--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/Transforms/IPO/ConstantMerge.cpp	Wed May 15 06:43:32 2013 +0900
@@ -0,0 +1,230 @@
+//===- 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
+// 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/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"
+using namespace llvm;
+
+STATISTIC(NumMerged, "Number of global constants merged");
+
+namespace {
+  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);
+
+    // Return true iff we can determine the alignment of this global variable.
+    bool hasKnownAlignment(GlobalVariable *GV) const;
+
+    // Return the alignment of the global, including converting the default
+    // alignment to a concrete value.
+    unsigned getAlignment(GlobalVariable *GV) const;
+
+    const DataLayout *TD;
+  };
+}
+
+char ConstantMerge::ID = 0;
+INITIALIZE_PASS(ConstantMerge, "constmerge",
+                "Merge Duplicate Global Constants", false, false)
+
+ModulePass *llvm::createConstantMergePass() { return new ConstantMerge(); }
+
+
+
+/// Find values that are marked as llvm.used.
+static void FindUsedValues(GlobalVariable *LLVMUsed,
+                           SmallPtrSet<const GlobalValue*, 8> &UsedValues) {
+  if (LLVMUsed == 0) 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);
+  }
+}
+
+// True if A is better than B.
+static bool IsBetterCannonical(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 TD || GV->getAlignment() != 0;
+}
+
+unsigned ConstantMerge::getAlignment(GlobalVariable *GV) const {
+  if (TD)
+    return TD->getPreferredAlignment(GV);
+  return GV->getAlignment();
+}
+
+bool ConstantMerge::runOnModule(Module &M) {
+  TD = getAnalysisIfAvailable<DataLayout>();
+
+  // 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 <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.
+      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 == 0 || IsBetterCannonical(*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++;
+
+      // 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;
+
+      // We can only replace constant with local linkage.
+      if (!GV->hasLocalLinkage())
+        continue;
+
+      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(
+            Replacements[i].first->getAlignment(),
+            Replacements[i].second->getAlignment()));
+      }
+
+      // 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();
+    }
+
+    NumMerged += Replacements.size();
+    Replacements.clear();
+  }
+}