ConstantMerge.cpp revision 7569322765651f19eea0609fb082e6b267d5d2b5 
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/DerivedTypes.h"
23#include "llvm/Module.h"
24#include "llvm/Pass.h"
25#include "llvm/ADT/DenseMap.h"
26#include "llvm/ADT/Statistic.h"
27using namespace llvm;
28
29STATISTIC(NumMerged, "Number of global constants merged");
30
31namespace {
32  struct 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;
44INITIALIZE_PASS(ConstantMerge, "constmerge",
45                "Merge Duplicate Global Constants", false, false);
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  DenseMap<Constant*, GlobalVariable*> CMap;
53
54  // Replacements - This vector contains a list of replacements to perform.
55  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> 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 in the default addres space.
81      if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
82          GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty())
83        continue;
84
85      Constant *Init = GV->getInitializer();
86
87      // Check to see if the initializer is already known.
88      GlobalVariable *&Slot = CMap[Init];
89
90      if (Slot == 0) {    // Nope, add it to the map.
91        Slot = GV;
92      } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
93        // Make all uses of the duplicate constant use the canonical version.
94        Replacements.push_back(std::make_pair(GV, Slot));
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      Replacements[i].first->eraseFromParent();
111    }
112
113    NumMerged += Replacements.size();
114    Replacements.clear();
115  }
116}
117