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//===- 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 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);
};
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();
}
}
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