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"
34 using namespace llvm;
35 
36 #define DEBUG_TYPE "wasm-fix-function-bitcasts"
37 
38 namespace {
39 class 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 
51 public:
52   static char ID;
53   FixFunctionBitcasts() : ModulePass(ID) {}
54 };
55 } // End anonymous namespace
56 
57 char FixFunctionBitcasts::ID = 0;
58 ModulePass *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.
64 static 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.
83 static 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 
123 bool 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