llvm/lib/Target/WebAssembly/WebAssemblyFixFunctionBitcasts.cpp

   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"
  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"
  33 using namespace llvm;
  34
  35 #define DEBUG_TYPE "wasm-fix-function-bitcasts"
  36
  37 namespace {
  38 class 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();
  45     ModulePass::getAnalysisUsage(AU);
  46   }
  47
  48   bool runOnModule(Module &M) override;
  49
  50 public:
  51   static char ID;
  52   FixFunctionBitcasts() : ModulePass(ID) {}
  53 };
  54 } // End anonymous namespace
  55
  56 char FixFunctionBitcasts::ID = 0;
  57 INITIALIZE_PASS(FixFunctionBitcasts, DEBUG_TYPE,
  58                 "Fix mismatching bitcasts for WebAssembly", false, false)
  59
  60 ModulePass *llvm::createWebAssemblyFixFunctionBitcasts() {
  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.
  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))
  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.
 109 static Function *createWrapper(Function *F, FunctionType *Ty) {
 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.
 118   SmallVector<Value *, 4> Args;
 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         BB->getInstList().push_back(PtrCast);
 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       BB->getInstList().push_back(Cast);
 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.
 219 static 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
 228 bool FixFunctionBitcasts::runOnModule(Module &M) {
 229   LLVM_DEBUG(dbgs() << "********** Fix Function Bitcasts **********\n");
 230
 231   Function *Main = nullptr;
 232   CallInst *CallMain = nullptr;
 233   SmallVector<std::pair<CallBase *, Function *>, 0> Uses;
 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),
 251                             PointerType::get(Type::getInt8PtrTy(C), 0)};
 252       FunctionType *MainTy = FunctionType::get(Type::getInt32Ty(C), MainArgTys,
 253                                                /*isVarArg=*/false);
 254       if (shouldFixMainFunction(F.getFunctionType(), MainTy)) {
 255         LLVM_DEBUG(dbgs() << "Found `main` function with incorrect type: "
 256                           << *F.getFunctionType() << "\n");
 257         Value *Args[] = {UndefValue::get(MainArgTys[0]),
 258                          UndefValue::get(MainArgTys[1])};
 259         Value *Casted =
 260             ConstantExpr::getBitCast(Main, PointerType::get(MainTy, 0));
 261         CallMain = CallInst::Create(MainTy, Casted, Args, "call_main");
 262         Uses.push_back(std::make_pair(CallMain, &F));
 263       }
 264     }
 265   }
 266
 267   DenseMap<std::pair<Function *, FunctionType *>, Function *> Wrappers;
 268
 269   for (auto &UseFunc : Uses) {
 270     CallBase *CB = UseFunc.first;
 271     Function *F = UseFunc.second;
 272     FunctionType *Ty = CB->getFunctionType();
 273
 274     auto Pair = Wrappers.insert(std::make_pair(std::make_pair(F, Ty), nullptr));
 275     if (Pair.second)
 276       Pair.first->second = createWrapper(F, Ty);
 277
 278     Function *Wrapper = Pair.first->second;
 279     if (!Wrapper)
 280       continue;
 281
 282     CB->setCalledOperand(Wrapper);
 283   }
 284
 285   // If we created a wrapper for main, rename the wrapper so that it's the
 286   // one that gets called from startup.
 287   if (CallMain) {
 288     Main->setName("__original_main");
 289     auto *MainWrapper =
 290         cast<Function>(CallMain->getCalledOperand()->stripPointerCasts());
 291     delete CallMain;
 292     if (Main->isDeclaration()) {
 293       // The wrapper is not needed in this case as we don't need to export
 294       // it to anyone else.
 295       MainWrapper->eraseFromParent();
 296     } else {
 297       // Otherwise give the wrapper the same linkage as the original main
 298       // function, so that it can be called from the same places.
 299       MainWrapper->setName("main");
 300       MainWrapper->setLinkage(Main->getLinkage());
 301       MainWrapper->setVisibility(Main->getVisibility());
 302     }
 303   }
 304
 305   return true;
 306 }