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