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"
28 Statistic<> NumMerged("constmerge", "Number of global constants merged");
30 struct ConstantMerge : public Pass {
31 // run - For this pass, process all of the globals in the module,
32 // eliminating duplicate constants.
37 RegisterOpt<ConstantMerge> X("constmerge","Merge Duplicate Global Constants");
40 Pass *createConstantMergePass() { return new ConstantMerge(); }
42 bool ConstantMerge::run(Module &M) {
43 std::map<Constant*, GlobalVariable*> CMap;
44 bool MadeChanges = false;
46 for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
47 // Only process constants with initializers
48 if (GV->isConstant() && GV->hasInitializer()) {
49 Constant *Init = GV->getInitializer();
51 // Check to see if the initializer is already known...
52 std::map<Constant*, GlobalVariable*>::iterator I = CMap.find(Init);
54 if (I == CMap.end()) { // Nope, add it to the map
55 CMap.insert(I, std::make_pair(Init, GV));
56 } else if (GV->hasInternalLinkage()) { // Yup, this is a duplicate!
57 // Make all uses of the duplicate constant use the canonical version...
58 GV->replaceAllUsesWith(I->second);
60 // Delete the global value from the module... and back up iterator to
61 // not skip the next global...
62 GV = --M.getGlobalList().erase(GV);
66 } else if (I->second->hasInternalLinkage()) {
67 // Make all uses of the duplicate constant use the canonical version...
68 I->second->replaceAllUsesWith(GV);
70 // Delete the global value from the module... and back up iterator to
71 // not skip the next global...
72 M.getGlobalList().erase(I->second);
83 } // End llvm namespace