ConstantMerge.cpp revision 7f8897f22e88271cfa114998a4d6088e7c8e8e11 
1//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file was developed by the LLVM research group and is distributed under
6// the University of Illinois Open Source 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#include "llvm/Transforms/IPO.h"
21#include "llvm/Module.h"
22#include "llvm/Pass.h"
23#include "llvm/ADT/Statistic.h"
24using namespace llvm;
25
26namespace {
27  Statistic<> NumMerged("constmerge", "Number of global constants merged");
28
29  struct ConstantMerge : public ModulePass {
30    // run - For this pass, process all of the globals in the module,
31    // eliminating duplicate constants.
32    //
33    bool runOnModule(Module &M);
34  };
35
36  RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
37}
38
39ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
40
41bool ConstantMerge::runOnModule(Module &M) {
42  // Map unique constant/section pairs to globals.  We don't want to merge
43  // globals in different sections.
44  std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
45
46  // Replacements - This vector contains a list of replacements to perform.
47  std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
48
49  bool MadeChange = false;
50
51  // Iterate constant merging while we are still making progress.  Merging two
52  // constants together may allow us to merge other constants together if the
53  // second level constants have initializers which point to the globals that
54  // were just merged.
55  while (1) {
56    // First pass: identify all globals that can be merged together, filling in
57    // the Replacements vector.  We cannot do the replacement in this pass
58    // because doing so may cause initializers of other globals to be rewritten,
59    // invalidating the Constant* pointers in CMap.
60    //
61    for (Module::global_iterator GV = M.global_begin(), E = M.global_end();
62         GV != E; ++GV)
63      // Only process constants with initializers.
64      if (GV->isConstant() && GV->hasInitializer()) {
65        Constant *Init = GV->getInitializer();
66
67        // Check to see if the initializer is already known.
68        GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
69
70        if (Slot == 0) {    // Nope, add it to the map.
71          Slot = GV;
72        } else if (GV->hasInternalLinkage()) {    // Yup, this is a duplicate!
73          // Make all uses of the duplicate constant use the canonical version.
74          Replacements.push_back(std::make_pair(GV, Slot));
75        } else if (GV->hasInternalLinkage()) {
76          // Make all uses of the duplicate constant use the canonical version.
77          Replacements.push_back(std::make_pair(Slot, GV));
78          Slot = GV;
79        }
80      }
81
82    if (Replacements.empty())
83      return MadeChange;
84    CMap.clear();
85
86    // Now that we have figured out which replacements must be made, do them all
87    // now.  This avoid invalidating the pointers in CMap, which are unneeded
88    // now.
89    for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
90      // Eliminate any uses of the dead global...
91      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
92
93      // Delete the global value from the module...
94      M.getGlobalList().erase(Replacements[i].first);
95    }
96
97    NumMerged += Replacements.size();
98    Replacements.clear();
99  }
100}
101