1 //===- ElimAvailExtern.cpp - DCE unreachable internal functions -----------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This transform is designed to eliminate available external global 10 // definitions from the program, turning them into declarations. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/IPO/ElimAvailExtern.h" 15 #include "llvm/ADT/Statistic.h" 16 #include "llvm/IR/Constant.h" 17 #include "llvm/IR/Function.h" 18 #include "llvm/IR/GlobalValue.h" 19 #include "llvm/IR/GlobalVariable.h" 20 #include "llvm/IR/Module.h" 21 #include "llvm/InitializePasses.h" 22 #include "llvm/Pass.h" 23 #include "llvm/Transforms/IPO.h" 24 #include "llvm/Transforms/Utils/GlobalStatus.h" 25 26 using namespace llvm; 27 28 #define DEBUG_TYPE "elim-avail-extern" 29 30 STATISTIC(NumFunctions, "Number of functions removed"); 31 STATISTIC(NumVariables, "Number of global variables removed"); 32 33 static bool eliminateAvailableExternally(Module &M) { 34 bool Changed = false; 35 36 // Drop initializers of available externally global variables. 37 for (GlobalVariable &GV : M.globals()) { 38 if (!GV.hasAvailableExternallyLinkage()) 39 continue; 40 if (GV.hasInitializer()) { 41 Constant *Init = GV.getInitializer(); 42 GV.setInitializer(nullptr); 43 if (isSafeToDestroyConstant(Init)) 44 Init->destroyConstant(); 45 } 46 GV.removeDeadConstantUsers(); 47 GV.setLinkage(GlobalValue::ExternalLinkage); 48 NumVariables++; 49 Changed = true; 50 } 51 52 // Drop the bodies of available externally functions. 53 for (Function &F : M) { 54 if (!F.hasAvailableExternallyLinkage()) 55 continue; 56 if (!F.isDeclaration()) 57 // This will set the linkage to external 58 F.deleteBody(); 59 F.removeDeadConstantUsers(); 60 NumFunctions++; 61 Changed = true; 62 } 63 64 return Changed; 65 } 66 67 PreservedAnalyses 68 EliminateAvailableExternallyPass::run(Module &M, ModuleAnalysisManager &) { 69 if (!eliminateAvailableExternally(M)) 70 return PreservedAnalyses::all(); 71 return PreservedAnalyses::none(); 72 } 73 74 namespace { 75 76 struct EliminateAvailableExternallyLegacyPass : public ModulePass { 77 static char ID; // Pass identification, replacement for typeid 78 79 EliminateAvailableExternallyLegacyPass() : ModulePass(ID) { 80 initializeEliminateAvailableExternallyLegacyPassPass( 81 *PassRegistry::getPassRegistry()); 82 } 83 84 // run - Do the EliminateAvailableExternally pass on the specified module, 85 // optionally updating the specified callgraph to reflect the changes. 86 bool runOnModule(Module &M) override { 87 if (skipModule(M)) 88 return false; 89 return eliminateAvailableExternally(M); 90 } 91 }; 92 93 } // end anonymous namespace 94 95 char EliminateAvailableExternallyLegacyPass::ID = 0; 96 97 INITIALIZE_PASS(EliminateAvailableExternallyLegacyPass, "elim-avail-extern", 98 "Eliminate Available Externally Globals", false, false) 99 100 ModulePass *llvm::createEliminateAvailableExternallyPass() { 101 return new EliminateAvailableExternallyLegacyPass(); 102 } 103