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 "Support/Statistic.h"
27 Statistic<> NumMerged("constmerge", "Number of global constants merged");
29 struct ConstantMerge : public Pass {
30 // run - For this pass, process all of the globals in the module,
31 // eliminating duplicate constants.
36 RegisterOpt<ConstantMerge> X("constmerge","Merge Duplicate Global Constants");
39 Pass *llvm::createConstantMergePass() { return new ConstantMerge(); }
41 bool ConstantMerge::run(Module &M) {
42 std::map<Constant*, GlobalVariable*> CMap;
44 // Replacements - This vector contains a list of replacements to perform.
45 std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
47 // First pass: identify all globals that can be merged together, filling in
48 // the Replacements vector. We cannot do the replacement in this pass because
49 // doing so may cause initializers of other globals to be rewritten,
50 // invalidating the Constant* pointers in CMap.
52 for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
53 // Only process constants with initializers
54 if (GV->isConstant() && GV->hasInitializer()) {
55 Constant *Init = GV->getInitializer();
57 // Check to see if the initializer is already known...
58 std::map<Constant*, GlobalVariable*>::iterator I = CMap.find(Init);
60 if (I == CMap.end()) { // Nope, add it to the map
61 CMap.insert(I, std::make_pair(Init, GV));
62 } else if (GV->hasInternalLinkage()) { // Yup, this is a duplicate!
63 // Make all uses of the duplicate constant use the canonical version...
64 Replacements.push_back(std::make_pair(GV, I->second));
65 } else if (I->second->hasInternalLinkage()) {
66 // Make all uses of the duplicate constant use the canonical version...
67 Replacements.push_back(std::make_pair(I->second, GV));
72 if (Replacements.empty()) return false;
75 // Now that we have figured out which replacements must be made, do them all
76 // now. This avoid invalidating the pointers in CMap, which are unneeded now.
77 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
78 // Eliminate any uses of the dead global...
79 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
81 // Delete the global value from the module...
82 M.getGlobalList().erase(Replacements[i].first);
85 NumMerged += Replacements.size();