//===- ConstantMerge.cpp - Merge duplicate global constants ---------------===//
//
+// The LLVM Compiler Infrastructure
+//
+// This file 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
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/IPO.h"
-#include "llvm/Module.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/PointerIntPair.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
-#include "Support/Statistic.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "constmerge"
+
+STATISTIC(NumMerged, "Number of global constants merged");
namespace {
- Statistic<> NumMerged("constmerge", "Number of global constants merged");
+ struct ConstantMerge : public ModulePass {
+ static char ID; // Pass identification, replacement for typeid
+ ConstantMerge() : ModulePass(ID) {
+ initializeConstantMergePass(*PassRegistry::getPassRegistry());
+ }
+
+ // For this pass, process all of the globals in the module, eliminating
+ // duplicate constants.
+ bool runOnModule(Module &M) override;
+
+ // Return true iff we can determine the alignment of this global variable.
+ bool hasKnownAlignment(GlobalVariable *GV) const;
- struct ConstantMerge : public Pass {
- // run - For this pass, process all of the globals in the module,
- // eliminating duplicate constants.
- //
- bool run(Module &M);
+ // Return the alignment of the global, including converting the default
+ // alignment to a concrete value.
+ unsigned getAlignment(GlobalVariable *GV) const;
+
+ const DataLayout *DL;
};
+}
+
+char ConstantMerge::ID = 0;
+INITIALIZE_PASS(ConstantMerge, "constmerge",
+ "Merge Duplicate Global Constants", false, false)
- RegisterOpt<ConstantMerge> X("constmerge","Merge Duplicate Global Constants");
+ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
+
+
+
+/// Find values that are marked as llvm.used.
+static void FindUsedValues(GlobalVariable *LLVMUsed,
+ SmallPtrSetImpl<const GlobalValue*> &UsedValues) {
+ if (!LLVMUsed) return;
+ ConstantArray *Inits = cast<ConstantArray>(LLVMUsed->getInitializer());
+
+ for (unsigned i = 0, e = Inits->getNumOperands(); i != e; ++i) {
+ Value *Operand = Inits->getOperand(i)->stripPointerCastsNoFollowAliases();
+ GlobalValue *GV = cast<GlobalValue>(Operand);
+ UsedValues.insert(GV);
+ }
}
-Pass *createConstantMergePass() { return new ConstantMerge(); }
+// True if A is better than B.
+static bool IsBetterCanonical(const GlobalVariable &A,
+ const GlobalVariable &B) {
+ if (!A.hasLocalLinkage() && B.hasLocalLinkage())
+ return true;
+
+ if (A.hasLocalLinkage() && !B.hasLocalLinkage())
+ return false;
+
+ return A.hasUnnamedAddr();
+}
+
+bool ConstantMerge::hasKnownAlignment(GlobalVariable *GV) const {
+ return DL || GV->getAlignment() != 0;
+}
+
+unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
+ unsigned Align = GV->getAlignment();
+ if (Align)
+ return Align;
+ if (DL)
+ return DL->getPreferredAlignment(GV);
+ return 0;
+}
+bool ConstantMerge::runOnModule(Module &M) {
+ DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
+ DL = DLP ? &DLP->getDataLayout() : nullptr;
-bool ConstantMerge::run(Module &M) {
- std::map<Constant*, GlobalVariable*> CMap;
- bool MadeChanges = false;
+ // 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);
- for (Module::giterator GV = M.gbegin(), E = M.gend(); GV != E; ++GV)
- // Only process constants with initializers
- if (GV->isConstant() && GV->hasInitializer()) {
+ // Map unique <constants, has-unknown-alignment> pairs to globals. We don't
+ // want to merge globals of unknown alignment with those of explicit
+ // alignment. If we have DataLayout, we always know the alignment.
+ DenseMap<PointerIntPair<Constant*, 1, bool>, 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: Find the canonical constants others will be merged with.
+ 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 address space.
+ if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
+ GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
+ // Don't touch values marked with attribute(used).
+ UsedGlobals.count(GV))
+ continue;
+
+ // This transformation is legal for weak ODR globals in the sense it
+ // doesn't change semantics, but we really don't want to perform it
+ // anyway; it's likely to pessimize code generation, and some tools
+ // (like the Darwin linker in cases involving CFString) don't expect it.
+ if (GV->isWeakForLinker())
+ continue;
+
Constant *Init = GV->getInitializer();
- // Check to see if the initializer is already known...
- std::map<Constant*, GlobalVariable*>::iterator I = CMap.find(Init);
+ // Check to see if the initializer is already known.
+ PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
+ GlobalVariable *&Slot = CMap[Pair];
+
+ // If this is the first constant we find or if the old one is local,
+ // replace with the current one. If the current is externally visible
+ // it cannot be replace, but can be the canonical constant we merge with.
+ if (!Slot || IsBetterCanonical(*GV, *Slot))
+ Slot = GV;
+ }
+
+ // Second: 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 (I == CMap.end()) { // Nope, add it to the map
- CMap.insert(I, std::make_pair(Init, GV));
- } else { // Yup, this is a duplicate!
- // Make all uses of the duplicate constant use the canonical version...
- GV->replaceAllUsesWith(I->second);
+ // Only process constants with initializers in the default address space.
+ if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
+ GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
+ // Don't touch values marked with attribute(used).
+ UsedGlobals.count(GV))
+ continue;
- // Delete the global value from the module... and back up iterator to
- // not skip the next global...
- GV = --M.getGlobalList().erase(GV);
+ // We can only replace constant with local linkage.
+ if (!GV->hasLocalLinkage())
+ continue;
- ++NumMerged;
- MadeChanges = true;
+ Constant *Init = GV->getInitializer();
+
+ // Check to see if the initializer is already known.
+ PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
+ GlobalVariable *Slot = CMap[Pair];
+
+ if (!Slot || Slot == GV)
+ continue;
+
+ if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
+ continue;
+
+ if (!GV->hasUnnamedAddr())
+ Slot->setUnnamedAddr(false);
+
+ // 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) {
+ // Bump the alignment if necessary.
+ if (Replacements[i].first->getAlignment() ||
+ Replacements[i].second->getAlignment()) {
+ Replacements[i].second->setAlignment(
+ std::max(getAlignment(Replacements[i].first),
+ getAlignment(Replacements[i].second)));
}
+
+ // Eliminate any uses of the dead global.
+ Replacements[i].first->replaceAllUsesWith(Replacements[i].second);
+
+ // Delete the global value from the module.
+ assert(Replacements[i].first->hasLocalLinkage() &&
+ "Refusing to delete an externally visible global variable.");
+ Replacements[i].first->eraseFromParent();
}
- return MadeChanges;
+ NumMerged += Replacements.size();
+ Replacements.clear();
+ }
}
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