ElimAvailExtern.cpp

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//===- ElimAvailExtern.cpp - DCE unreachable internal functions -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This transform is designed to eliminate available external global
// definitions from the program, turning them into declarations.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/IPO/ElimAvailExtern.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils/GlobalStatus.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "elim-avail-extern"
 
STATISTIC(NumFunctions, "Number of functions removed");
STATISTIC(NumVariables, "Number of global variables removed");
 
static bool eliminateAvailableExternally(Module &M) {
  bool Changed = false;
 
  // Drop initializers of available externally global variables.
  for (GlobalVariable &GV : M.globals()) {
    if (!GV.hasAvailableExternallyLinkage())
      continue;
    if (GV.hasInitializer()) {
      Constant *Init = GV.getInitializer();
      GV.setInitializer(nullptr);
      if (isSafeToDestroyConstant(Init))
        Init->destroyConstant();
    }
    GV.removeDeadConstantUsers();
    GV.setLinkage(GlobalValue::ExternalLinkage);
    NumVariables++;
    Changed = true;
  }
 
  // Drop the bodies of available externally functions.
  for (Function &F : M) {
    if (!F.hasAvailableExternallyLinkage())
      continue;
    if (!F.isDeclaration())
      // This will set the linkage to external
      F.deleteBody();
    F.removeDeadConstantUsers();
    NumFunctions++;
    Changed = true;
  }
 
  return Changed;
}
 
PreservedAnalyses
EliminateAvailableExternallyPass::run(Module &M, ModuleAnalysisManager &) {
  if (!eliminateAvailableExternally(M))
    return PreservedAnalyses::all();
  return PreservedAnalyses::none();
}