1 //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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/DerivedTypes.h"
23 #include "llvm/Module.h"
24 #include "llvm/Pass.h"
25 #include "llvm/ADT/DenseMap.h"
26 #include "llvm/ADT/Statistic.h"
29 STATISTIC(NumMerged, "Number of global constants merged");
32 struct ConstantMerge : public ModulePass {
33 static char ID; // Pass identification, replacement for typeid
34 ConstantMerge() : ModulePass(&ID) {}
36 // run - For this pass, process all of the globals in the module,
37 // eliminating duplicate constants.
39 bool runOnModule(Module &M);
43 char ConstantMerge::ID = 0;
44 INITIALIZE_PASS(ConstantMerge, "constmerge",
45 "Merge Duplicate Global Constants", false, false);
47 ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
49 bool ConstantMerge::runOnModule(Module &M) {
50 // Map unique constant/section pairs to globals. We don't want to merge
51 // globals in different sections.
52 DenseMap<Constant*, GlobalVariable*> CMap;
54 // Replacements - This vector contains a list of replacements to perform.
55 SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;
57 bool MadeChange = false;
59 // Iterate constant merging while we are still making progress. Merging two
60 // constants together may allow us to merge other constants together if the
61 // second level constants have initializers which point to the globals that
64 // First pass: identify all globals that can be merged together, filling in
65 // the Replacements vector. We cannot do the replacement in this pass
66 // because doing so may cause initializers of other globals to be rewritten,
67 // invalidating the Constant* pointers in CMap.
69 for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
71 GlobalVariable *GV = GVI++;
73 // If this GV is dead, remove it.
74 GV->removeDeadConstantUsers();
75 if (GV->use_empty() && GV->hasLocalLinkage()) {
76 GV->eraseFromParent();
80 // Only process constants with initializers in the default addres space.
81 if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
82 GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty())
85 Constant *Init = GV->getInitializer();
87 // Check to see if the initializer is already known.
88 GlobalVariable *&Slot = CMap[Init];
90 if (Slot == 0) { // Nope, add it to the map.
92 } else if (GV->hasLocalLinkage()) { // Yup, this is a duplicate!
93 // Make all uses of the duplicate constant use the canonical version.
94 Replacements.push_back(std::make_pair(GV, Slot));
98 if (Replacements.empty())
102 // Now that we have figured out which replacements must be made, do them all
103 // now. This avoid invalidating the pointers in CMap, which are unneeded
105 for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
106 // Eliminate any uses of the dead global.
107 Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
109 // Delete the global value from the module.
110 Replacements[i].first->eraseFromParent();
113 NumMerged += Replacements.size();
114 Replacements.clear();