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 bool ConstantMerge::runOnModule(Module &M) {
  69   // Find all the globals that are marked "used".  These cannot be merged.
  70   SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
  71   FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
  72   FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
  73
  74   // Map unique constant/section pairs to globals.  We don't want to merge
  75   // globals in different sections.
  76   DenseMap<Constant*, GlobalVariable*> CMap;
  77
  78   // Replacements - This vector contains a list of replacements to perform.
  79   SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
  80
  81   bool MadeChange = false;
  82
  83   // Iterate constant merging while we are still making progress.  Merging two
  84   // constants together may allow us to merge other constants together if the
  85   // second level constants have initializers which point to the globals that
  86   // were just merged.
  87   while (1) {
  88     // First pass: identify all globals that can be merged together, filling in
  89     // the Replacements vector.  We cannot do the replacement in this pass
  90     // because doing so may cause initializers of other globals to be rewritten,
  91     // invalidating the Constant* pointers in CMap.
  92     //
  93     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
  94          GVI != E; ) {
  95       GlobalVariable *GV = GVI++;
  96
  97       // If this GV is dead, remove it.
  98       GV->removeDeadConstantUsers();
  99       if (GV->use_empty() && GV->hasLocalLinkage()) {
 100         GV->eraseFromParent();
 101         continue;
 102       }
 103
 104       // Only process constants with initializers in the default address space.
 105       if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
 106           GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
 107           // Don't touch values marked with attribute(used).
 108           UsedGlobals.count(GV))
 109         continue;
 110
 111       // Start by filling slots with only the globals we aren't allowed to
 112       // delete because they're externally visible.
 113       if (GV->hasLocalLinkage())
 114         continue;
 115
 116       Constant *Init = GV->getInitializer();
 117
 118       // Check to see if the initializer is already known.
 119       GlobalVariable *&Slot = CMap[Init];
 120
 121       if (Slot == 0) {    // Nope, add it to the map.
 122         Slot = GV;
 123       }
 124     }
 125
 126     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
 127          GVI != E; ) {
 128       GlobalVariable *GV = GVI++;
 129
 130       // Only process constants with initializers in the default address space.
 131       if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
 132           GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
 133           // Don't touch values marked with attribute(used).
 134           UsedGlobals.count(GV))
 135         continue;
 136
 137       // Only look at the remaining globals now.
 138       if (!GV->hasLocalLinkage())
 139         continue;
 140
 141       Constant *Init = GV->getInitializer();
 142
 143       // Check to see if the initializer is already known.
 144       GlobalVariable *&Slot = CMap[Init];
 145
 146       if (Slot == 0) {    // Nope, add it to the map.
 147         Slot = GV;
 148       } else {            // Yup, this is a duplicate!
 149         // Make all uses of the duplicate constant use the canonical version.
 150         Replacements.push_back(std::make_pair(GV, Slot));
 151       }
 152     }
 153
 154     if (Replacements.empty())
 155       return MadeChange;
 156     CMap.clear();
 157
 158     // Now that we have figured out which replacements must be made, do them all
 159     // now.  This avoid invalidating the pointers in CMap, which are unneeded
 160     // now.
 161     for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
 162       // Eliminate any uses of the dead global.
 163       Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
 164
 165       // Delete the global value from the module.
 166       assert(Replacements[i].first->hasLocalLinkage() &&
 167              "Refusing to delete an externally visible global variable.");
 168       Replacements[i].first->eraseFromParent();
 169     }
 170
 171     NumMerged += Replacements.size();
 172     Replacements.clear();
 173   }
 174 }