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.h"
17#include "llvm/ADT/Statistic.h"
18#include "llvm/IR/Constants.h"
19#include "llvm/IR/Module.h"
20#include "llvm/Transforms/Utils/GlobalStatus.h"
21#include "llvm/Pass.h"
22using namespace llvm;
23
24#define DEBUG_TYPE "elim-avail-extern"
25
26STATISTIC(NumFunctions, "Number of functions removed");
27STATISTIC(NumVariables, "Number of global variables removed");
28
29namespace {
30struct EliminateAvailableExternally : public ModulePass {
31  static char ID; // Pass identification, replacement for typeid
32  EliminateAvailableExternally() : ModulePass(ID) {
33    initializeEliminateAvailableExternallyPass(
34        *PassRegistry::getPassRegistry());
35  }
36
37  // run - Do the EliminateAvailableExternally pass on the specified module,
38  // optionally updating the specified callgraph to reflect the changes.
39  //
40  bool runOnModule(Module &M) override;
41};
42}
43
44char EliminateAvailableExternally::ID = 0;
45INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
46                "Eliminate Available Externally Globals", false, false)
47
48ModulePass *llvm::createEliminateAvailableExternallyPass() {
49  return new EliminateAvailableExternally();
50}
51
52bool EliminateAvailableExternally::runOnModule(Module &M) {
53  bool Changed = false;
54
55  // Drop initializers of available externally global variables.
56  for (GlobalVariable &GV : M.globals()) {
57    if (!GV.hasAvailableExternallyLinkage())
58      continue;
59    if (GV.hasInitializer()) {
60      Constant *Init = GV.getInitializer();
61      GV.setInitializer(nullptr);
62      if (isSafeToDestroyConstant(Init))
63        Init->destroyConstant();
64    }
65    GV.removeDeadConstantUsers();
66    GV.setLinkage(GlobalValue::ExternalLinkage);
67    NumVariables++;
68    Changed = true;
69  }
70
71  // Drop the bodies of available externally functions.
72  for (Function &F : M) {
73    if (!F.hasAvailableExternallyLinkage())
74      continue;
75    if (!F.isDeclaration())
76      // This will set the linkage to external
77      F.deleteBody();
78    F.removeDeadConstantUsers();
79    NumFunctions++;
80    Changed = true;
81  }
82
83  return Changed;
84}
85