1 //===- CtorUtils.cpp - Helpers for working with global_ctors ----*- C++ -*-===// 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 // This file defines functions that are used to process llvm.global_ctors. 10 // 11 //===----------------------------------------------------------------------===// 12 13 #include "llvm/Transforms/Utils/CtorUtils.h" 14 #include "llvm/ADT/BitVector.h" 15 #include "llvm/IR/Constants.h" 16 #include "llvm/IR/Function.h" 17 #include "llvm/IR/GlobalVariable.h" 18 #include "llvm/IR/Module.h" 19 #include "llvm/Support/Debug.h" 20 #include "llvm/Support/raw_ostream.h" 21 #include <numeric> 22 23 #define DEBUG_TYPE "ctor_utils" 24 25 using namespace llvm; 26 27 /// Given a specified llvm.global_ctors list, remove the listed elements. 28 static void removeGlobalCtors(GlobalVariable *GCL, const BitVector &CtorsToRemove) { 29 // Filter out the initializer elements to remove. 30 ConstantArray *OldCA = cast<ConstantArray>(GCL->getInitializer()); 31 SmallVector<Constant *, 10> CAList; 32 for (unsigned I = 0, E = OldCA->getNumOperands(); I < E; ++I) 33 if (!CtorsToRemove.test(I)) 34 CAList.push_back(OldCA->getOperand(I)); 35 36 // Create the new array initializer. 37 ArrayType *ATy = 38 ArrayType::get(OldCA->getType()->getElementType(), CAList.size()); 39 Constant *CA = ConstantArray::get(ATy, CAList); 40 41 // If we didn't change the number of elements, don't create a new GV. 42 if (CA->getType() == OldCA->getType()) { 43 GCL->setInitializer(CA); 44 return; 45 } 46 47 // Create the new global and insert it next to the existing list. 48 GlobalVariable *NGV = 49 new GlobalVariable(CA->getType(), GCL->isConstant(), GCL->getLinkage(), 50 CA, "", GCL->getThreadLocalMode()); 51 GCL->getParent()->getGlobalList().insert(GCL->getIterator(), NGV); 52 NGV->takeName(GCL); 53 54 // Nuke the old list, replacing any uses with the new one. 55 if (!GCL->use_empty()) { 56 Constant *V = NGV; 57 if (V->getType() != GCL->getType()) 58 V = ConstantExpr::getBitCast(V, GCL->getType()); 59 GCL->replaceAllUsesWith(V); 60 } 61 GCL->eraseFromParent(); 62 } 63 64 /// Given a llvm.global_ctors list that we can understand, 65 /// return a list of the functions and null terminator as a vector. 66 static std::vector<std::pair<uint32_t, Function *>> 67 parseGlobalCtors(GlobalVariable *GV) { 68 ConstantArray *CA = cast<ConstantArray>(GV->getInitializer()); 69 std::vector<std::pair<uint32_t, Function *>> Result; 70 Result.reserve(CA->getNumOperands()); 71 for (auto &V : CA->operands()) { 72 ConstantStruct *CS = cast<ConstantStruct>(V); 73 Result.emplace_back(cast<ConstantInt>(CS->getOperand(0))->getZExtValue(), 74 dyn_cast<Function>(CS->getOperand(1))); 75 } 76 return Result; 77 } 78 79 /// Find the llvm.global_ctors list. 80 static GlobalVariable *findGlobalCtors(Module &M) { 81 GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors"); 82 if (!GV) 83 return nullptr; 84 85 // Verify that the initializer is simple enough for us to handle. We are 86 // only allowed to optimize the initializer if it is unique. 87 if (!GV->hasUniqueInitializer()) 88 return nullptr; 89 90 // If there are no ctors, then the initializer might be null/undef/poison. 91 // Ignore anything but an array. 92 ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer()); 93 if (!CA) 94 return nullptr; 95 96 for (auto &V : CA->operands()) { 97 if (isa<ConstantAggregateZero>(V)) 98 continue; 99 ConstantStruct *CS = cast<ConstantStruct>(V); 100 if (isa<ConstantPointerNull>(CS->getOperand(1))) 101 continue; 102 103 // Can only handle global constructors with no arguments. 104 Function *F = dyn_cast<Function>(CS->getOperand(1)); 105 if (!F || F->arg_size() != 0) 106 return nullptr; 107 } 108 return GV; 109 } 110 111 /// Call "ShouldRemove" for every entry in M's global_ctor list and remove the 112 /// entries for which it returns true. Return true if anything changed. 113 bool llvm::optimizeGlobalCtorsList( 114 Module &M, function_ref<bool(uint32_t, Function *)> ShouldRemove) { 115 GlobalVariable *GlobalCtors = findGlobalCtors(M); 116 if (!GlobalCtors) 117 return false; 118 119 std::vector<std::pair<uint32_t, Function *>> Ctors = 120 parseGlobalCtors(GlobalCtors); 121 if (Ctors.empty()) 122 return false; 123 124 bool MadeChange = false; 125 // Loop over global ctors, optimizing them when we can. 126 BitVector CtorsToRemove(Ctors.size()); 127 std::vector<size_t> CtorsByPriority(Ctors.size()); 128 std::iota(CtorsByPriority.begin(), CtorsByPriority.end(), 0); 129 stable_sort(CtorsByPriority, [&](size_t LHS, size_t RHS) { 130 return Ctors[LHS].first < Ctors[RHS].first; 131 }); 132 for (unsigned CtorIndex : CtorsByPriority) { 133 const uint32_t Priority = Ctors[CtorIndex].first; 134 Function *F = Ctors[CtorIndex].second; 135 if (!F) 136 continue; 137 138 LLVM_DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n"); 139 140 // If we can evaluate the ctor at compile time, do. 141 if (ShouldRemove(Priority, F)) { 142 Ctors[CtorIndex].second = nullptr; 143 CtorsToRemove.set(CtorIndex); 144 MadeChange = true; 145 continue; 146 } 147 } 148 149 if (!MadeChange) 150 return false; 151 152 removeGlobalCtors(GlobalCtors, CtorsToRemove); 153 return true; 154 } 155