LLVM 18.0.0git
WebAssemblyFixFunctionBitcasts.cpp
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1//===-- WebAssemblyFixFunctionBitcasts.cpp - Fix function bitcasts --------===//
2//
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
6//
7//===----------------------------------------------------------------------===//
8///
9/// \file
10/// Fix bitcasted functions.
11///
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.
15///
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.
18///
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
21/// common cases.
22///
23//===----------------------------------------------------------------------===//
24
25#include "WebAssembly.h"
26#include "llvm/IR/Constants.h"
28#include "llvm/IR/Module.h"
29#include "llvm/IR/Operator.h"
30#include "llvm/Pass.h"
31#include "llvm/Support/Debug.h"
33using namespace llvm;
34
35#define DEBUG_TYPE "wasm-fix-function-bitcasts"
36
37namespace {
38class FixFunctionBitcasts final : public ModulePass {
39 StringRef getPassName() const override {
40 return "WebAssembly Fix Function Bitcasts";
41 }
42
43 void getAnalysisUsage(AnalysisUsage &AU) const override {
44 AU.setPreservesCFG();
46 }
47
48 bool runOnModule(Module &M) override;
49
50public:
51 static char ID;
52 FixFunctionBitcasts() : ModulePass(ID) {}
53};
54} // End anonymous namespace
55
56char FixFunctionBitcasts::ID = 0;
57INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE,
58 "Fix mismatching bitcasts for WebAssembly", false, false)
59
61 return new FixFunctionBitcasts();
62}
63
64// Recursively descend the def-use lists from V to find non-bitcast users of
65// bitcasts of V.
66static 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))
72 findUses(A, F, Uses);
73 else if (auto *CB = dyn_cast<CallBase>(U)) {
74 Value *Callee = CB->getCalledOperand();
75 if (Callee != V)
76 // Skip calls where the function isn't the callee
77 continue;
78 if (CB->getFunctionType() == F.getValueType())
79 // Skip uses that are immediately called
80 continue;
81 Uses.push_back(std::make_pair(CB, &F));
82 }
83 }
84}
85
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 undef
90// - Return value is not needed: drop it
91// - Return value needed but not present: supply an undef
92//
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
95// instead).
96//
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).
105//
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.
110 Module *M = F->getParent();
111
112 Function *Wrapper = Function::Create(Ty, Function::PrivateLinkage,
113 F->getName() + "_bitcast", M);
114 BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
115 const DataLayout &DL = BB->getModule()->getDataLayout();
116
117 // Determine what arguments to pass.
119 Function::arg_iterator AI = Wrapper->arg_begin();
120 Function::arg_iterator AE = Wrapper->arg_end();
121 FunctionType::param_iterator PI = F->getFunctionType()->param_begin();
122 FunctionType::param_iterator PE = F->getFunctionType()->param_end();
123 bool TypeMismatch = false;
124 bool WrapperNeeded = false;
125
126 Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
127 Type *RtnType = Ty->getReturnType();
128
129 if ((F->getFunctionType()->getNumParams() != Ty->getNumParams()) ||
130 (F->getFunctionType()->isVarArg() != Ty->isVarArg()) ||
131 (ExpectedRtnType != RtnType))
132 WrapperNeeded = true;
133
134 for (; AI != AE && PI != PE; ++AI, ++PI) {
135 Type *ArgType = AI->getType();
136 Type *ParamType = *PI;
137
138 if (ArgType == ParamType) {
139 Args.push_back(&*AI);
140 } else {
141 if (CastInst::isBitOrNoopPointerCastable(ArgType, ParamType, DL)) {
142 Instruction *PtrCast =
143 CastInst::CreateBitOrPointerCast(AI, ParamType, "cast");
144 PtrCast->insertInto(BB, BB->end());
145 Args.push_back(PtrCast);
146 } else if (ArgType->isStructTy() || ParamType->isStructTy()) {
147 LLVM_DEBUG(dbgs() << "createWrapper: struct param type in bitcast: "
148 << F->getName() << "\n");
149 WrapperNeeded = false;
150 } else {
151 LLVM_DEBUG(dbgs() << "createWrapper: arg type mismatch calling: "
152 << F->getName() << "\n");
153 LLVM_DEBUG(dbgs() << "Arg[" << Args.size() << "] Expected: "
154 << *ParamType << " Got: " << *ArgType << "\n");
155 TypeMismatch = true;
156 break;
157 }
158 }
159 }
160
161 if (WrapperNeeded && !TypeMismatch) {
162 for (; PI != PE; ++PI)
163 Args.push_back(UndefValue::get(*PI));
164 if (F->isVarArg())
165 for (; AI != AE; ++AI)
166 Args.push_back(&*AI);
167
168 CallInst *Call = CallInst::Create(F, Args, "", BB);
169
170 Type *ExpectedRtnType = F->getFunctionType()->getReturnType();
171 Type *RtnType = Ty->getReturnType();
172 // Determine what value to return.
