lib/Transforms/IPO/ConstantMerge.cpp

   1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file defines the interface to a pass that merges duplicate global
  11 // constants together into a single constant that is shared.  This is useful
  12 // because some passes (ie TraceValues) insert a lot of string constants into
  13 // the program, regardless of whether or not an existing string is available.
  14 //
  15 // Algorithm: ConstantMerge is designed to build up a map of available constants
  16 // and eliminate duplicates when it is initialized.
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #define DEBUG_TYPE "constmerge"
  21 #include "llvm/Transforms/IPO.h"
  22 #include "llvm/Constants.h"
  23 #include "llvm/DerivedTypes.h"
  24 #include "llvm/Module.h"
  25 #include "llvm/Pass.h"
  26 #include "llvm/ADT/DenseMap.h"
  27 #include "llvm/ADT/SmallPtrSet.h"
  28 #include "llvm/ADT/Statistic.h"
  29 using namespace llvm;
  30
  31 STATISTIC(NumMerged, "Number of global constants merged");
  32
  33 namespace {
  34   struct ConstantMerge : public ModulePass {
  35     static char ID; // Pass identification, replacement for typeid
  36     ConstantMerge() : ModulePass(ID) {
  37       initializeConstantMergePass(*PassRegistry::getPassRegistry());
  38     }
  39
  40     // run - For this pass, process all of the globals in the module,
  41     // eliminating duplicate constants.
  42     //
  43     bool runOnModule(Module &M);
  44   };
  45 }
  46
  47 char ConstantMerge::ID = 0;
  48 INITIALIZE_PASS(ConstantMerge, "constmerge",
  49                 "Merge Duplicate Global Constants", false, false)
  50
  51 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
  52
  53
  54
  55 /// Find values that are marked as llvm.used.
  56 static void FindUsedValues(GlobalVariable *LLVMUsed,
  57                            SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
  58   if (LLVMUsed == 0) return;
  59   ConstantArray *Inits = dyn_cast<ConstantArray>(LLVMUsed->getInitializer());
  60   if (Inits == 0) return;
  61
  62   for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
  63     if (GlobalValue *GV =
  64         dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
  65       UsedValues.insert(GV);
  66 }
  67
  68 // True if A is better than B.
  69 static bool IsBetterCannonical(const GlobalVariable &A,
  70                                const GlobalVariable &B) {
  71   if (!A.hasLocalLinkage() && B.hasLocalLinkage())
  72     return true;
  73
  74   if (A.hasLocalLinkage() && !B.hasLocalLinkage())
  75     return false;
  76
  77   return A.hasUnnamedAddr();
  78 }
  79
  80 bool ConstantMerge::runOnModule(Module &M) {
  81   // Find all the globals that are marked "used".  These cannot be merged.
  82   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
  83   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
  84   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
  85
  86   // Map unique constant/section pairs to globals.  We don't want to merge
  87   // globals in different sections.
  88   DenseMap<Constant*, GlobalVariable*> CMap;
  89
  90   // Replacements - This vector contains a list of replacements to perform.
  91   SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
  92
  93   bool MadeChange = false;
  94
  95   // Iterate constant merging while we are still making progress.  Merging two
  96   // constants together may allow us to merge other constants together if the
  97   // second level constants have initializers which point to the globals that
  98   // were just merged.
  99   while (1) {
 100
 101     // First: Find the canonical constants others will be merged with.
 102     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
 103          GVI != E; ) {
 104       GlobalVariable *GV = GVI++;
 105
 106       // If this GV is dead, remove it.
 107       GV->removeDeadConstantUsers();
 108       if (GV->use_empty() && GV->hasLocalLinkage()) {
 109         GV->eraseFromParent();
 110         continue;
 111       }
 112
 113       // Only process constants with initializers in the default address space.
 114       if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
 115           GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
 116           // Don't touch values marked with attribute(used).
 117           UsedGlobals.count(GV))
 118         continue;
 119
 120       Constant *Init = GV->getInitializer();
 121
 122       // Check to see if the initializer is already known.
 123       GlobalVariable *&Slot = CMap[Init];
 124
 125       // If this is the first constant we find or if the old on is local,
 126       // replace with the current one. It the current is externally visible
 127       // it cannot be replace, but can be the canonical constant we merge with.
 128       if (Slot == 0 || IsBetterCannonical(*GV, *Slot)) {
 129         Slot = GV;
 130       }
 131     }
 132
 133     // Second: identify all globals that can be merged together, filling in
 134     // the Replacements vector.  We cannot do the replacement in this pass
 135     // because doing so may cause initializers of other globals to be rewritten,
 136     // invalidating the Constant* pointers in CMap.
 137     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
 138          GVI != E; ) {
 139       GlobalVariable *GV = GVI++;
 140
 141       // Only process constants with initializers in the default address space.
 142       if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
 143           GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
 144           // Don't touch values marked with attribute(used).
 145           UsedGlobals.count(GV))
 146         continue;
 147
 148       // We can only replace constant with local linkage.
 149       if (!GV->hasLocalLinkage())
 150         continue;
 151
 152       Constant *Init = GV->getInitializer();
 153
 154       // Check to see if the initializer is already known.
 155       GlobalVariable *Slot = CMap[Init];
 156
 157       if (!Slot || Slot == GV)
 158         continue;
 159
 160       if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
 161         continue;
 162
 163       if (!GV->hasUnnamedAddr())
 164         Slot->setUnnamedAddr(false);
 165
 166       // Make all uses of the duplicate constant use the canonical version.
 167       Replacements.push_back(std::make_pair(GV, Slot));
 168     }
 169
 170     if (Replacements.empty())
 171       return MadeChange;
 172     CMap.clear();
 173
 174     // Now that we have figured out which replacements must be made, do them all
 175     // now.  This avoid invalidating the pointers in CMap, which are unneeded
 176     // now.
 177     for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
 178       // Eliminate any uses of the dead global.
 179       Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
 180
 181       // Delete the global value from the module.
 182       assert(Replacements[i].first->hasLocalLinkage() &&
 183              "Refusing to delete an externally visible global variable.");
 184       Replacements[i].first->eraseFromParent();
 185     }
 186
 187     NumMerged += Replacements.size();
 188     Replacements.clear();
 189   }
 190 }