ConstantMerge.cpp revision 081ce940e7351e90fff829320b7dc6738a6b3815 
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"
26using namespace llvm;
27
28STATISTIC(NumMerged, "Number of global constants merged");
29
30namespace {
31  struct VISIBILITY_HIDDEN ConstantMerge : public ModulePass {
32    static char ID; // Pass identification, replacement for typeid
33    ConstantMerge() : ModulePass((intptr_t)&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  char ConstantMerge::ID = 0;
42  RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
43}
44
45ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
46
47bool ConstantMerge::runOnModule(Module &M) {
48  // Map unique constant/section pairs to globals.  We don't want to merge
49  // globals in different sections.
50  std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
51
52  // Replacements - This vector contains a list of replacements to perform.
53  std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
54
55  bool MadeChange = false;
56
57  // Iterate constant merging while we are still making progress.  Merging two
58  // constants together may allow us to merge other constants together if the
59  // second level constants have initializers which point to the globals that
60  // were just merged.
61  while (1) {
62    // First pass: identify all globals that can be merged together, filling in
63    // the Replacements vector.  We cannot do the replacement in this pass
64    // because doing so may cause initializers of other globals to be rewritten,
65    // invalidating the Constant* pointers in CMap.
66    //
67    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
68         GVI != E; ) {
69      GlobalVariable *GV = GVI++;
70
71      // If this GV is dead, remove it.
72      GV->removeDeadConstantUsers();
73      if (GV->use_empty() && GV->hasInternalLinkage()) {
74        GV->eraseFromParent();
75        continue;
76      }
77
78      // Only process constants with initializers.
79      if (GV->isConstant() && GV->hasInitializer()) {
80        Constant *Init = GV->getInitializer();
81
82        // Check to see if the initializer is already known.
83        GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
84
85        if (Slot == 0) {    // Nope, add it to the map.
86          Slot = GV;
87        } else if (GV->hasInternalLinkage()) {    // Yup, this is a duplicate!
88          // Make all uses of the duplicate constant use the canonical version.
89          Replacements.push_back(std::make_pair(GV, Slot));
90        } else if (GV->hasInternalLinkage()) {
91          // Make all uses of the duplicate constant use the canonical version.
92          Replacements.push_back(std::make_pair(Slot, GV));
93          Slot = GV;
94        }
95      }
96    }
97
98    if (Replacements.empty())
99      return MadeChange;
100    CMap.clear();
101
102    // Now that we have figured out which replacements must be made, do them all
103    // now.  This avoid invalidating the pointers in CMap, which are unneeded
104    // now.
105    for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
106      // Eliminate any uses of the dead global...
107      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
108
109      // Delete the global value from the module...
110      M.getGlobalList().erase(Replacements[i].first);
111    }
112
113    NumMerged += Replacements.size();
114    Replacements.clear();
115  }
116}
117