[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

Author: Dan Gohman

llvm/lib/Target/WebAssembly/CMakeLists.txt

@@ -17,6 +17,7 @@ add_llvm_target(WebAssemblyCodeGen
   WebAssemblyExplicitLocals.cpp
   WebAssemblyFastISel.cpp
   WebAssemblyFixIrreducibleControlFlow.cpp
+  WebAssemblyFixFunctionBitcasts.cpp
   WebAssemblyFrameLowering.cpp
   WebAssemblyISelDAGToDAG.cpp
   WebAssemblyISelLowering.cpp

llvm/lib/Target/WebAssembly/WebAssembly.h

@@ -28,6 +28,7 @@ class FunctionPass;
 // LLVM IR passes.
 ModulePass *createWebAssemblyLowerEmscriptenEHSjLj(bool DoEH, bool DoSjLj);
 void initializeWebAssemblyLowerEmscriptenEHSjLjPass(PassRegistry &);
+ModulePass *createWebAssemblyFixFunctionBitcasts();
 FunctionPass *createWebAssemblyOptimizeReturned();
 
 // ISel and immediate followup passes.

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;
+}

llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp

@@ -163,6 +163,10 @@ void WebAssemblyPassConfig::addIRPasses() {
     // 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());

llvm/test/CodeGen/WebAssembly/function-bitcasts.ll

@@ -0,0 +1,56 @@
+; RUN: llc < %s -asm-verbose=false | FileCheck %s
+
+; Test that function pointer casts are replaced with wrappers.
+
+target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
+target triple = "wasm32-unknown-unknown"
+
+; CHECK-LABEL: test:
+; CHECK-NEXT: call        .Lbitcast@FUNCTION{{$}}
+; CHECK-NEXT: call        .Lbitcast.1@FUNCTION{{$}}
+; CHECK-NEXT: i32.const   $push[[L0:[0-9]*]]=, 0
+; CHECK-NEXT: call        .Lbitcast.2@FUNCTION, $pop[[L0]]{{$}}
+; CHECK-NEXT: i32.call    $drop=, .Lbitcast.3@FUNCTION{{$}}
+; CHECK-NEXT: call        foo2@FUNCTION{{$}}
+; CHECK-NEXT: call        foo3@FUNCTION{{$}}
+; CHECK-NEXT: .endfunc
+
+; CHECK-LABEL: .Lbitcast:
+; CHECK-NEXT: .local      i32
+; CHECK-NEXT: call        has_i32_arg@FUNCTION, $0{{$}}
+; CHECK-NEXT: .endfunc
+
+; CHECK-LABEL: .Lbitcast.1:
+; CHECK-NEXT: call        $drop=, has_i32_ret@FUNCTION{{$}}
+; CHECK-NEXT: .endfunc
+
+; CHECK-LABEL: .Lbitcast.2:
+; CHECK-NEXT: .param      i32
+; CHECK-NEXT: call        foo0@FUNCTION{{$}}
+; CHECK-NEXT: .endfunc
+
+; CHECK-LABEL: .Lbitcast.3:
+; CHECK-NEXT: .result     i32
+; CHECK-NEXT: .local      i32
+; CHECK-NEXT: call        foo1@FUNCTION{{$}}
+; CHECK-NEXT: copy_local  $push0=, $0
+; CHECK-NEXT: .endfunc
+
+declare void @has_i32_arg(i32)
+declare i32 @has_i32_ret()
+
+declare void @foo0()
+declare void @foo1()
+declare void @foo2()
+declare void @foo3()
+
+define void @test() {
+entry:
+  call void bitcast (void (i32)* @has_i32_arg to void ()*)()
+  call void bitcast (i32 ()* @has_i32_ret to void ()*)()
+  call void bitcast (void ()* @foo0 to void (i32)*)(i32 0)
+  %t = call i32 bitcast (void ()* @foo1 to i32 ()*)()
+  call void bitcast (void ()* @foo2 to void ()*)()
+  call void @foo3()
+  ret void
+}