1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
3 // The LLVM Compiler Infrastructure
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.
8 //===----------------------------------------------------------------------===//
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.
15 // Algorithm: ConstantMerge is designed to build up a map of available constants
16 // and eliminate duplicates when it is initialized.
18 //===----------------------------------------------------------------------===//
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"
28 STATISTIC(NumMerged, "Number of global constants merged");
31 struct VISIBILITY_HIDDEN ConstantMerge : public ModulePass {
32 static char ID; // Pass identification, replacement for typeid
33 ConstantMerge() : ModulePass((intptr_t)&ID) {}
35 // run - For this pass, process all of the globals in the module,
36 // eliminating duplicate constants.
38 bool runOnModule(Module &M);
41 char ConstantMerge::ID = 0;
42 RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
45 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
47 bool 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;
52 // Replacements - This vector contains a list of replacements to perform.
53 std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
55 bool MadeChange = false;
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
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.
67 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
69 GlobalVariable *GV = GVI++;
71 // If this GV is dead, remove it.
72 GV->removeDeadConstantUsers();
73 if (GV->use_empty() && GV->hasInternalLinkage()) {
74 GV->eraseFromParent();
78 // Only process constants with initializers.
79 if (GV->isConstant() && GV->hasInitializer()) {
80 Constant *Init = GV->getInitializer();
82 // Check to see if the initializer is already known.
83 GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
85 if (Slot == 0) { // Nope, add it to the map.
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));
98 if (Replacements.empty())
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
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);
109 // Delete the global value from the module...
110 M.getGlobalList().erase(Replacements[i].first);
113 NumMerged += Replacements.size();
114 Replacements.clear();