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/Pass.h"
  22 #include "llvm/Transforms/IPO.h"
  23 #include "llvm/Transforms/Utils/GlobalStatus.h"
  24
  25 using namespace llvm;
  26
  27 #define DEBUG_TYPE "elim-avail-extern"
  28
  29 STATISTIC(NumFunctions, "Number of functions removed");
  30 STATISTIC(NumVariables, "Number of global variables removed");
  31
  32 static bool eliminateAvailableExternally(Module &M) {
  33   bool Changed = false;
  34
  35   // Drop initializers of available externally global variables.
  36   for (GlobalVariable &GV : M.globals()) {
  37     if (!GV.hasAvailableExternallyLinkage())
  38       continue;
  39     if (GV.hasInitializer()) {
  40       Constant *Init = GV.getInitializer();
  41       GV.setInitializer(nullptr);
  42       if (isSafeToDestroyConstant(Init))
  43         Init->destroyConstant();
  44     }
  45     GV.removeDeadConstantUsers();
  46     GV.setLinkage(GlobalValue::ExternalLinkage);
  47     NumVariables++;
  48     Changed = true;
  49   }
  50
  51   // Drop the bodies of available externally functions.
  52   for (Function &F : M) {
  53     if (!F.hasAvailableExternallyLinkage())
  54       continue;
  55     if (!F.isDeclaration())
  56       // This will set the linkage to external
  57       F.deleteBody();
  58     F.removeDeadConstantUsers();
  59     NumFunctions++;
  60     Changed = true;
  61   }
  62
  63   return Changed;
  64 }
  65
  66 PreservedAnalyses
  67 EliminateAvailableExternallyPass::run(Module &M, ModuleAnalysisManager &) {
  68   if (!eliminateAvailableExternally(M))
  69     return PreservedAnalyses::all();
  70   return PreservedAnalyses::none();
  71 }
  72
  73 namespace {
  74
  75 struct EliminateAvailableExternallyLegacyPass : public ModulePass {
  76   static char ID; // Pass identification, replacement for typeid
  77
  78   EliminateAvailableExternallyLegacyPass() : ModulePass(ID) {
  79     initializeEliminateAvailableExternallyLegacyPassPass(
  80         *PassRegistry::getPassRegistry());
  81   }
  82
  83   // run - Do the EliminateAvailableExternally pass on the specified module,
  84   // optionally updating the specified callgraph to reflect the changes.
  85   bool runOnModule(Module &M) override {
  86     if (skipModule(M))
  87       return false;
  88     return eliminateAvailableExternally(M);
  89   }
  90 };
  91
  92 } // end anonymous namespace
  93
  94 char EliminateAvailableExternallyLegacyPass::ID = 0;
  95
  96 INITIALIZE_PASS(EliminateAvailableExternallyLegacyPass, "elim-avail-extern",
  97                 "Eliminate Available Externally Globals", false, false)
  98
  99 ModulePass *llvm::createEliminateAvailableExternallyPass() {
 100   return new EliminateAvailableExternallyLegacyPass();
 101 }