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
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 
44 char EliminateAvailableExternally::ID = 0;
45 INITIALIZE_PASS(EliminateAvailableExternally, "elim-avail-extern",
46                 "Eliminate Available Externally Globals", false, false)
47 
48 ModulePass *llvm::createEliminateAvailableExternallyPass() {
49   return new EliminateAvailableExternally();
50 }
51 
52 static void convertAliasToDeclaration(GlobalAlias &GA, Module &M) {
53   GlobalValue *GVal = GA.getBaseObject();
54   GlobalValue *NewGV;
55   if (auto *GVar = dyn_cast<GlobalVariable>(GVal)) {
56     GlobalVariable *NewGVar = new GlobalVariable(
57         M, GVar->getType()->getElementType(), GVar->isConstant(),
58         GVar->getLinkage(), /*init*/ nullptr, GA.getName(), GVar,
59         GVar->getThreadLocalMode(), GVar->getType()->getAddressSpace());
60     NewGV = NewGVar;
61     NewGV->copyAttributesFrom(GVar);
62   } else {
63     auto *F = dyn_cast<Function>(GVal);
64     assert(F);
65     Function *NewF = Function::Create(F->getFunctionType(), F->getLinkage(),
66                                       GA.getName(), &M);
67     NewGV = NewF;
68     NewGV->copyAttributesFrom(F);
69   }
70   GA.replaceAllUsesWith(ConstantExpr::getBitCast(NewGV, GA.getType()));
71   GA.eraseFromParent();
72 }
73 
74 bool EliminateAvailableExternally::runOnModule(Module &M) {
75   bool Changed = false;
76 
77   // Convert any aliases that alias with an available externally
78   // value (which will be turned into declarations later on in this routine)
79   // into declarations themselves. All aliases must be definitions, and
80   // must alias with a definition. So this involves creating a declaration
81   // equivalent to the alias's base object.
82   for (Module::alias_iterator I = M.alias_begin(), E = M.alias_end(); I != E;) {
83     // Increment the iterator first since we may delete the current alias.
84     GlobalAlias &GA = *(I++);
85     GlobalValue *GVal = GA.getBaseObject();
86     if (!GVal->hasAvailableExternallyLinkage())
87       continue;
88     convertAliasToDeclaration(GA, M);
89     Changed = true;
90   }
91 
92   // Drop initializers of available externally global variables.
93   for (GlobalVariable &GV : M.globals()) {
94     if (!GV.hasAvailableExternallyLinkage())
95       continue;
96     if (GV.hasInitializer()) {
97       Constant *Init = GV.getInitializer();
98       GV.setInitializer(nullptr);
99       if (isSafeToDestroyConstant(Init))
100         Init->destroyConstant();
101     }
102     GV.removeDeadConstantUsers();
103     GV.setLinkage(GlobalValue::ExternalLinkage);
104     NumVariables++;
105     Changed = true;
106   }
107 
108   // Drop the bodies of available externally functions.
109   for (Function &F : M) {
110     if (!F.hasAvailableExternallyLinkage())
111       continue;
112     if (!F.isDeclaration())
113       // This will set the linkage to external
114       F.deleteBody();
115     F.removeDeadConstantUsers();
116     NumFunctions++;
117     Changed = true;
118   }
119 
120   return Changed;
121 }
122