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//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
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// The LLVM Compiler Infrastructure
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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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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// 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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#define DEBUG_TYPE "constmerge"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/Statistic.h"
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STATISTIC(NumMerged, "Number of global constants merged");
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struct ConstantMerge : public ModulePass {
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static char ID; // Pass identification, replacement for typeid
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ConstantMerge() : ModulePass(&ID) {}
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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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bool runOnModule(Module &M);
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char ConstantMerge::ID = 0;
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static RegisterPass<ConstantMerge>
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X("constmerge", "Merge Duplicate Global Constants");
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ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
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bool ConstantMerge::runOnModule(Module &M) {
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// Map unique constant/section pairs to globals. We don't want to merge
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// globals in different sections.
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DenseMap<Constant*, GlobalVariable*> CMap;
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// Replacements - This vector contains a list of replacements to perform.
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SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> 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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// 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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for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
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GlobalVariable *GV = GVI++;
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// If this GV is dead, remove it.
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GV->removeDeadConstantUsers();
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if (GV->use_empty() && GV->hasLocalLinkage()) {
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GV->eraseFromParent();
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// Only process constants with initializers in the default addres space.
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if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
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GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty())
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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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GlobalVariable *&Slot = CMap[Init];
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if (Slot == 0) { // Nope, add it to the map.
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} else if (GV->hasLocalLinkage()) { // 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, Slot));
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if (Replacements.empty())
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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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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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Replacements[i].first->eraseFromParent();
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NumMerged += Replacements.size();
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Replacements.clear();