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/Instructions.h"
19 #include "llvm/IR/Module.h"
20 #include "llvm/Transforms/Utils/CtorUtils.h"
21 #include "llvm/Transforms/Utils/GlobalStatus.h"
22 #include "llvm/Pass.h"
23 using namespace llvm;
24 
25 #define DEBUG_TYPE "elim-avail-extern"
26 
27 STATISTIC(NumAliases  , "Number of global aliases removed");
28 STATISTIC(NumFunctions, "Number of functions removed");
29 STATISTIC(NumVariables, "Number of global variables removed");
30 
31 namespace {
32   struct EliminateAvailableExternally : public ModulePass {
33     static char ID; // Pass identification, replacement for typeid
34     EliminateAvailableExternally() : ModulePass(ID) {
35       initializeEliminateAvailableExternallyPass(
36           *PassRegistry::getPassRegistry());
37     }
38 
39     // run - Do the EliminateAvailableExternally pass on the specified module,
40     // optionally updating the specified callgraph to reflect the changes.
41     //
42     bool runOnModule(Module &M) override;
43   };
44 }
45 
46 char EliminateAvailableExternally::ID = 0;
47 INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
48                 "Eliminate Available Externally Globals", false, false)
49 
50 ModulePass *llvm::createEliminateAvailableExternallyPass() {
51   return new EliminateAvailableExternally();
52 }
53 
54 bool EliminateAvailableExternally::runOnModule(Module &M) {
55   bool Changed = false;
56 
57   // Drop initializers of available externally global variables.
58   for (Module::global_iterator I = M.global_begin(), E = M.global_end();
59        I != E; ++I) {
60     if (!I->hasAvailableExternallyLinkage())
61       continue;
62     if (I->hasInitializer()) {
63       Constant *Init = I->getInitializer();
64       I->setInitializer(nullptr);
65       if (isSafeToDestroyConstant(Init))
66         Init->destroyConstant();
67     }
68     I->removeDeadConstantUsers();
69     I->setLinkage(GlobalValue::ExternalLinkage);
70     I->setVisibility(GlobalValue::DefaultVisibility);
71     NumVariables++;
72   }
73 
74   // Drop the bodies of available externally functions.
75   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
76     if (!I->hasAvailableExternallyLinkage())
77       continue;
78     if (!I->isDeclaration())
79       // This will set the linkage to external
80       I->deleteBody();
81     I->setVisibility(GlobalValue::DefaultVisibility);
82     I->removeDeadConstantUsers();
83     NumFunctions++;
84   }
85 
86   // Drop targets of available externally aliases.
87   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;
88        ++I) {
89     if (!I->hasAvailableExternallyLinkage())
90       continue;
91     I->setAliasee(nullptr);
92     I->removeDeadConstantUsers();
93     I->setLinkage(GlobalValue::ExternalLinkage);
94     I->setVisibility(GlobalValue::DefaultVisibility);
95     NumAliases++;
96   }
97 
98   return Changed;
99 }
100