1 //===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
10 /// Fix bitcasted functions.
12 /// WebAssembly requires caller and callee signatures to match, however in LLVM,
13 /// some amount of slop is vaguely permitted. Detect mismatch by looking for
14 /// bitcasts of functions and rewrite them to use wrapper functions instead.
16 /// This doesn't catch all cases, such as when a function's address is taken in
17 /// one place and casted in another, but it works for many common cases.
19 /// Note that LLVM already optimizes away function bitcasts in common cases by
20 /// dropping arguments as needed, so this pass only ends up getting used in less
23 //===----------------------------------------------------------------------===//
25 #include "WebAssembly.h"
26 #include "llvm/IR/Constants.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/IR/Operator.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/raw_ostream.h"
35 #define DEBUG_TYPE "wasm-fix-function-bitcasts"
38 class FixFunctionBitcasts final
: public ModulePass
{
39 StringRef
getPassName() const override
{
40 return "WebAssembly Fix Function Bitcasts";
43 void getAnalysisUsage(AnalysisUsage
&AU
) const override
{
45 ModulePass::getAnalysisUsage(AU
);
48 bool runOnModule(Module
&M
) override
;
52 FixFunctionBitcasts() : ModulePass(ID
) {}
54 } // End anonymous namespace
56 char FixFunctionBitcasts::ID
= 0;
57 INITIALIZE_PASS(FixFunctionBitcasts
, DEBUG_TYPE
,
58 "Fix mismatching bitcasts for WebAssembly", false, false)
60 ModulePass
*llvm::createWebAssemblyFixFunctionBitcasts() {
61 return new FixFunctionBitcasts();
64 // Recursively descend the def-use lists from V to find non-bitcast users of
66 static void findUses(Value
*V
, Function
&F
,
67 SmallVectorImpl
<std::pair
<CallBase
*, Function
*>> &Uses
) {
68 for (User
*U
: V
->users()) {
69 if (auto *BC
= dyn_cast
<BitCastOperator
>(U
))
70 findUses(BC
, F
, Uses
);
71 else if (auto *A
= dyn_cast
<GlobalAlias
>(U
))
73 else if (auto *CB
= dyn_cast
<CallBase
>(U
)) {
74 Value
*Callee
= CB
->getCalledOperand();
76 // Skip calls where the function isn't the callee
78 if (CB
->getFunctionType() == F
.getValueType())
79 // Skip uses that are immediately called
81 Uses
.push_back(std::make_pair(CB
, &F
));
86 // Create a wrapper function with type Ty that calls F (which may have a
87 // different type). Attempt to support common bitcasted function idioms:
88 // - Call with more arguments than needed: arguments are dropped
89 // - Call with fewer arguments than needed: arguments are filled in with poison
90 // - Return value is not needed: drop it
91 // - Return value needed but not present: supply a poison value
93 // If the all the argument types of trivially castable to one another (i.e.
94 // I32 vs pointer type) then we don't create a wrapper at all (return nullptr
97 // If there is a type mismatch that we know would result in an invalid wasm
98 // module then generate wrapper that contains unreachable (i.e. abort at
99 // runtime). Such programs are deep into undefined behaviour territory,
100 // but we choose to fail at runtime rather than generate and invalid module
101 // or fail at compiler time. The reason we delay the error is that we want
102 // to support the CMake which expects to be able to compile and link programs
103 // that refer to functions with entirely incorrect signatures (this is how
104 // CMake detects the existence of a function in a toolchain).
106 // For bitcasts that involve struct types we don't know at this stage if they
107 // would be equivalent at the wasm level and so we can't know if we need to
108 // generate a wrapper.
109 static Function
*createWrapper(Function
*F
, FunctionType
*Ty
) {
110 Module
*M
= F
->getParent();
112 Function
*Wrapper
= Function::Create(Ty
, Function::PrivateLinkage
,
113 F
->getName() + "_bitcast", M
);
114 Wrapper
->setAttributes(F
->getAttributes());
115 BasicBlock
*BB
= BasicBlock::Create(M
->getContext(), "body", Wrapper
);
116 const DataLayout
&DL
= BB
->getDataLayout();
118 // Determine what arguments to pass.
