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