1 //===-- ElimAvailExtern.cpp - DCE unreachable internal functions ----------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This transform is designed to eliminate available external global
11 // definitions from the program, turning them into declarations.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/IPO.h"
16 #include "llvm/ADT/Statistic.h"
17 #include "llvm/IR/Constants.h"
18 #include "llvm/IR/Module.h"
19 #include "llvm/Transforms/Utils/GlobalStatus.h"
20 #include "llvm/Pass.h"
21 using namespace llvm;
22 
23 #define DEBUG_TYPE "elim-avail-extern"
24 
25 STATISTIC(NumFunctions, "Number of functions removed");
26 STATISTIC(NumVariables, "Number of global variables removed");
27 
28 namespace {
29   struct EliminateAvailableExternally : public ModulePass {
30     static char ID; // Pass identification, replacement for typeid
31     EliminateAvailableExternally() : ModulePass(ID) {
32       initializeEliminateAvailableExternallyPass(
33           *PassRegistry::getPassRegistry());
34     }
35 
36     // run - Do the EliminateAvailableExternally pass on the specified module,
37     // optionally updating the specified callgraph to reflect the changes.
38     //
39     bool runOnModule(Module &M) override;
40   };
41 }
42 
43 char EliminateAvailableExternally::ID = 0;
44 INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
45                 "Eliminate Available Externally Globals", false, false)
46 
47 ModulePass *llvm::createEliminateAvailableExternallyPass() {
48   return new EliminateAvailableExternally();
49 }
50 
51 bool EliminateAvailableExternally::runOnModule(Module &M) {
52   bool Changed = false;
53 
54   // Drop initializers of available externally global variables.
55   for (GlobalVariable &GV :M.globals()) {
56     if (!GV.hasAvailableExternallyLinkage())
57       continue;
58     if (GV.hasInitializer()) {
59       Constant *Init = GV.getInitializer();
60       GV.setInitializer(nullptr);
61       if (isSafeToDestroyConstant(Init))
62         Init->destroyConstant();
63     }
64     GV.removeDeadConstantUsers();
65     GV.setLinkage(GlobalValue::ExternalLinkage);
66     NumVariables++;
67   }
68 
69   // Drop the bodies of available externally functions.
70   for (Function &F : M) {
71     if (!F.hasAvailableExternallyLinkage())
72       continue;
73     if (!F.isDeclaration())
74       // This will set the linkage to external
75       F.deleteBody();
76     F.removeDeadConstantUsers();
77     NumFunctions++;
78   }
79 
80   return Changed;
81 }
82