173 if (RtnType->isVoidTy()) {
174 ReturnInst::Create(M->getContext(), BB);
175 } else if (ExpectedRtnType->isVoidTy()) {
176 LLVM_DEBUG(dbgs() << "Creating dummy return: " << *RtnType << "\n");
177 ReturnInst::Create(M->getContext(), UndefValue::get(RtnType), BB);
178 } else if (RtnType == ExpectedRtnType) {
179 ReturnInst::Create(M->getContext(), Call, BB);
180 } else if (CastInst::isBitOrNoopPointerCastable(ExpectedRtnType, RtnType,
181 DL)) {
182 Instruction *Cast =
183 CastInst::CreateBitOrPointerCast(Call, RtnType, "cast");
184 Cast->insertInto(BB, BB->end());
185 ReturnInst::Create(M->getContext(), Cast, BB);
186 } else if (RtnType->isStructTy() || ExpectedRtnType->isStructTy()) {
187 LLVM_DEBUG(dbgs() << "createWrapper: struct return type in bitcast: "
188 << F->getName() << "\n");
189 WrapperNeeded = false;
190 } else {
191 LLVM_DEBUG(dbgs() << "createWrapper: return type mismatch calling: "
192 << F->getName() << "\n");
193 LLVM_DEBUG(dbgs() << "Expected: " << *ExpectedRtnType
194 << " Got: " << *RtnType << "\n");
195 TypeMismatch = true;
196 }
197 }
198
199 if (TypeMismatch) {
200 // Create a new wrapper that simply contains `unreachable`.
201 Wrapper->eraseFromParent();
202 Wrapper = Function::Create(Ty, Function::PrivateLinkage,
203 F->getName() + "_bitcast_invalid", M);
204 BasicBlock *BB = BasicBlock::Create(M->getContext(), "body", Wrapper);
205 new UnreachableInst(M->getContext(), BB);
206 Wrapper->setName(F->getName() + "_bitcast_invalid");
207 } else if (!WrapperNeeded) {
208 LLVM_DEBUG(dbgs() << "createWrapper: no wrapper needed: " << F->getName()
209 << "\n");
210 Wrapper->eraseFromParent();
211 return nullptr;
212 }
213 LLVM_DEBUG(dbgs() << "createWrapper: " << F->getName() << "\n");
214 return Wrapper;
215}
216
217// Test whether a main function with type FuncTy should be rewritten to have
218// type MainTy.
219static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy) {
220 // Only fix the main function if it's the standard zero-arg form. That way,
221 // the standard cases will work as expected, and users will see signature
222 // mismatches from the linker for non-standard cases.
223 return FuncTy->getReturnType() == MainTy->getReturnType() &&
224 FuncTy->getNumParams() == 0 &&
225 !FuncTy->isVarArg();
226}
227
228bool FixFunctionBitcasts::runOnModule(Module &M) {
229 LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n");
230
231 Function *Main = nullptr;
232 CallInst *CallMain = nullptr;
234
235 // Collect all the places that need wrappers.
236 for (Function &F : M) {
237 // Skip to fix when the function is swiftcc because swiftcc allows
238 // bitcast type difference for swiftself and swifterror.
