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 bool MadeChange = false;
49 // Iterate constant merging while we are still making progress. Merging two
50 // constants together may allow us to merge other constants together if the
51 // second level constants have initializers which point to the globals that
54 // First pass: identify all globals that can be merged together, filling in
55 // the Replacements vector. We cannot do the replacement in this pass
56 // because doing so may cause initializers of other globals to be rewritten,
57 // invalidating the Constant* pointers in CMap.
59 for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
60 // Only process constants with initializers
61 if (GV->isConstant() && GV->hasInitializer()) {
62 Constant *Init = GV->getInitializer();
64 // Check to see if the initializer is already known...
65 std::map<Constant*, GlobalVariable*>::iterator I = CMap.find(Init);
67 if (I == CMap.end()) { // Nope, add it to the map
68 CMap.insert(I, std::make_pair(Init, GV));
69 } else if (GV->hasInternalLinkage()) { // Yup, this is a duplicate!
70 // Make all uses of the duplicate constant use the canonical version.
71 Replacements.push_back(std::make_pair(GV, I->second));
72 } else if (I->second->hasInternalLinkage()) {
73 // Make all uses of the duplicate constant use the canonical version.
74 Replacements.push_back(std::make_pair(I->second, GV));
79 if (Replacements.empty())
83 // Now that we have figured out which replacements must be made, do them all
84 // now. This avoid invalidating the pointers in CMap, which are unneeded
86 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
87 // Eliminate any uses of the dead global...
88 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
90 // Delete the global value from the module...
91 M.getGlobalList().erase(Replacements[i].first);
94 NumMerged += Replacements.size();