bool ConstantMerge::runOnModule(Module &M) { // Find all the globals that are marked "used". These cannot be merged. SmallPtrSet<const GlobalValue*, 8> UsedGlobals; FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals); FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals); // Map unique constant/section pairs to globals. We don't want to merge // globals in different sections. DenseMap<Constant*, GlobalVariable*> CMap; // Replacements - This vector contains a list of replacements to perform. SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> 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->hasLocalLinkage()) { GV->eraseFromParent(); continue; } // Only process constants with initializers in the default addres space. if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() || GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty() || // Don't touch values marked with attribute(used). UsedGlobals.count(GV)) continue; Constant *Init = GV->getInitializer(); // Check to see if the initializer is already known. GlobalVariable *&Slot = CMap[Init]; if (Slot == 0) { // Nope, add it to the map. Slot = GV; } else if (GV->hasLocalLinkage()) { // Yup, this is a duplicate! // Make all uses of the duplicate constant use the canonical version. Replacements.push_back(std::make_pair(GV, Slot)); } } 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. Replacements[i].first->eraseFromParent(); } NumMerged += Replacements.size(); Replacements.clear(); } }