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"
22 using namespace llvm;
23 
24 #define DEBUG_TYPE "elim-avail-extern"
25 
26 STATISTIC(NumFunctions, "Number of functions removed");
27 STATISTIC(NumVariables, "Number of global variables removed");
28 
29 namespace {
30 struct EliminateAvailableExternally : public ModulePass {
31   static char ID; // Pass identification, replacement for typeid
EliminateAvailableExternally__anon4676bb110111::EliminateAvailableExternally32   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 
44 char EliminateAvailableExternally::ID = 0;
45 INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
46                 "Eliminate Available Externally Globals", false, false)
47 
createEliminateAvailableExternallyPass()48 ModulePass *llvm::createEliminateAvailableExternallyPass() {
49   return new EliminateAvailableExternally();
50 }
51 
runOnModule(Module & M)52 bool 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