ConstantMerge.cpp revision 249ddbfb29dfc2335f98984878c52b2c77703417 
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((intptr_t)&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  char ConstantMerge::ID = 0;
43  RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
44}
45
46ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
47
48bool 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->hasInternalLinkage()) {
75        GV->eraseFromParent();
76        continue;
77      }
78
79      // Only process constants with initializers.
80      if (GV->isConstant() && GV->hasInitializer()) {
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->hasInternalLinkage()) {    // 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}
114