llvm/llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp

//===-- 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"
usingnamespacellvm;

#define DEBUG_TYPE …

namespace {
class FixFunctionBitcasts final : public ModulePass { … };
} // End anonymous namespace

char FixFunctionBitcasts::ID = …;
INITIALIZE_PASS(…)

ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() { … }

// 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) { … }

// 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) { … }

// Test whether a main function with type FuncTy should be rewritten to have
// type MainTy.
static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy) { … }

bool FixFunctionBitcasts::runOnModule(Module &M) { … }