WebAssemblyFixFunctionBitcasts.cpp revision 311818 
1//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9///
10/// \file
11/// \brief Fix bitcasted functions.
12///
13/// WebAssembly requires caller and callee signatures to match, however in LLVM,
14/// some amount of slop is vaguely permitted. Detect mismatch by looking for
15/// bitcasts of functions and rewrite them to use wrapper functions instead.
16///
17/// This doesn't catch all cases, such as when a function's address is taken in
18/// one place and casted in another, but it works for many common cases.
19///
20/// Note that LLVM already optimizes away function bitcasts in common cases by
21/// dropping arguments as needed, so this pass only ends up getting used in less
22/// common cases.
23///
24//===----------------------------------------------------------------------===//
25
26#include "WebAssembly.h"
27#include "llvm/IR/Constants.h"
28#include "llvm/IR/Instructions.h"
29#include "llvm/IR/Module.h"
30#include "llvm/IR/Operator.h"
31#include "llvm/Pass.h"
32#include "llvm/Support/Debug.h"
33#include "llvm/Support/raw_ostream.h"
34using namespace llvm;
35
36#define DEBUG_TYPE "wasm-fix-function-bitcasts"
37
38namespace {
39class FixFunctionBitcasts final : public ModulePass {
40  StringRef getPassName() const override {
41    return "WebAssembly Fix Function Bitcasts";
42  }
43
44  void getAnalysisUsage(AnalysisUsage &AU) const override {
45    AU.setPreservesCFG();
46    ModulePass::getAnalysisUsage(AU);
47  }
48
49  bool runOnModule(Module &M) override;
50
51public:
52  static char ID;
53  FixFunctionBitcasts() : ModulePass(ID) {}
54};
55} // End anonymous namespace
56
57char FixFunctionBitcasts::ID = 0;
58ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
59  return new FixFunctionBitcasts();
60}
61
62// Recursively descend the def-use lists from V to find non-bitcast users of
63// bitcasts of V.
64static void FindUses(Value *V, Function &F,
65                     SmallVectorImpl<std::pair<Use *, Function *>> &Uses) {
66  for (Use &U : V->uses()) {
67    if (BitCastOperator *BC = dyn_cast<BitCastOperator>(U.getUser()))
68      FindUses(BC, F, Uses);
69    else if (U.get()->getType() != F.getType())
70      Uses.push_back(std::make_pair(&U, &F));
71  }
72}
73
74// Create a wrapper function with type Ty that calls F (which may have a
75// different type). Attempt to support common bitcasted function idioms:
76//  - Call with more arguments than needed: arguments are dropped
77//  - Call with fewer arguments than needed: arguments are filled in with undef
78//  - Return value is not needed: drop it
79//  - Return value needed but not present: supply an undef
80//
81// For now, return nullptr without creating a wrapper if the wrapper cannot
82// be generated due to incompatible types.
83static Function *CreateWrapper(Function *F, FunctionType *Ty) {
84  Module *M = F->getParent();
85
86  Function *Wrapper =
87      Function::Create(Ty, Function::PrivateLinkage, "bitcast", M);
88  BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
89
90  // Determine what arguments to pass.
91  SmallVector<Value *, 4> Args;
92  Function::arg_iterator AI = Wrapper->arg_begin();
93  FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
94  FunctionType::param_iterator PE = F->getFunctionType()->param_end();
95  for (; AI != Wrapper->arg_end() && PI != PE; ++AI, ++PI) {
96    if (AI->getType() != *PI) {
97      Wrapper->eraseFromParent();
98      return nullptr;
99    }
100    Args.push_back(&*AI);
101  }
102  for (; PI != PE; ++PI)
103    Args.push_back(UndefValue::get(*PI));
104
105  CallInst *Call = CallInst::Create(F, Args, "", BB);
106
107  // Determine what value to return.
108  if (Ty->getReturnType()->isVoidTy())
109    ReturnInst::Create(M->getContext(), BB);
110  else if (F->getFunctionType()->getReturnType()->isVoidTy())
111    ReturnInst::Create(M->getContext(), UndefValue::get(Ty->getReturnType()),
112                       BB);
113  else if (F->getFunctionType()->getReturnType() == Ty->getReturnType())
114    ReturnInst::Create(M->getContext(), Call, BB);
115  else {
116    Wrapper->eraseFromParent();
117    return nullptr;
118  }
119
120  return Wrapper;
121}
122
123bool FixFunctionBitcasts::runOnModule(Module &M) {
124  SmallVector<std::pair<Use *, Function *>, 0> Uses;
125
126  // Collect all the places that need wrappers.
127  for (Function &F : M)
128    FindUses(&F, F, Uses);
129
130  DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
131
132  for (auto &UseFunc : Uses) {
133    Use *U = UseFunc.first;
134    Function *F = UseFunc.second;
135    PointerType *PTy = cast<PointerType>(U->get()->getType());
136    FunctionType *Ty = dyn_cast<FunctionType>(PTy->getElementType());
137
138    // If the function is casted to something like i8* as a "generic pointer"
139    // to be later casted to something else, we can't generate a wrapper for it.
140    // Just ignore such casts for now.
141    if (!Ty)
142      continue;
143
144    auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
145    if (Pair.second)
146      Pair.first->second = CreateWrapper(F, Ty);
147
148    Function *Wrapper = Pair.first->second;
149    if (!Wrapper)
150      continue;
151
152    if (isa<Constant>(U->get()))
153      U->get()->replaceAllUsesWith(Wrapper);
154    else
155      U->set(Wrapper);
156  }
157
158  return true;
159}
160