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