llvm/lib/Transforms/IPO/ConstantMerge.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-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines the interface to a pass that merges duplicate global
  10 // constants together into a single constant that is shared.  This is useful
  11 // because some passes (ie TraceValues) insert a lot of string constants into
  12 // 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 constants
  15 // 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/InitializePasses.h"
  32 #include "llvm/Pass.h"
  33 #include "llvm/Support/Casting.h"
  34 #include "llvm/Transforms/IPO.h"
  35 #include <algorithm>
  36 #include <cassert>
  37 #include <utility>
  38
  39 using namespace llvm;
  40
  41 #define DEBUG_TYPE "constmerge"
  42
  43 STATISTIC(NumIdenticalMerged, "Number of identical global constants merged");
  44
  45 /// Find values that are marked as llvm.used.
  46 static void FindUsedValues(GlobalVariable *LLVMUsed,
  47                            SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
  48   if (!LLVMUsed) return;
  49   ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
  50
  51   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
  52     Value *Operand = Inits->getOperand(i)->stripPointerCasts();
  53     GlobalValue *GV = cast<GlobalValue>(Operand);
  54     UsedValues.insert(GV);
  55   }
  56 }
  57
  58 // True if A is better than B.
  59 static bool IsBetterCanonical(const GlobalVariable &A,
  60                               const GlobalVariable &B) {
  61   if (!A.hasLocalLinkage() && B.hasLocalLinkage())
  62     return true;
  63
  64   if (A.hasLocalLinkage() && !B.hasLocalLinkage())
  65     return false;
  66
  67   return A.hasGlobalUnnamedAddr();
  68 }
  69
  70 static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
  71   SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
  72   GV->getAllMetadata(MDs);
  73   for (const auto &V : MDs)
  74     if (V.first != LLVMContext::MD_dbg)
  75       return true;
  76   return false;
  77 }
  78
  79 static void copyDebugLocMetadata(const GlobalVariable *From,
  80                                  GlobalVariable *To) {
  81   SmallVector<DIGlobalVariableExpression *, 1> MDs;
  82   From->getDebugInfo(MDs);
  83   for (auto *MD : MDs)
  84     To->addDebugInfo(MD);
  85 }
  86
  87 static Align getAlign(GlobalVariable *GV) {
  88   return GV->getAlign().value_or(
  89       GV->getParent()->getDataLayout().getPreferredAlign(GV));
  90 }
  91
  92 static bool
  93 isUnmergeableGlobal(GlobalVariable *GV,
  94                     const SmallPtrSetImpl<const GlobalValue *> &UsedGlobals) {
  95   // Only process constants with initializers in the default address space.
  96   return !GV->isConstant() || !GV->hasDefinitiveInitializer() ||
  97          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
  98          // Don't touch thread-local variables.
  99          GV->isThreadLocal() ||
 100          // Don't touch values marked with attribute(used).
 101          UsedGlobals.count(GV);
 102 }
 103
 104 enum class CanMerge { No, Yes };
 105 static CanMerge makeMergeable(GlobalVariable *Old, GlobalVariable *New) {
 106   if (!Old->hasGlobalUnnamedAddr() && !New->hasGlobalUnnamedAddr())
 107     return CanMerge::No;
 108   if (hasMetadataOtherThanDebugLoc(Old))
 109     return CanMerge::No;
 110   assert(!hasMetadataOtherThanDebugLoc(New));
 111   if (!Old->hasGlobalUnnamedAddr())
 112     New->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
 113   return CanMerge::Yes;
 114 }
 115
 116 static void replace(Module &M, GlobalVariable *Old, GlobalVariable *New) {
 117   Constant *NewConstant = New;
 118
 119   LLVM_DEBUG(dbgs() << "Replacing global: @" << Old->getName() << " -> @"
 120                     << New->getName() << "\n");
 121
 122   // Bump the alignment if necessary.
 123   if (Old->getAlign() || New->getAlign())
 124     New->setAlignment(std::max(getAlign(Old), getAlign(New)));
 125
 126   copyDebugLocMetadata(Old, New);
 127   Old->replaceAllUsesWith(NewConstant);
 128
 129   // Delete the global value from the module.
 130   assert(Old->hasLocalLinkage() &&
 131          "Refusing to delete an externally visible global variable.");
 132   Old->eraseFromParent();
 133 }
 134
 135 static bool mergeConstants(Module &M) {
 136   // Find all the globals that are marked "used".  These cannot be merged.
 137   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
 138   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
 139   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
 140
 141   // Map unique constants to globals.
 142   DenseMap<Constant *, GlobalVariable *> CMap;
 143
 144   SmallVector<std::pair<GlobalVariable *, GlobalVariable *>, 32>
 145       SameContentReplacements;
 146
 147   size_t ChangesMade = 0;
 148   size_t OldChangesMade = 0;
 149
 150   // Iterate constant merging while we are still making progress.  Merging two
 151   // constants together may allow us to merge other constants together if the
 152   // second level constants have initializers which point to the globals that
 153   // were just merged.
