Imported Upstream version 6.0.0.172

Former-commit-id: f3cc9b82f3e5bd8f0fd3ebc098f789556b44e9cd

external/llvm/lib/Transforms/IPO/ConstantMerge.cpp

@@ -0,0 +1,260 @@
+//===- 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.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/ConstantMerge.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/GlobalValue.h"
+#include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Transforms/IPO.h"
+#include <algorithm>
+#include <cassert>
+#include <utility>
+
+using namespace llvm;
+
+#define DEBUG_TYPE "constmerge"
+
+STATISTIC(NumMerged, "Number of global constants merged");
+
+/// 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);
+  }
+}
+
+// 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.hasGlobalUnnamedAddr();
+}
+
+static bool hasMetadataOtherThanDebugLoc(const GlobalVariable *GV) {
+  SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
+  GV->getAllMetadata(MDs);
+  for (const auto &V : MDs)
+    if (V.first != LLVMContext::MD_dbg)
+      return true;
+  return false;
+}
+
+static void copyDebugLocMetadata(const GlobalVariable *From,
+                                 GlobalVariable *To) {
+  SmallVector<DIGlobalVariableExpression *, 1> MDs;
+  From->getDebugInfo(MDs);
+  for (auto MD : MDs)
+    To->addDebugInfo(MD);
+}
+
+static unsigned getAlignment(GlobalVariable *GV) {
+  unsigned Align = GV->getAlignment();
+  if (Align)
+    return Align;
+  return GV->getParent()->getDataLayout().getPreferredAlignment(GV);
+}
+
+static bool mergeConstants(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 constants to globals.
+  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 (true) {
+    // 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;
+
+      // Don't touch globals with metadata other then !dbg.
+      if (hasMetadataOtherThanDebugLoc(GV))
+        continue;
+
+      Constant *Init = GV->getInitializer();
+
+      // Check to see if the initializer is already known.
+      GlobalVariable *&Slot = CMap[Init];
+
+      // 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++;
+
+      // 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.
+      GlobalVariable *Slot = CMap[Init];
+
+      if (!Slot || Slot == GV)
+        continue;
+
+      if (!Slot->hasGlobalUnnamedAddr() && !GV->hasGlobalUnnamedAddr())
+        continue;
+
+      if (hasMetadataOtherThanDebugLoc(GV))
+        continue;
+
+      if (!GV->hasGlobalUnnamedAddr())
+        Slot->setUnnamedAddr(GlobalValue::UnnamedAddr::None);
+
+      // 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)));
+      }
+
+      copyDebugLocMetadata(Replacements[i].first, 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();
+    }
+
+    NumMerged += Replacements.size();
+    Replacements.clear();
+  }
+}
+
+PreservedAnalyses ConstantMergePass::run(Module &M, ModuleAnalysisManager &) {
+  if (!mergeConstants(M))
+    return PreservedAnalyses::all();
+  return PreservedAnalyses::none();
+}
+
+namespace {
+
+struct ConstantMergeLegacyPass : public ModulePass {
+  static char ID; // Pass identification, replacement for typeid
+
+  ConstantMergeLegacyPass() : ModulePass(ID) {
+    initializeConstantMergeLegacyPassPass(*PassRegistry::getPassRegistry());
+  }
+
+  // For this pass, process all of the globals in the module, eliminating
+  // duplicate constants.
+  bool runOnModule(Module &M) override {
+    if (skipModule(M))
+      return false;
+    return mergeConstants(M);
+  }
+};
+
+} // end anonymous namespace
+
+char ConstantMergeLegacyPass::ID = 0;
+
+INITIALIZE_PASS(ConstantMergeLegacyPass, "constmerge",
+                "Merge Duplicate Global Constants", false, false)
+
+ModulePass *llvm::createConstantMergePass() {
+  return new ConstantMergeLegacyPass();
+}