1 //===-- WebAssemblyAddMissingPrototypes.cpp - Fix prototypeless functions -===// 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 /// Add prototypes to prototypes-less functions. 11 /// 12 /// WebAssembly has strict function prototype checking so we need functions 13 /// declarations to match the call sites. Clang treats prototype-less functions 14 /// as varargs (foo(...)) which happens to work on existing platforms but 15 /// doesn't under WebAssembly. This pass will find all the call sites of each 16 /// prototype-less function, ensure they agree, and then set the signature 17 /// on the function declaration accordingly. 18 /// 19 //===----------------------------------------------------------------------===// 20 21 #include "WebAssembly.h" 22 #include "llvm/IR/Constants.h" 23 #include "llvm/IR/IRBuilder.h" 24 #include "llvm/IR/Module.h" 25 #include "llvm/IR/Operator.h" 26 #include "llvm/Pass.h" 27 #include "llvm/Support/Debug.h" 28 #include "llvm/Transforms/Utils/Local.h" 29 #include "llvm/Transforms/Utils/ModuleUtils.h" 30 using namespace llvm; 31 32 #define DEBUG_TYPE "wasm-add-missing-prototypes" 33 34 namespace { 35 class WebAssemblyAddMissingPrototypes final : public ModulePass { getPassName() const36 StringRef getPassName() const override { 37 return "Add prototypes to prototypes-less functions"; 38 } 39 getAnalysisUsage(AnalysisUsage & AU) const40 void getAnalysisUsage(AnalysisUsage &AU) const override { 41 AU.setPreservesCFG(); 42 ModulePass::getAnalysisUsage(AU); 43 } 44 45 bool runOnModule(Module &M) override; 46 47 public: 48 static char ID; WebAssemblyAddMissingPrototypes()49 WebAssemblyAddMissingPrototypes() : ModulePass(ID) {} 50 }; 51 } // End anonymous namespace 52 53 char WebAssemblyAddMissingPrototypes::ID = 0; 54 INITIALIZE_PASS(WebAssemblyAddMissingPrototypes, DEBUG_TYPE, 55 "Add prototypes to prototypes-less functions", false, false) 56 createWebAssemblyAddMissingPrototypes()57 ModulePass *llvm::createWebAssemblyAddMissingPrototypes() { 58 return new WebAssemblyAddMissingPrototypes(); 59 } 60 runOnModule(Module & M)61 bool WebAssemblyAddMissingPrototypes::runOnModule(Module &M) { 62 LLVM_DEBUG(dbgs() << "********** Add Missing Prototypes **********\n"); 63 64 std::vector<std::pair<Function *, Function *>> Replacements; 65 66 // Find all the prototype-less function declarations 67 for (Function &F : M) { 68 if (!F.isDeclaration() || !F.hasFnAttribute("no-prototype")) 69 continue; 70 71 LLVM_DEBUG(dbgs() << "Found no-prototype function: " << F.getName() 72 << "\n"); 73 74 // When clang emits prototype-less C functions it uses (...), i.e. varargs 75 // function that take no arguments (have no sentinel). When we see a 76 // no-prototype attribute we expect the function have these properties. 77 if (!F.isVarArg()) 78 report_fatal_error( 79 "Functions with 'no-prototype' attribute must take varargs: " + 80 F.getName()); 81 unsigned NumParams = F.getFunctionType()->getNumParams(); 82 if (NumParams != 0) { 83 if (!(NumParams == 1 && F.arg_begin()->hasStructRetAttr())) 84 report_fatal_error("Functions with 'no-prototype' attribute should " 85 "not have params: " + 86 F.getName()); 87 } 88 89 // Find calls of this function, looking through bitcasts. 90 SmallVector<CallBase *> Calls; 91 SmallVector<Value *> Worklist; 92 Worklist.push_back(&F); 93 while (!Worklist.empty()) { 94 Value *V = Worklist.pop_back_val(); 95 for (User *U : V->users()) { 96 if (auto *BC = dyn_cast<BitCastOperator>(U)) 97 Worklist.push_back(BC); 98 else if (auto *CB = dyn_cast<CallBase>(U)) 99 if (CB->getCalledOperand() == V) 100 Calls.push_back(CB); 101 } 102 } 103 104 // Create a function prototype based on the first call site that we find. 105 FunctionType *NewType = nullptr; 106 for (CallBase *CB : Calls) { 107 LLVM_DEBUG(dbgs() << "prototype-less call of " << F.getName() << ":\n"); 108 LLVM_DEBUG(dbgs() << *CB << "\n"); 109 FunctionType *DestType = CB->getFunctionType(); 110 if (!NewType) { 111 // Create a new function with the correct type 112 NewType = DestType; 113 LLVM_DEBUG(dbgs() << "found function type: " << *NewType << "\n"); 114 } else if (NewType != DestType) { 115 errs() << "warning: prototype-less function used with " 116 "conflicting signatures: " 117 << F.getName() << "\n"; 118 LLVM_DEBUG(dbgs() << " " << *DestType << "\n"); 119 LLVM_DEBUG(dbgs() << " " << *NewType << "\n"); 120 } 121 } 122 123 if (!NewType) { 124 LLVM_DEBUG( 125 dbgs() << "could not derive a function prototype from usage: " + 126 F.getName() + "\n"); 127 // We could not derive a type for this function. In this case strip 128 // the isVarArg and make it a simple zero-arg function. This has more 129 // chance of being correct. The current signature of (...) is illegal in 130 // C since it doesn't have any arguments before the "...", we this at 131 // least makes it possible for this symbol to be resolved by the linker. 132 NewType = FunctionType::get(F.getFunctionType()->getReturnType(), false); 133 } 134 135 Function *NewF = 136 Function::Create(NewType, F.getLinkage(), F.getName() + ".fixed_sig"); 137 NewF->setAttributes(F.getAttributes()); 138 NewF->removeFnAttr("no-prototype"); 139 NewF->IsNewDbgInfoFormat = F.IsNewDbgInfoFormat; 140 Replacements.emplace_back(&F, NewF); 141 } 142 143 for (auto &Pair : Replacements) { 144 Function *OldF = Pair.first; 145 Function *NewF = Pair.second; 146 std::string Name = std::string(OldF->getName()); 147 M.getFunctionList().push_back(NewF); 148 OldF->replaceAllUsesWith( 149 ConstantExpr::getPointerBitCastOrAddrSpaceCast(NewF, OldF->getType())); 150 OldF->eraseFromParent(); 151 NewF->setName(Name); 152 } 153 154 return !Replacements.empty(); 155 } 156