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