119 SmallVector
<Value
*, 4> Args
;
120 Function::arg_iterator AI
= Wrapper
->arg_begin();
121 Function::arg_iterator AE
= Wrapper
->arg_end();
122 FunctionType::param_iterator PI
= F
->getFunctionType()->param_begin();
123 FunctionType::param_iterator PE
= F
->getFunctionType()->param_end();
124 bool TypeMismatch
= false;
125 bool WrapperNeeded
= false;
127 Type
*ExpectedRtnType
= F
->getFunctionType()->getReturnType();
128 Type
*RtnType
= Ty
->getReturnType();
130 if ((F
->getFunctionType()->getNumParams() != Ty
->getNumParams()) ||
131 (F
->getFunctionType()->isVarArg() != Ty
->isVarArg()) ||
132 (ExpectedRtnType
!= RtnType
))
133 WrapperNeeded
= true;
135 for (; AI
!= AE
&& PI
!= PE
; ++AI
, ++PI
) {
136 Type
*ArgType
= AI
->getType();
137 Type
*ParamType
= *PI
;
139 if (ArgType
== ParamType
) {
140 Args
.push_back(&*AI
);
142 if (CastInst::isBitOrNoopPointerCastable(ArgType
, ParamType
, DL
)) {
143 Instruction
*PtrCast
=
144 CastInst::CreateBitOrPointerCast(AI
, ParamType
, "cast");
145 PtrCast
->insertInto(BB
, BB
->end());
146 Args
.push_back(PtrCast
);
147 } else if (ArgType
->isStructTy() || ParamType
->isStructTy()) {
148 LLVM_DEBUG(dbgs() << "createWrapper: struct param type in bitcast: "
149 << F
->getName() << "\n");
150 WrapperNeeded
= false;
152 LLVM_DEBUG(dbgs() << "createWrapper: arg type mismatch calling: "
153 << F
->getName() << "\n");
154 LLVM_DEBUG(dbgs() << "Arg[" << Args
.size() << "] Expected: "
155 << *ParamType
<< " Got: " << *ArgType
<< "\n");
162 if (WrapperNeeded
&& !TypeMismatch
) {
163 for (; PI
!= PE
; ++PI
)
164 Args
.push_back(PoisonValue::get(*PI
));
166 for (; AI
!= AE
; ++AI
)
167 Args
.push_back(&*AI
);
169 CallInst
*Call
= CallInst::Create(F
, Args
, "", BB
);
171 Type
*ExpectedRtnType
= F
->getFunctionType()->getReturnType();
172 Type
*RtnType
= Ty
->getReturnType();
173 // Determine what value to return.
174 if (RtnType
->isVoidTy()) {
175 ReturnInst::Create(M
->getContext(), BB
);
176 } else if (ExpectedRtnType
->isVoidTy()) {
177 LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType
<< "\n");
178 ReturnInst::Create(M
->getContext(), PoisonValue::get(RtnType
), BB
);
179 } else if (RtnType
== ExpectedRtnType
) {
180 ReturnInst::Create(M
->getContext(), Call
, BB
);
181 } else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType
, RtnType
,
184 CastInst::CreateBitOrPointerCast(Call
, RtnType
, "cast");
185 Cast
->insertInto(BB
, BB
->end());
186 ReturnInst::Create(M
->getContext(), Cast
, BB
);
187 } else if (RtnType
->isStructTy() || ExpectedRtnType
->isStructTy()) {
188 LLVM_DEBUG(dbgs() << "createWrapper: struct return type in bitcast: "
189 << F
->getName() << "\n");
190 WrapperNeeded
= false;
192 LLVM_DEBUG(dbgs() << "createWrapper: return type mismatch calling: "
193 << F
->getName() << "\n");
194 LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType
195 << " Got: " << *RtnType
<< "\n");
201 // Create a new wrapper that simply contains `unreachable`.
