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 #include "llvm/Transforms/IPO.h"
21 #include "llvm/Module.h"
22 #include "llvm/Pass.h"
23 #include "llvm/ADT/Statistic.h"
27 Statistic<> NumMerged("constmerge", "Number of global constants merged");
29 struct ConstantMerge : public ModulePass {
30 // run - For this pass, process all of the globals in the module,
31 // eliminating duplicate constants.
33 bool runOnModule(Module &M);
36 RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
39 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
41 bool 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;
46 // Replacements - This vector contains a list of replacements to perform.
47 std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
49 bool MadeChange = false;
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
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.
61 for (Module::global_iterator GV = M.global_begin(), E = M.global_end();
63 // Only process constants with initializers.
64 if (GV->isConstant() && GV->hasInitializer()) {
65 Constant *Init = GV->getInitializer();
67 // Check to see if the initializer is already known.
68 GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
70 if (Slot == 0) { // Nope, add it to the map.
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));
82 if (Replacements.empty())
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
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);
93 // Delete the global value from the module...
94 M.getGlobalList().erase(Replacements[i].first);
97 NumMerged += Replacements.size();