Add new EliminateAvailableExternally module pass, which is performed in

O2 compiles just before GlobalDCE, unless we are preparing for LTO.

This pass eliminates available externally globals (turning them into
declarations), regardless of whether they are dead/unreferenced, since
we are guaranteed to have a copy available elsewhere at link time.
This enables additional opportunities for GlobalDCE.

If we are preparing for LTO (e.g. a -flto -c compile), the pass is not
included as we want to preserve available externally functions for possible
link time inlining. The FE indicates whether we are doing an -flto compile
via the new PrepareForLTO flag on the PassManagerBuilder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239480 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/include/llvm/InitializePasses.h b/include/llvm/InitializePasses.h
index 4f95c886800feea870c5b0eeb2506d89154ea098..f6b497dc78ce00147d47ff62bff989b4ad9f63c0 100644 (file)
--- a/include/llvm/InitializePasses.h
+++ b/include/llvm/InitializePasses.h
@@ -130,6 +130,7 @@ void initializeSanitizerCoverageModulePass(PassRegistry&);
 void initializeDataFlowSanitizerPass(PassRegistry&);
 void initializeScalarizerPass(PassRegistry&);
 void initializeEarlyCSELegacyPassPass(PassRegistry &);
+void initializeEliminateAvailableExternallyPass(PassRegistry&);
 void initializeExpandISelPseudosPass(PassRegistry&);
 void initializeFunctionAttrsPass(PassRegistry&);
 void initializeGCMachineCodeAnalysisPass(PassRegistry&);

diff --git a/include/llvm/Transforms/IPO.h b/include/llvm/Transforms/IPO.h
index fbd999cbc946968c46f6f558ad708e9decf82ac2..2ea47301bb4cadea42dd6a200c6f1193a97a4adb 100644 (file)
--- a/include/llvm/Transforms/IPO.h
+++ b/include/llvm/Transforms/IPO.h
@@ -70,6 +70,12 @@ ModulePass *createGlobalOptimizerPass();
 ///
 ModulePass *createGlobalDCEPass();
 
+//===----------------------------------------------------------------------===//
+/// This transform is designed to eliminate available external globals
+/// (functions or global variables)
+///
+ModulePass *createEliminateAvailableExternallyPass();
+
 //===----------------------------------------------------------------------===//
 /// createGVExtractionPass - If deleteFn is true, this pass deletes
 /// the specified global values. Otherwise, it deletes as much of the module as

diff --git a/include/llvm/Transforms/IPO/PassManagerBuilder.h b/include/llvm/Transforms/IPO/PassManagerBuilder.h
index 5d574ae0bf0f298b7b975dd563960cee3bad1243..1334dd0da23c30d9035e738cedbac5bed99f82ef 100644 (file)
--- a/include/llvm/Transforms/IPO/PassManagerBuilder.h
+++ b/include/llvm/Transforms/IPO/PassManagerBuilder.h
@@ -121,6 +121,7 @@ public:
   bool VerifyInput;
   bool VerifyOutput;
   bool MergeFunctions;
+  bool PrepareForLTO;
 
 private:
   /// ExtensionList - This is list of all of the extensions that are registered.

diff --git a/lib/Transforms/IPO/CMakeLists.txt b/lib/Transforms/IPO/CMakeLists.txt
index 3df17b920a958acf7398495364536556440bae22..336dac45e13af4c74b05484be5c74a5a04ef079e 100644 (file)
--- a/lib/Transforms/IPO/CMakeLists.txt
+++ b/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/lib/Transforms/IPO/ElimAvailExtern.cpp b/lib/Transforms/IPO/ElimAvailExtern.cpp
new file mode 100644 (file)
index 0000000..e70ab87
--- /dev/null
+++ b/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/lib/Transforms/IPO/PassManagerBuilder.cpp b/lib/Transforms/IPO/PassManagerBuilder.cpp
index 963f1bb13aafb4712bdc5317eb9c9a18fe9fe084..a067a4b45254edd073610d7c1e2bdb37c0144b7c 100644 (file)
--- a/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/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
     }