llvm/lib/Transforms/IPO/ElimAvailExtern.cpp

   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 }