152 lines · cpp
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-exception6//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 25using namespace llvm;26 27/// Given a specified llvm.global_ctors list, remove the listed elements.28static 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 = new GlobalVariable(49 CA->getType(), GCL->isConstant(), GCL->getLinkage(), CA, "",50 GCL->getThreadLocalMode(), GCL->getAddressSpace());51 GCL->getParent()->insertGlobalVariable(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 GCL->replaceAllUsesWith(NGV);57 58 GCL->eraseFromParent();59}60 61/// Given a llvm.global_ctors list that we can understand,62/// return a list of the functions and null terminator as a vector.63static std::vector<std::pair<uint32_t, Function *>>64parseGlobalCtors(GlobalVariable *GV) {65 ConstantArray *CA = cast<ConstantArray>(GV->getInitializer());66 std::vector<std::pair<uint32_t, Function *>> Result;67 Result.reserve(CA->getNumOperands());68 for (auto &V : CA->operands()) {69 ConstantStruct *CS = cast<ConstantStruct>(V);70 Result.emplace_back(cast<ConstantInt>(CS->getOperand(0))->getZExtValue(),71 dyn_cast<Function>(CS->getOperand(1)));72 }73 return Result;74}75 76/// Find the llvm.global_ctors list.77static GlobalVariable *findGlobalCtors(Module &M) {78 GlobalVariable *GV = M.getGlobalVariable("llvm.global_ctors");79 if (!GV)80 return nullptr;81 82 // Verify that the initializer is simple enough for us to handle. We are83 // only allowed to optimize the initializer if it is unique.84 if (!GV->hasUniqueInitializer())85 return nullptr;86 87 // If there are no ctors, then the initializer might be null/undef/poison.88 // Ignore anything but an array.89 ConstantArray *CA = dyn_cast<ConstantArray>(GV->getInitializer());90 if (!CA)91 return nullptr;92 93 for (auto &V : CA->operands()) {94 if (isa<ConstantAggregateZero>(V))95 continue;96 ConstantStruct *CS = cast<ConstantStruct>(V);97 if (isa<ConstantPointerNull>(CS->getOperand(1)))98 continue;99 100 // Can only handle global constructors with no arguments.101 Function *F = dyn_cast<Function>(CS->getOperand(1));102 if (!F || F->arg_size() != 0)103 return nullptr;104 }105 return GV;106}107 108/// Call "ShouldRemove" for every entry in M's global_ctor list and remove the109/// entries for which it returns true. Return true if anything changed.110bool llvm::optimizeGlobalCtorsList(111 Module &M, function_ref<bool(uint32_t, Function *)> ShouldRemove) {112 GlobalVariable *GlobalCtors = findGlobalCtors(M);113 if (!GlobalCtors)114 return false;115 116 std::vector<std::pair<uint32_t, Function *>> Ctors =117 parseGlobalCtors(GlobalCtors);118 if (Ctors.empty())119 return false;120 121 bool MadeChange = false;122 // Loop over global ctors, optimizing them when we can.123 BitVector CtorsToRemove(Ctors.size());124 std::vector<size_t> CtorsByPriority(Ctors.size());125 std::iota(CtorsByPriority.begin(), CtorsByPriority.end(), 0);126 stable_sort(CtorsByPriority, [&](size_t LHS, size_t RHS) {127 return Ctors[LHS].first < Ctors[RHS].first;128 });129 for (unsigned CtorIndex : CtorsByPriority) {130 const uint32_t Priority = Ctors[CtorIndex].first;131 Function *F = Ctors[CtorIndex].second;132 if (!F)133 continue;134 135 LLVM_DEBUG(dbgs() << "Optimizing Global Constructor: " << *F << "\n");136 137 // If we can evaluate the ctor at compile time, do.138 if (ShouldRemove(Priority, F)) {139 Ctors[CtorIndex].second = nullptr;140 CtorsToRemove.set(CtorIndex);141 MadeChange = true;142 continue;143 }144 }145 146 if (!MadeChange)147 return false;148 149 removeGlobalCtors(GlobalCtors, CtorsToRemove);150 return true;151}152