3 files changed, 109 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/IPO/CMakeLists.txt b/llvm/lib/Transforms/IPO/CMakeLists.txt
index 3df17b920a9..336dac45e13 100644
--- a/llvm/lib/Transforms/IPO/CMakeLists.txt
+++ b/llvm/lib/Transforms/IPO/CMakeLists.txt
@@ -3,6 +3,7 @@ add_llvm_library(LLVMipo
   BarrierNoopPass.cpp
   ConstantMerge.cpp
   DeadArgumentElimination.cpp
+  ElimAvailExtern.cpp
   ExtractGV.cpp
   FunctionAttrs.cpp
   GlobalDCE.cpp
diff --git a/llvm/lib/Transforms/IPO/ElimAvailExtern.cpp b/llvm/lib/Transforms/IPO/ElimAvailExtern.cpp
new file mode 100644
index 00000000000..e70ab871b88
--- /dev/null
+++ b/llvm/lib/Transforms/IPO/ElimAvailExtern.cpp
@@ -0,0 +1,96 @@
+//===-- ElimAvailExtern.cpp - DCE unreachable internal functions ----------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This transform is designed to eliminate available external global
+// definitions from the program, turning them into declarations.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Transforms/Utils/CtorUtils.h"
+#include "llvm/Transforms/Utils/GlobalStatus.h"
+#include "llvm/Pass.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "elim-avail-extern"
+
+STATISTIC(NumAliases  , "Number of global aliases removed");
+STATISTIC(NumFunctions, "Number of functions removed");
+STATISTIC(NumVariables, "Number of global variables removed");
+
+namespace {
+  struct EliminateAvailableExternally : public ModulePass {
+    static char ID; // Pass identification, replacement for typeid
+    EliminateAvailableExternally() : ModulePass(ID) {
+      initializeEliminateAvailableExternallyPass(
+          *PassRegistry::getPassRegistry());
+    }
+
+    // run - Do the EliminateAvailableExternally pass on the specified module,
+    // optionally updating the specified callgraph to reflect the changes.
+    //
+    bool runOnModule(Module &M) override;
+  };
+}
+
+char EliminateAvailableExternally::ID = 0;
+INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
+                "Eliminate Available Externally Globals", false, false)
+
+ModulePass *llvm::createEliminateAvailableExternallyPass() {
+  return new EliminateAvailableExternally();
+}
+
+bool EliminateAvailableExternally::runOnModule(Module &M) {
+  bool Changed = false;
+
+  // Drop initializers of available externally global variables.
+  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I) {
+    if (!I->hasAvailableExternallyLinkage())
+      continue;
+    if (I->hasInitializer()) {
+      Constant *Init = I->getInitializer();
+      I->setInitializer(nullptr);
+      if (isSafeToDestroyConstant(Init))
+        Init->destroyConstant();
+    }
+    I->removeDeadConstantUsers();
+    I->setLinkage(GlobalValue::ExternalLinkage);
+    NumVariables++;
+  }
+
+  // Drop the bodies of available externally functions.
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
+    if (!I->hasAvailableExternallyLinkage())
+      continue;
+    if (!I->isDeclaration())
+      // This will set the linkage to external
+      I->deleteBody();
+    I->removeDeadConstantUsers();
+    NumFunctions++;
+  }
+
+  // Drop targets of available externally aliases.
+  for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
+       ++I) {
+    if (!I->hasAvailableExternallyLinkage())
+      continue;
+    I->setAliasee(nullptr);
+    I->removeDeadConstantUsers();
+    I->setLinkage(GlobalValue::ExternalLinkage);
+    NumAliases++;
+  }
+
+  return Changed;
+}
diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index 963f1bb13aa..a067a4b4525 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -105,6 +105,7 @@ PassManagerBuilder::PassManagerBuilder() {
     VerifyInput = false;
     VerifyOutput = false;
     MergeFunctions = false;
+    PrepareForLTO = false;
 }
 
 PassManagerBuilder::~PassManagerBuilder() {
@@ -401,6 +402,17 @@ void PassManagerBuilder::populateModulePassManager(
     // GlobalOpt already deletes dead functions and globals, at -O2 try a
     // late pass of GlobalDCE.  It is capable of deleting dead cycles.
     if (OptLevel > 1) {
+      if (!PrepareForLTO) {
+        // Remove avail extern fns and globals definitions if we aren't
+        // compiling an object file for later LTO. For LTO we want to preserve
+        // these so they are eligible for inlining at link-time. Note if they
+        // are unreferenced they will be removed by GlobalDCE below, so
+        // this only impacts referenced available externally globals.
+        // Eventually they will be suppressed during codegen, but eliminating
+        // here enables more opportunity for GlobalDCE as it may make
+        // globals referenced by available external functions dead.
+        MPM.add(createEliminateAvailableExternallyPass());
+      }
       MPM.add(createGlobalDCEPass());         // Remove dead fns and globals.
       MPM.add(createConstantMergePass());     // Merge dup global constants
     }