202 Wrapper
->eraseFromParent();
203 Wrapper
= Function::Create(Ty
, Function::PrivateLinkage
,
204 F
->getName() + "_bitcast_invalid", M
);
205 Wrapper
->setAttributes(F
->getAttributes());
206 BasicBlock
*BB
= BasicBlock::Create(M
->getContext(), "body", Wrapper
);
207 new UnreachableInst(M
->getContext(), BB
);
208 Wrapper
->setName(F
->getName() + "_bitcast_invalid");
209 } else if (!WrapperNeeded
) {
210 LLVM_DEBUG(dbgs() << "createWrapper: no wrapper needed: " << F
->getName()
212 Wrapper
->eraseFromParent();
215 LLVM_DEBUG(dbgs() << "createWrapper: " << F
->getName() << "\n");
219 // Test whether a main function with type FuncTy should be rewritten to have
221 static bool shouldFixMainFunction(FunctionType
*FuncTy
, FunctionType
*MainTy
) {
222 // Only fix the main function if it's the standard zero-arg form. That way,
223 // the standard cases will work as expected, and users will see signature
224 // mismatches from the linker for non-standard cases.
225 return FuncTy
->getReturnType() == MainTy
->getReturnType() &&
226 FuncTy
->getNumParams() == 0 &&
230 bool FixFunctionBitcasts::runOnModule(Module
&M
) {
231 LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n");
233 Function
*Main
= nullptr;
234 CallInst
*CallMain
= nullptr;
235 SmallVector
<std::pair
<CallBase
*, Function
*>, 0> Uses
;
237 // Collect all the places that need wrappers.
238 for (Function
&F
: M
) {
239 // Skip to fix when the function is swiftcc because swiftcc allows
240 // bitcast type difference for swiftself and swifterror.
241 if (F
.getCallingConv() == CallingConv::Swift
)
243 findUses(&F
, F
, Uses
);
245 // If we have a "main" function, and its type isn't
246 // "int main(int argc, char *argv[])", create an artificial call with it
247 // bitcasted to that type so that we generate a wrapper for it, so that
248 // the C runtime can call it.
249 if (F
.getName() == "main") {
251 LLVMContext
&C
= M
.getContext();
252 Type
*MainArgTys
[] = {Type::getInt32Ty(C
), PointerType::get(C
, 0)};
253 FunctionType
*MainTy
= FunctionType::get(Type::getInt32Ty(C
), MainArgTys
,
255 if (shouldFixMainFunction(F
.getFunctionType(), MainTy
)) {
256 LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
257 << *F
.getFunctionType() << "\n");
258 Value
*Args
[] = {PoisonValue::get(MainArgTys
[0]),
259 PoisonValue::get(MainArgTys
[1])};
260 CallMain
= CallInst::Create(MainTy
, Main
, Args
, "call_main");
261 Uses
.push_back(std::make_pair(CallMain
, &F
));
266 DenseMap
<std::pair
<Function
*, FunctionType
*>, Function
*> Wrappers
;
268 for (auto &UseFunc
: Uses
) {
269 CallBase
*CB
= UseFunc
.first
;
270 Function
*F
= UseFunc
.second
;
271 FunctionType
*Ty
= CB
->getFunctionType();
273 auto Pair
= Wrappers
.insert(std::make_pair(std::make_pair(F
, Ty
), nullptr));
275 Pair
.first
->second
= createWrapper(F
, Ty
);
277 Function
*Wrapper
= Pair
.first
->second
;
281 CB
->setCalledOperand(Wrapper
);
284 // If we created a wrapper for main, rename the wrapper so that it's the
285 // one that gets called from startup.
287 Main
->setName("__original_main");
289 cast
<Function
>(CallMain
->getCalledOperand()->stripPointerCasts());
291 if (Main
->isDeclaration()) {
292 // The wrapper is not needed in this case as we don't need to export
293 // it to anyone else.
294 MainWrapper
->eraseFromParent();
296 // Otherwise give the wrapper the same linkage as the original main
297 // function, so that it can be called from the same places.
298 MainWrapper
->setName("main");
299 MainWrapper
->setLinkage(Main
->getLinkage());
300 MainWrapper
->setVisibility(Main
->getVisibility());