239 if (F.getCallingConv() == CallingConv::Swift)
240 continue;
241 findUses(&F, F, Uses);
242
243 // If we have a "main" function, and its type isn't
244 // "int main(int argc, char *argv[])", create an artificial call with it
245 // bitcasted to that type so that we generate a wrapper for it, so that
246 // the C runtime can call it.
247 if (F.getName() == "main") {
248 Main = &F;
249 LLVMContext &C = M.getContext();
250 Type *MainArgTys[] = {Type::getInt32Ty(C), PointerType::get(C, 0)};
251 FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
252 /*isVarArg=*/false);
253 if (shouldFixMainFunction(F.getFunctionType(), MainTy)) {
254 LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
255 << *F.getFunctionType() << "\n");
256 Value *Args[] = {UndefValue::get(MainArgTys[0]),
257 UndefValue::get(MainArgTys[1])};
258 CallMain = CallInst::Create(MainTy, Main, Args, "call_main");
259 Uses.push_back(std::make_pair(CallMain, &F));
260 }
261 }
262 }
263
265
266 for (auto &UseFunc : Uses) {
267 CallBase *CB = UseFunc.first;
268 Function *F = UseFunc.second;
269 FunctionType *Ty = CB->getFunctionType();
270
271 auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
272 if (Pair.second)
273 Pair.first->second = createWrapper(F, Ty);
274
275 Function *Wrapper = Pair.first->second;
276 if (!Wrapper)
277 continue;
278
280 }
281
282 // If we created a wrapper for main, rename the wrapper so that it's the
283 // one that gets called from startup.
284 if (CallMain) {
285 Main->setName("__original_main");
286 auto *MainWrapper =
287 cast<Function>(CallMain->getCalledOperand()->stripPointerCasts());
288 delete CallMain;
289 if (Main->isDeclaration()) {
290 // The wrapper is not needed in this case as we don't need to export
291 // it to anyone else.
292 MainWrapper->eraseFromParent();
293 } else {
294 // Otherwise give the wrapper the same linkage as the original main
295 // function, so that it can be called from the same places.
296 MainWrapper->setName("main");
297 MainWrapper->setLinkage(Main->getLinkage());
298 MainWrapper->setVisibility(Main->getVisibility());
299 }
300 }
301
302 return true;
303}
MachineBasicBlock MachineBasicBlock::iterator DebugLoc DL
amdgpu aa AMDGPU Address space based Alias Analysis Wrapper
static GCRegistry::Add< ErlangGC > A("erlang", "erlang-compatible garbage collector")
This file contains the declarations for the subclasses of Constant, which represent the different fla...
#define LLVM_DEBUG(X)
Definition: Debug.h:101
Rewrite Partial Register Uses
#define F(x, y, z)
Definition: MD5.cpp:55
Module.h This file contains the declarations for the Module class.
#define INITIALIZE_PASS(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:38
static void findUses(Value *V, Function &F, SmallVectorImpl< std::pair< CallBase *, Function * > > &Uses)
static bool shouldFixMainFunction(FunctionType *FuncTy, FunctionType *MainTy)
static Function * createWrapper(Function *F, FunctionType *Ty)
This file contains the entry points for global functions defined in the LLVM WebAssembly back-end.
Represent the analysis usage information of a pass.
void setPreservesCFG()
This function should be called by the pass, iff they do not:
Definition: Pass.cpp:269
This class represents an incoming formal argument to a Function.
Definition: Argument.h:28
LLVM Basic Block Representation.
Definition: BasicBlock.h:60
iterator end()
Definition: BasicBlock.h:450
static BasicBlock * Create(LLVMContext &Context, const Twine &Name="", Function *Parent=nullptr, BasicBlock *InsertBefore=nullptr)
Creates a new BasicBlock.
Definition: BasicBlock.h:206
const Module * getModule() const
Return the module owning the function this basic block belongs to, or nullptr if the function does no...
Definition: BasicBlock.cpp:328
Base class for all callable instructions (InvokeInst and CallInst) Holds everything related to callin...
