[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

1 files changed, 148 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp
new file mode 100644
index 00000000000..dfad5b8facd
--- /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;
+}