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 "llvm/Support/Compiler.h"
  26 #include <map>
  27 using namespace llvm;
  28
  29 STATISTIC(NumMerged, "Number of global constants merged");
  30
  31 namespace {
  32   struct VISIBILITY_HIDDEN ConstantMerge : public ModulePass {
  33     static char ID; // Pass identification, replacement for typeid
  34     ConstantMerge() : ModulePass(&ID) {}
  35
  36     // run - For this pass, process all of the globals in the module,
  37     // eliminating duplicate constants.
  38     //
  39     bool runOnModule(Module &M);
  40   };
  41 }
  42
  43 char ConstantMerge::ID = 0;
  44 static RegisterPass<ConstantMerge>
  45 X("constmerge", "Merge Duplicate Global Constants");
  46
  47 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
  48
  49 bool ConstantMerge::runOnModule(Module &M) {
  50   // Map unique constant/section pairs to globals.  We don't want to merge
  51   // globals in different sections.
  52   std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
  53
  54   // Replacements - This vector contains a list of replacements to perform.
  55   std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
  56
  57   bool MadeChange = false;
  58
  59   // Iterate constant merging while we are still making progress.  Merging two
  60   // constants together may allow us to merge other constants together if the
  61   // second level constants have initializers which point to the globals that
  62   // were just merged.
  63   while (1) {
  64     // First pass: identify all globals that can be merged together, filling in
  65     // the Replacements vector.  We cannot do the replacement in this pass
  66     // because doing so may cause initializers of other globals to be rewritten,
  67     // invalidating the Constant* pointers in CMap.
  68     //
  69     for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
  70          GVI != E; ) {
  71       GlobalVariable *GV = GVI++;
  72
  73       // If this GV is dead, remove it.
  74       GV->removeDeadConstantUsers();
  75       if (GV->use_empty() && GV->hasLocalLinkage()) {
  76         GV->eraseFromParent();
  77         continue;
  78       }
  79
  80       // Only process constants with initializers.
  81       if (GV->isConstant() && GV->hasInitializer()) {
  82         Constant *Init = GV->getInitializer();
  83
  84         // Check to see if the initializer is already known.
  85         GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
  86
  87         if (Slot == 0) {    // Nope, add it to the map.
  88           Slot = GV;
  89         } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
  90           // Make all uses of the duplicate constant use the canonical version.
  91           Replacements.push_back(std::make_pair(GV, Slot));
  92         }
  93       }
  94     }
  95
  96     if (Replacements.empty())
  97       return MadeChange;
  98     CMap.clear();
  99
 100     // Now that we have figured out which replacements must be made, do them all
 101     // now.  This avoid invalidating the pointers in CMap, which are unneeded
 102     // now.
 103     for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
 104       // Eliminate any uses of the dead global...
 105       Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
 106
 107       // Delete the global value from the module...
 108       M.getGlobalList().erase(Replacements[i].first);
 109     }
 110
 111     NumMerged += Replacements.size();
 112     Replacements.clear();
 113   }
 114 }