Definition: InstrTypes.h:1227
Value * getCalledOperand() const
Definition: InstrTypes.h:1442
FunctionType * getFunctionType() const
Definition: InstrTypes.h:1307
void setCalledOperand(Value *V)
Definition: InstrTypes.h:1485
This class represents a function call, abstracting a target machine's calling convention.
static CallInst * Create(FunctionType *Ty, Value *F, const Twine &NameStr="", Instruction *InsertBefore=nullptr)
static CastInst * CreateBitOrPointerCast(Value *S, Type *Ty, const Twine &Name="", Instruction *InsertBefore=nullptr)
Create a BitCast, a PtrToInt, or an IntToPTr cast instruction.
static bool isBitOrNoopPointerCastable(Type *SrcTy, Type *DestTy, const DataLayout &DL)
Check whether a bitcast, inttoptr, or ptrtoint cast between these types is valid and a no-op.
A parsed version of the target data layout string in and methods for querying it.
Definition: DataLayout.h:110
std::pair< iterator, bool > insert(const std::pair< KeyT, ValueT > &KV)
Definition: DenseMap.h:220
Type::subtype_iterator param_iterator
Definition: DerivedTypes.h:126
static Function * Create(FunctionType *Ty, LinkageTypes Linkage, unsigned AddrSpace, const Twine &N="", Module *M=nullptr)
Definition: Function.h:162
VisibilityTypes getVisibility() const
Definition: GlobalValue.h:244
bool isDeclaration() const
Return true if the primary definition of this global value is outside of the current translation unit...
Definition: Globals.cpp:273
LinkageTypes getLinkage() const
Definition: GlobalValue.h:541
InstListType::iterator eraseFromParent()
This method unlinks 'this' from the containing basic block and deletes it.
Definition: Instruction.cpp:93
InstListType::iterator insertInto(BasicBlock *ParentBB, InstListType::iterator It)
Inserts an unlinked instruction into ParentBB at position It and returns the iterator of the inserted...
This is an important class for using LLVM in a threaded context.
Definition: LLVMContext.h:67
ModulePass class - This class is used to implement unstructured interprocedural optimizations and ana...
Definition: Pass.h:251
virtual bool runOnModule(Module &M)=0
runOnModule - Virtual method overriden by subclasses to process the module being operated on.
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
const DataLayout & getDataLayout() const
Get the data layout for the module's target platform.
Definition: Module.h:275
virtual void getAnalysisUsage(AnalysisUsage &) const
getAnalysisUsage - This function should be overriden by passes that need analysis information to do t...
Definition: Pass.cpp:98
virtual StringRef getPassName() const
getPassName - Return a nice clean name for a pass.
Definition: Pass.cpp:81
static ReturnInst * Create(LLVMContext &C, Value *retVal=nullptr, Instruction *InsertBefore=nullptr)
This class consists of common code factored out of the SmallVector class to reduce code duplication b...
Definition: SmallVector.h:577
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:50
The instances of the Type class are immutable: once they are created, they are never changed.
Definition: Type.h:45
bool isStructTy() const
True if this is an instance of StructType.
Definition: Type.h:249
static IntegerType * getInt32Ty(LLVMContext &C)
bool isVoidTy() const
Return true if this is 'void'.
Definition: Type.h:140
static UndefValue * get(Type *T)
Static factory methods - Return an 'undef' object of the specified type.
Definition: Constants.cpp:1724
This function has undefined behavior.
LLVM Value Representation.
Definition: Value.h:74
Type * getType() const
All values are typed, get the type of this value.
Definition: Value.h:255
void setName(const Twine &Name)
Change the name of the value.
Definition: Value.cpp:377
const Value * stripPointerCasts() const
Strip off pointer casts, all-zero GEPs and address space casts.
Definition: Value.cpp:693
constexpr char Args[]
Key for Kernel::Metadata::mArgs.
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
@ Swift
Calling convention for Swift.
Definition: CallingConv.h:69
@ C
The default llvm calling convention, compatible with C.
Definition: CallingConv.h:34
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
ModulePass * createWebAssemblyFixFunctionBitcasts()