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"
27 STATISTIC(NumMerged, "Number of global constants merged");
30 struct ConstantMerge : public ModulePass {
31 // run - For this pass, process all of the globals in the module,
32 // eliminating duplicate constants.
34 bool runOnModule(Module &M);
37 RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants");
40 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
42 bool ConstantMerge::runOnModule(Module &M) {
43 // Map unique constant/section pairs to globals. We don't want to merge
44 // globals in different sections.
45 std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap;
47 // Replacements - This vector contains a list of replacements to perform.
48 std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements;
50 bool MadeChange = false;
52 // Iterate constant merging while we are still making progress. Merging two
53 // constants together may allow us to merge other constants together if the
54 // second level constants have initializers which point to the globals that
57 // First pass: identify all globals that can be merged together, filling in
58 // the Replacements vector. We cannot do the replacement in this pass
59 // because doing so may cause initializers of other globals to be rewritten,
60 // invalidating the Constant* pointers in CMap.
62 for (Module::global_iterator GV = M.global_begin(), E = M.global_end();
64 // Only process constants with initializers.
65 if (GV->isConstant() && GV->hasInitializer()) {
66 Constant *Init = GV->getInitializer();
68 // Check to see if the initializer is already known.
69 GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())];
71 if (Slot == 0) { // Nope, add it to the map.
73 } else if (GV->hasInternalLinkage()) { // Yup, this is a duplicate!
74 // Make all uses of the duplicate constant use the canonical version.
75 Replacements.push_back(std::make_pair(GV, Slot));
76 } else if (GV->hasInternalLinkage()) {
77 // Make all uses of the duplicate constant use the canonical version.
78 Replacements.push_back(std::make_pair(Slot, GV));
83 if (Replacements.empty())
87 // Now that we have figured out which replacements must be made, do them all
88 // now. This avoid invalidating the pointers in CMap, which are unneeded
90 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
91 // Eliminate any uses of the dead global...
92 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
94 // Delete the global value from the module...
95 M.getGlobalList().erase(Replacements[i].first);
98 NumMerged += Replacements.size();