[WebAssembly] Add a pass to create wrappers for function bitcasts.
WebAssembly requires caller and callee signatures to match exactly. In LLVM,
there are a variety of circumstances where signatures may be mismatched in
practice, and one can bitcast a function address to another type to call it
as that type. This patch adds a pass which replaces bitcasted function
addresses with wrappers to replace the bitcasts.
This doesn't catch everything, but it does match many common cases.
llvm-svn: 291315
diff --git a/llvm/lib/Target/WebAssembly/CMakeLists.txt b/llvm/lib/Target/WebAssembly/CMakeLists.txt
index f4d4638..d9c53ec 100644
--- a/llvm/lib/Target/WebAssembly/CMakeLists.txt
+++ b/llvm/lib/Target/WebAssembly/CMakeLists.txt
@@ -17,6 +17,7 @@
WebAssemblyExplicitLocals.cpp
WebAssemblyFastISel.cpp
WebAssemblyFixIrreducibleControlFlow.cpp
+ WebAssemblyFixFunctionBitcasts.cpp
WebAssemblyFrameLowering.cpp
WebAssemblyISelDAGToDAG.cpp
WebAssemblyISelLowering.cpp
diff --git a/llvm/lib/Target/WebAssembly/WebAssembly.h b/llvm/lib/Target/WebAssembly/WebAssembly.h
index 09c35b4..8738263 100644
--- a/llvm/lib/Target/WebAssembly/WebAssembly.h
+++ b/llvm/lib/Target/WebAssembly/WebAssembly.h
@@ -28,6 +28,7 @@
// LLVM IR passes.
ModulePass *createWebAssemblyLowerEmscriptenEHSjLj(bool DoEH, bool DoSjLj);
void initializeWebAssemblyLowerEmscriptenEHSjLjPass(PassRegistry &);
+ModulePass *createWebAssemblyFixFunctionBitcasts();
FunctionPass *createWebAssemblyOptimizeReturned();
// ISel and immediate followup passes.
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp
new file mode 100644
index 0000000..dfad5b8
--- /dev/null
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp
@@ -0,0 +1,148 @@
+//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief 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;
+
+ SmallVector<std::pair<Use *, Function *>, 0> Uses;
+
+public:
+ static char ID;
+ FixFunctionBitcasts() : ModulePass(ID) {}
+};
+} // End anonymous namespace
+
+char FixFunctionBitcasts::ID = 0;
+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<Use *, Function *>> &Uses) {
+ for (Use &U : V->uses()) {
+ if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser()))
+ FindUses(BC, F, Uses);
+ else if (U.get()->getType() != F.getType())
+ Uses.push_back(std::make_pair(&U, &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
+static Function *CreateWrapper(Function *F, FunctionType *Ty) {
+ Module *M = F->getParent();
+
+ Function *Wrapper =
+ Function::Create(Ty, Function::PrivateLinkage, "bitcast", M);
+ BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
+
+ // Determine what arguments to pass.
+ SmallVector<Value *, 4> Args;
+ Function::arg_iterator AI = Wrapper->arg_begin();
+ FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
+ FunctionType::param_iterator PE = F->getFunctionType()->param_end();
+ for (; AI != Wrapper->arg_end() && PI != PE; ++AI, ++PI) {
+ assert(AI->getType() == *PI &&
+ "mismatched argument types not supported yet");
+ Args.push_back(&*AI);
+ }
+ for (; PI != PE; ++PI)
+ Args.push_back(UndefValue::get(*PI));
+
+ CallInst *Call = CallInst::Create(F, Args, "", BB);
+
+ // Determine what value to return.
+ if (Ty->getReturnType()->isVoidTy())
+ ReturnInst::Create(M->getContext(), BB);
+ else if (F->getFunctionType()->getReturnType()->isVoidTy())
+ ReturnInst::Create(M->getContext(), UndefValue::get(Ty->getReturnType()),
+ BB);
+ else if (F->getFunctionType()->getReturnType() == Ty->getReturnType())
+ ReturnInst::Create(M->getContext(), Call, BB);
+ else
+ llvm_unreachable("mismatched return types not supported yet");
+
+ return Wrapper;
+}
+
+bool FixFunctionBitcasts::runOnModule(Module &M) {
+ // Collect all the places that need wrappers.
+ for (Function &F : M)
+ FindUses(&F, F, Uses);
+
+ DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
+
+ for (auto &UseFunc : Uses) {
+ Use *U = UseFunc.first;
+ Function *F = UseFunc.second;
+ PointerType *PTy = cast<PointerType>(U->get()->getType());
+ FunctionType *Ty = dyn_cast<FunctionType>(PTy->getElementType());
+
+ // If the function is casted to something like i8* as a "generic pointer"
+ // to be later casted to something else, we can't generate a wrapper for it.
+ // Just ignore such casts for now.
+ if (!Ty)
+ continue;
+
+ auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
+ if (Pair.second)
+ Pair.first->second = CreateWrapper(F, Ty);
+
+ if (isa<Constant>(U->get()))
+ U->get()->replaceAllUsesWith(Pair.first->second);
+ else
+ U->set(Pair.first->second);
+ }
+
+ return true;
+}
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
index b61bc0a..f5ef35a 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
@@ -163,6 +163,10 @@
// control specifically what gets lowered.
addPass(createAtomicExpandPass(TM));
+ // Fix function bitcasts, as WebAssembly requires caller and callee signatures
+ // to match.
+ addPass(createWebAssemblyFixFunctionBitcasts());
+
// Optimize "returned" function attributes.
if (getOptLevel() != CodeGenOpt::None)
addPass(createWebAssemblyOptimizeReturned());