| |
| auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr)); |
| if (Pair.second) |
| Pair.first->second = createWrapper(F, Ty); |
| |
| Function *Wrapper = Pair.first->second; |
| if (!Wrapper) |
| continue; |
| |
| CB->setCalledOperand(Wrapper); |
| } |
| |
| // If we created a wrapper for main, rename the wrapper so that it's the |
| // one that gets called from startup. |
| if (CallMain) { |
| Main->setName("__original_main"); |
| auto *MainWrapper = |
| cast<Function>(CallMain->getCalledOperand()->stripPointerCasts()); |
| delete CallMain; |
| if (Main->isDeclaration()) { |
| // The wrapper is not needed in this case as we don't need to export |
| // it to anyone else. |
| MainWrapper->eraseFromParent(); |
| } else { |
| // Otherwise give the wrapper the same linkage as the original main |
| // function, so that it can be called from the same places. |
| MainWrapper->setName("main"); |
| MainWrapper->setLinkage(Main->getLinkage()); |
| MainWrapper->setVisibility(Main->getVisibility()); |
| } |
| } |
| |
| return true; |
| } |