From 3d41fcb3da4442cb63e20b67579e0316e8edceab Mon Sep 17 00:00:00 2001 From: Teresa Johnson Date: Mon, 6 Jul 2015 16:22:42 +0000 Subject: [PATCH] Resubmit "Add new EliminateAvailableExternally module pass" (r239480) This change includes a fix for https://code.google.com/p/chromium/issues/detail?id=499508#c3, which required updating the visibility for symbols with eliminated definitions. --Original Commit Message-- 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@241466 91177308-0d34-0410-b5e6-96231b3b80d8 --- include/llvm/InitializePasses.h | 1 + include/llvm/Transforms/IPO.h | 6 ++ .../llvm/Transforms/IPO/PassManagerBuilder.h | 1 + lib/Transforms/IPO/CMakeLists.txt | 1 + lib/Transforms/IPO/ElimAvailExtern.cpp | 99 +++++++++++++++++++ lib/Transforms/IPO/PassManagerBuilder.cpp | 12 +++ 6 files changed, 120 insertions(+) create mode 100644 lib/Transforms/IPO/ElimAvailExtern.cpp diff --git a/include/llvm/InitializePasses.h b/include/llvm/InitializePasses.h index 74fbc0f94b0..da614e46384 100644 --- 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 fbd999cbc94..2ea47301bb4 100644 --- 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 5d574ae0bf0..1334dd0da23 100644 --- 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 3df17b920a9..336dac45e13 100644 --- 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 index 00000000000..1566e85e99b --- /dev/null +++ b/lib/Transforms/IPO/ElimAvailExtern.cpp @@ -0,0 +1,99 @@ +//===-- 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); + I->setVisibility(GlobalValue::DefaultVisibility); + 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->setVisibility(GlobalValue::DefaultVisibility); + 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); + I->setVisibility(GlobalValue::DefaultVisibility); + NumAliases++; + } + + return Changed; +} diff --git a/lib/Transforms/IPO/PassManagerBuilder.cpp b/lib/Transforms/IPO/PassManagerBuilder.cpp index 963f1bb13aa..a067a4b4525 100644 --- 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 } -- 2.34.1