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230 lines
8.3 KiB
C++
230 lines
8.3 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 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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//===----------------------------------------------------------------------===//
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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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#define DEBUG_TYPE "constmerge"
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#include "llvm/Transforms/IPO.h"
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#include "llvm/Constants.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/Target/TargetData.h"
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#include "llvm/ADT/DenseMap.h"
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#include "llvm/ADT/PointerIntPair.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/Statistic.h"
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using namespace llvm;
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STATISTIC(NumMerged, "Number of global constants merged");
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namespace {
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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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initializeConstantMergePass(*PassRegistry::getPassRegistry());
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}
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// For this pass, process all of the globals in the module, eliminating
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// duplicate constants.
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bool runOnModule(Module &M);
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// Return true iff we can determine the alignment of this global variable.
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bool hasKnownAlignment(GlobalVariable *GV) const;
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// Return the alignment of the global, including converting the default
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// alignment to a concrete value.
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unsigned getAlignment(GlobalVariable *GV) const;
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const TargetData *TD;
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};
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}
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char ConstantMerge::ID = 0;
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INITIALIZE_PASS(ConstantMerge, "constmerge",
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"Merge Duplicate Global Constants", false, false)
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ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
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/// Find values that are marked as llvm.used.
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static void FindUsedValues(GlobalVariable *LLVMUsed,
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SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
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if (LLVMUsed == 0) return;
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ConstantArray *Inits = dyn_cast<ConstantArray>(LLVMUsed->getInitializer());
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if (Inits == 0) return;
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for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i)
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if (GlobalValue *GV =
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dyn_cast<GlobalValue>(Inits->getOperand(i)->stripPointerCasts()))
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UsedValues.insert(GV);
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}
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// True if A is better than B.
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static bool IsBetterCannonical(const GlobalVariable &A,
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const GlobalVariable &B) {
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if (!A.hasLocalLinkage() && B.hasLocalLinkage())
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return true;
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if (A.hasLocalLinkage() && !B.hasLocalLinkage())
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return false;
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return A.hasUnnamedAddr();
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}
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bool ConstantMerge::hasKnownAlignment(GlobalVariable *GV) const {
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return TD || GV->getAlignment() != 0;
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}
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unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
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if (TD)
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return TD->getPreferredAlignment(GV);
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return GV->getAlignment();
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}
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bool ConstantMerge::runOnModule(Module &M) {
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TD = getAnalysisIfAvailable<TargetData>();
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// Find all the globals that are marked "used". These cannot be merged.
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SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
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FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
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FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
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// Map unique <constants, has-unknown-alignment> pairs to globals. We don't
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// want to merge globals of unknown alignment with those of explicit
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// alignment. If we have TargetData, we always know the alignment.
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DenseMap<PointerIntPair<Constant*, 1, bool>, 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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// were just merged.
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while (1) {
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// First: Find the canonical constants others will be merged with.
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for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
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GVI != E; ) {
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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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continue;
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}
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// Only process constants with initializers in the default address space.
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if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
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GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
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// Don't touch values marked with attribute(used).
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UsedGlobals.count(GV))
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continue;
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// This transformation is legal for weak ODR globals in the sense it
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// doesn't change semantics, but we really don't want to perform it
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// anyway; it's likely to pessimize code generation, and some tools
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// (like the Darwin linker in cases involving CFString) don't expect it.
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if (GV->isWeakForLinker())
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continue;
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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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PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
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GlobalVariable *&Slot = CMap[Pair];
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// If this is the first constant we find or if the old one is local,
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// replace with the current one. If the current is externally visible
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// it cannot be replace, but can be the canonical constant we merge with.
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if (Slot == 0 || IsBetterCannonical(*GV, *Slot))
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Slot = GV;
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}
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// Second: 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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GVI != E; ) {
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GlobalVariable *GV = GVI++;
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// Only process constants with initializers in the default address space.
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if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
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GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
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// Don't touch values marked with attribute(used).
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UsedGlobals.count(GV))
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continue;
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// We can only replace constant with local linkage.
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if (!GV->hasLocalLinkage())
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continue;
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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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PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
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GlobalVariable *Slot = CMap[Pair];
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if (!Slot || Slot == GV)
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continue;
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if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
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continue;
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if (!GV->hasUnnamedAddr())
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Slot->setUnnamedAddr(false);
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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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}
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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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// Bump the alignment if necessary.
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if (Replacements[i].first->getAlignment() ||
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Replacements[i].second->getAlignment()) {
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Replacements[i].second->setAlignment(std::max(
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Replacements[i].first->getAlignment(),
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Replacements[i].second->getAlignment()));
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}
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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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assert(Replacements[i].first->hasLocalLinkage() &&
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"Refusing to delete an externally visible global variable.");
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Replacements[i].first->eraseFromParent();
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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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