ConstantMerge.cpp revision 5e423ede22161ecfb96aeb6bc35745b3f26bd629 
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>
27using namespace llvm;
28
29STATISTIC(NumMerged, "Number of global constants merged");
30
31namespace {
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
43char ConstantMerge::ID = 0;
44static RegisterPass<ConstantMerge>
45X("constmerge", "Merge Duplicate Global Constants");
46
47ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
48
49bool 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->hasDefinitiveInitializer()) {
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}
115