| //===- ConstantMerge.cpp - Merge duplicate global constants ---------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file was developed by the LLVM research group and is distributed under |
| // the University of Illinois Open Source License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines the interface to a pass that merges duplicate global |
| // constants together into a single constant that is shared. This is useful |
| // because some passes (ie TraceValues) insert a lot of string constants into |
| // the program, regardless of whether or not an existing string is available. |
| // |
| // Algorithm: ConstantMerge is designed to build up a map of available constants |
| // and eliminate duplicates when it is initialized. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "constmerge" |
| #include "llvm/Transforms/IPO.h" |
| #include "llvm/Module.h" |
| #include "llvm/Pass.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Support/Compiler.h" |
| using namespace llvm; |
| |
| STATISTIC(NumMerged, "Number of global constants merged"); |
| |
| namespace { |
| struct VISIBILITY_HIDDEN ConstantMerge : public ModulePass { |
| static const char ID; // Pass identifcation, replacement for typeid |
| ConstantMerge() : ModulePass((intptr_t)&ID) {} |
| |
| // run - For this pass, process all of the globals in the module, |
| // eliminating duplicate constants. |
| // |
| bool runOnModule(Module &M); |
| }; |
| |
| const char ConstantMerge::ID = 0; |
| RegisterPass<ConstantMerge>X("constmerge","Merge Duplicate Global Constants"); |
| } |
| |
| ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); } |
| |
| bool ConstantMerge::runOnModule(Module &M) { |
| // Map unique constant/section pairs to globals. We don't want to merge |
| // globals in different sections. |
| std::map<std::pair<Constant*, std::string>, GlobalVariable*> CMap; |
| |
| // Replacements - This vector contains a list of replacements to perform. |
| std::vector<std::pair<GlobalVariable*, GlobalVariable*> > Replacements; |
| |
| bool MadeChange = false; |
| |
| // Iterate constant merging while we are still making progress. Merging two |
| // constants together may allow us to merge other constants together if the |
| // second level constants have initializers which point to the globals that |
| // were just merged. |
| while (1) { |
| // First pass: identify all globals that can be merged together, filling in |
| // the Replacements vector. We cannot do the replacement in this pass |
| // because doing so may cause initializers of other globals to be rewritten, |
| // invalidating the Constant* pointers in CMap. |
| // |
| for (Module::global_iterator GVI = M.global_begin(), E = M.global_end(); |
| GVI != E; ) { |
| GlobalVariable *GV = GVI++; |
| |
| // If this GV is dead, remove it. |
| GV->removeDeadConstantUsers(); |
| if (GV->use_empty() && GV->hasInternalLinkage()) { |
| GV->eraseFromParent(); |
| continue; |
| } |
| |
| // Only process constants with initializers. |
| if (GV->isConstant() && GV->hasInitializer()) { |
| Constant *Init = GV->getInitializer(); |
| |
| // Check to see if the initializer is already known. |
| GlobalVariable *&Slot = CMap[std::make_pair(Init, GV->getSection())]; |
| |
| if (Slot == 0) { // Nope, add it to the map. |
| Slot = GV; |
| } else if (GV->hasInternalLinkage()) { // Yup, this is a duplicate! |
| // Make all uses of the duplicate constant use the canonical version. |
| Replacements.push_back(std::make_pair(GV, Slot)); |
| } else if (GV->hasInternalLinkage()) { |
| // Make all uses of the duplicate constant use the canonical version. |
| Replacements.push_back(std::make_pair(Slot, GV)); |
| Slot = GV; |
| } |
| } |
| } |
| |
| if (Replacements.empty()) |
| return MadeChange; |
| CMap.clear(); |
| |
| // Now that we have figured out which replacements must be made, do them all |
| // now. This avoid invalidating the pointers in CMap, which are unneeded |
| // now. |
| for (unsigned i = 0, e = Replacements.size(); i != e; ++i) { |
| // Eliminate any uses of the dead global... |
| Replacements[i].first->replaceAllUsesWith(Replacements[i].second); |
| |
| // Delete the global value from the module... |
| M.getGlobalList().erase(Replacements[i].first); |
| } |
| |
| NumMerged += Replacements.size(); |
| Replacements.clear(); |
| } |
| } |