251 lines · cpp
1//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//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 the interface to a pass that merges duplicate global10// constants together into a single constant that is shared. This is useful11// because some passes (ie TraceValues) insert a lot of string constants into12// the program, regardless of whether or not an existing string is available.13//14// Algorithm: ConstantMerge is designed to build up a map of available constants15// and eliminate duplicates when it is initialized.16//17//===----------------------------------------------------------------------===//18 19#include "llvm/Transforms/IPO/ConstantMerge.h"20#include "llvm/ADT/DenseMap.h"21#include "llvm/ADT/SmallPtrSet.h"22#include "llvm/ADT/SmallVector.h"23#include "llvm/ADT/Statistic.h"24#include "llvm/IR/Constants.h"25#include "llvm/IR/DataLayout.h"26#include "llvm/IR/DerivedTypes.h"27#include "llvm/IR/GlobalValue.h"28#include "llvm/IR/GlobalVariable.h"29#include "llvm/IR/LLVMContext.h"30#include "llvm/IR/Module.h"31#include "llvm/Support/Casting.h"32#include "llvm/Support/Debug.h"33#include "llvm/Transforms/IPO.h"34#include <algorithm>35#include <cassert>36#include <utility>37 38using namespace llvm;39 40#define DEBUG_TYPE "constmerge"41 42STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");43 44/// Find values that are marked as llvm.used.45static void FindUsedValues(GlobalVariable *LLVMUsed,46 SmallPtrSetImpl<const GlobalValue*> &UsedValues) {47 if (!LLVMUsed) return;48 ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());49 50 for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {51 Value *Operand = Inits->getOperand(i)->stripPointerCasts();52 GlobalValue *GV = cast<GlobalValue>(Operand);53 UsedValues.insert(GV);54 }55}56 57// True if A is better than B.58static bool IsBetterCanonical(const GlobalVariable &A,59 const GlobalVariable &B) {60 if (!A.hasLocalLinkage() && B.hasLocalLinkage())61 return true;62 63 if (A.hasLocalLinkage() && !B.hasLocalLinkage())64 return false;65 66 return A.hasGlobalUnnamedAddr();67}68 69static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {70 SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;71 GV->getAllMetadata(MDs);72 for (const auto &V : MDs)73 if (V.first != LLVMContext::MD_dbg)74 return true;75 return false;76}77 78static void copyDebugLocMetadata(const GlobalVariable *From,79 GlobalVariable *To) {80 SmallVector<DIGlobalVariableExpression *, 1> MDs;81 From->getDebugInfo(MDs);82 for (auto *MD : MDs)83 To->addDebugInfo(MD);84}85 86static Align getAlign(GlobalVariable *GV) {87 return GV->getAlign().value_or(88 GV->getDataLayout().getPreferredAlign(GV));89}90 91static bool92isUnmergeableGlobal(GlobalVariable *GV,93 const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {94 // Only process constants with initializers in the default address space.95 return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||96 GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||97 // Don't touch thread-local variables.98 GV->isThreadLocal() ||99 // Don't touch values marked with attribute(used).100 UsedGlobals.count(GV);101}102 103enum class CanMerge { No, Yes };104static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {105 if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())106 return CanMerge::No;107 if (hasMetadataOtherThanDebugLoc(Old))108 return CanMerge::No;109 assert(!hasMetadataOtherThanDebugLoc(New));110 if (!Old->hasGlobalUnnamedAddr())111 New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);112 return CanMerge::Yes;113}114 115static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {116 Constant *NewConstant = New;117 118 LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"119 << New->getName() << "\n");120 121 // Bump the alignment if necessary.122 if (Old->getAlign() || New->getAlign())123 New->setAlignment(std::max(getAlign(Old), getAlign(New)));124 125 copyDebugLocMetadata(Old, New);126 Old->replaceAllUsesWith(NewConstant);127 128 // Delete the global value from the module.129 assert(Old->hasLocalLinkage() &&130 "Refusing to delete an externally visible global variable.");131 Old->eraseFromParent();132}133 134static bool mergeConstants(Module &M) {135 // Find all the globals that are marked "used". These cannot be merged.136 SmallPtrSet<const GlobalValue*, 8> UsedGlobals;137 FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);138 FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);139 140 // Map unique constants to globals.141 DenseMap<Constant *, GlobalVariable *> CMap;142 143 SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>144 SameContentReplacements;145 146 size_t ChangesMade = 0;147 size_t OldChangesMade = 0;148 149 // Iterate constant merging while we are still making progress. Merging two150 // constants together may allow us to merge other constants together if the151 // second level constants have initializers which point to the globals that152 // were just merged.153 while (true) {154 // Find the canonical constants others will be merged with.155 for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {156 // If this GV is dead, remove it.157 GV.removeDeadConstantUsers();158 if (GV.use_empty() && GV.hasLocalLinkage()) {159 GV.eraseFromParent();160 ++ChangesMade;161 continue;162 }163 164 if (isUnmergeableGlobal(&GV, UsedGlobals))165 continue;166 167 // This transformation is legal for weak ODR globals in the sense it168 // doesn't change semantics, but we really don't want to perform it169 // anyway; it's likely to pessimize code generation, and some tools170 // (like the Darwin linker in cases involving CFString) don't expect it.171 if (GV.isWeakForLinker())172 continue;173 174 // Don't touch globals with metadata other then !dbg.175 if (hasMetadataOtherThanDebugLoc(&GV))176 continue;177 178 Constant *Init = GV.getInitializer();179 180 // Check to see if the initializer is already known.181 GlobalVariable *&Slot = CMap[Init];182 183 // If this is the first constant we find or if the old one is local,184 // replace with the current one. If the current is externally visible185 // it cannot be replace, but can be the canonical constant we merge with.186 bool FirstConstantFound = !Slot;187 if (FirstConstantFound || IsBetterCanonical(GV, *Slot)) {188 Slot = &GV;189 LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV.getName()190 << (FirstConstantFound ? "\n" : " (updated)\n"));191 }192 }193 194 // Identify all globals that can be merged together, filling in the195 // SameContentReplacements vector. We cannot do the replacement in this pass196 // because doing so may cause initializers of other globals to be rewritten,197 // invalidating the Constant* pointers in CMap.198 for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {199 if (isUnmergeableGlobal(&GV, UsedGlobals))200 continue;201 202 // We can only replace constant with local linkage.203 if (!GV.hasLocalLinkage())204 continue;205 206 Constant *Init = GV.getInitializer();207 208 // Check to see if the initializer is already known.209 auto Found = CMap.find(Init);210 if (Found == CMap.end())211 continue;212 213 GlobalVariable *Slot = Found->second;214 if (Slot == &GV)215 continue;216 217 if (makeMergeable(&GV, Slot) == CanMerge::No)218 continue;219 220 // Make all uses of the duplicate constant use the canonical version.221 LLVM_DEBUG(dbgs() << "Will replace: @" << GV.getName() << " -> @"222 << Slot->getName() << "\n");223 SameContentReplacements.push_back(std::make_pair(&GV, Slot));224 }225 226 // Now that we have figured out which replacements must be made, do them all227 // now. This avoid invalidating the pointers in CMap, which are unneeded228 // now.229 for (const auto &[Old, New] : SameContentReplacements) {230 replace(M, Old, New);231 ++ChangesMade;232 ++NumIdenticalMerged;233 }234 235 if (ChangesMade == OldChangesMade)236 break;237 OldChangesMade = ChangesMade;238 239 SameContentReplacements.clear();240 CMap.clear();241 }242 243 return ChangesMade;244}245 246PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {247 if (!mergeConstants(M))248 return PreservedAnalyses::all();249 return PreservedAnalyses::none();250}251