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/Module.h"
  23 #include "llvm/Pass.h"
  24 #include "llvm/ADT/Statistic.h"
  25 #include <map>
  26 using namespace llvm;
  27
  28 STATISTIC(NumMerged, "Number of global constants merged");
  29
  30 namespace {
  31   struct ConstantMerge : public ModulePass {
  32     static char ID; // Pass identification, replacement for typeid
  33     ConstantMerge() : ModulePass(&ID) {}
  34
  35     // run - For this pass, process all of the globals in the module,
  36     // eliminating duplicate constants.
  37     //
  38     bool runOnModule(Module &M);
  39   };
  40 }
  41
  42 char ConstantMerge::ID = 0;
  43 static RegisterPass<ConstantMerge>
  44 X("constmerge", "Merge Duplicate Global Constants");
  45
  46 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
  47
  48 bool ConstantMerge::runOnModule(Module &M) {
  49   // Map unique constant/section pairs to globals.  We don't want to merge
  50   // globals in different sections.
  51   std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
  52
  53   // Replacements - This vector contains a list of replacements to perform.
  54   std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
  55
  56   bool MadeChange = false;
  57
  58   // Iterate constant merging while we are still making progress.  Merging two
  59   // constants together may allow us to merge other constants together if the
  60   // second level constants have initializers which point to the globals that
  61   // were just merged.
  62   while (1) {
  63     // First pass: identify all globals that can be merged together, filling in
  64     // the Replacements vector.  We cannot do the replacement in this pass
  65     // because doing so may cause initializers of other globals to be rewritten,
  66     // invalidating the Constant* pointers in CMap.
  67     //
  68     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
  69          GVI != E; ) {
  70       GlobalVariable *GV = GVI++;
  71
  72       // If this GV is dead, remove it.
  73       GV->removeDeadConstantUsers();
  74       if (GV->use_empty() && GV->hasLocalLinkage()) {
  75         GV->eraseFromParent();
  76         continue;
  77       }
  78
  79       // Only process constants with initializers.
  80       if (GV->isConstant() && GV->hasDefinitiveInitializer()) {
  81         Constant *Init = GV->getInitializer();
  82
  83         // Check to see if the initializer is already known.
  84         GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
  85
  86         if (Slot == 0) {    // Nope, add it to the map.
  87           Slot = GV;
  88         } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
  89           // Make all uses of the duplicate constant use the canonical version.
  90           Replacements.push_back(std::make_pair(GV, Slot));
  91         }
  92       }
  93     }
  94
  95     if (Replacements.empty())
  96       return MadeChange;
  97     CMap.clear();
  98
  99     // Now that we have figured out which replacements must be made, do them all
 100     // now.  This avoid invalidating the pointers in CMap, which are unneeded
 101     // now.
 102     for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
 103       // Eliminate any uses of the dead global...
 104       Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
 105
 106       // Delete the global value from the module...
 107       M.getGlobalList().erase(Replacements[i].first);
 108     }
 109
 110     NumMerged += Replacements.size();
 111     Replacements.clear();
 112   }
 113 }