 154   while (true) {
 155     // Find the canonical constants others will be merged with.
 156     for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
 157       // If this GV is dead, remove it.
 158       GV.removeDeadConstantUsers();
 159       if (GV.use_empty() && GV.hasLocalLinkage()) {
 160         GV.eraseFromParent();
 161         ++ChangesMade;
 162         continue;
 163       }
 164
 165       if (isUnmergeableGlobal(&GV, UsedGlobals))
 166         continue;
 167
 168       // This transformation is legal for weak ODR globals in the sense it
 169       // doesn't change semantics, but we really don't want to perform it
 170       // anyway; it's likely to pessimize code generation, and some tools
 171       // (like the Darwin linker in cases involving CFString) don't expect it.
 172       if (GV.isWeakForLinker())
 173         continue;
 174
 175       // Don't touch globals with metadata other then !dbg.
 176       if (hasMetadataOtherThanDebugLoc(&GV))
 177         continue;
 178
 179       Constant *Init = GV.getInitializer();
 180
 181       // Check to see if the initializer is already known.
 182       GlobalVariable *&Slot = CMap[Init];
 183
 184       // If this is the first constant we find or if the old one is local,
 185       // replace with the current one. If the current is externally visible
 186       // it cannot be replace, but can be the canonical constant we merge with.
 187       bool FirstConstantFound = !Slot;
 188       if (FirstConstantFound || IsBetterCanonical(GV, *Slot)) {
 189         Slot = &GV;
 190         LLVM_DEBUG(dbgs() << "Cmap[" << *Init << "] = " << GV.getName()
 191                           << (FirstConstantFound ? "\n" : " (updated)\n"));
 192       }
 193     }
 194
 195     // Identify all globals that can be merged together, filling in the
 196     // SameContentReplacements vector. We cannot do the replacement in this pass
 197     // because doing so may cause initializers of other globals to be rewritten,
 198     // invalidating the Constant* pointers in CMap.
 199     for (GlobalVariable &GV : llvm::make_early_inc_range(M.globals())) {
 200       if (isUnmergeableGlobal(&GV, UsedGlobals))
 201         continue;
 202
 203       // We can only replace constant with local linkage.
 204       if (!GV.hasLocalLinkage())
 205         continue;
 206
 207       Constant *Init = GV.getInitializer();
 208
 209       // Check to see if the initializer is already known.
 210       auto Found = CMap.find(Init);
 211       if (Found == CMap.end())
 212         continue;
 213
 214       GlobalVariable *Slot = Found->second;
 215       if (Slot == &GV)
 216         continue;
 217
 218       if (makeMergeable(&GV, Slot) == CanMerge::No)
 219         continue;
 220
 221       // Make all uses of the duplicate constant use the canonical version.
 222       LLVM_DEBUG(dbgs() << "Will replace: @" << GV.getName() << " -> @"
 223                         << Slot->getName() << "\n");
 224       SameContentReplacements.push_back(std::make_pair(&GV, Slot));
 225     }
 226
 227     // Now that we have figured out which replacements must be made, do them all
 228     // now.  This avoid invalidating the pointers in CMap, which are unneeded
 229     // now.
 230     for (unsigned i = 0, e = SameContentReplacements.size(); i != e; ++i) {
 231       GlobalVariable *Old = SameContentReplacements[i].first;
 232       GlobalVariable *New = SameContentReplacements[i].second;
 233       replace(M, Old, New);
 234       ++ChangesMade;
 235       ++NumIdenticalMerged;
 236     }
 237
 238     if (ChangesMade == OldChangesMade)
 239       break;
 240     OldChangesMade = ChangesMade;
 241
 242     SameContentReplacements.clear();
 243     CMap.clear();
 244   }
 245
 246   return ChangesMade;
 247 }
 248
 249 PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
 250   if (!mergeConstants(M))
 251     return PreservedAnalyses::all();
 252   return PreservedAnalyses::none();
 253 }
 254
 255 namespace {
 256
 257 struct ConstantMergeLegacyPass : public ModulePass {
 258   static char ID; // Pass identification, replacement for typeid
 259
 260   ConstantMergeLegacyPass() : ModulePass(ID) {
 261     initializeConstantMergeLegacyPassPass(*PassRegistry::getPassRegistry());
 262   }
 263
 264   // For this pass, process all of the globals in the module, eliminating
 265   // duplicate constants.
 266   bool runOnModule(Module &M) override {
 267     if (skipModule(M))
 268       return false;
 269     return mergeConstants(M);
 270   }
 271 };
 272
 273 } // end anonymous namespace
 274
 275 char ConstantMergeLegacyPass::ID = 0;
 276
 277 INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
 278                 "Merge Duplicate Global Constants", false, false)
 279
 280 ModulePass *llvm::createConstantMergePass() {
 281   return new ConstantMergeLegacyPass();
 282 }