1 //===- ElimAvailExtern.cpp - DCE unreachable internal functions -----------===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This transform is designed to eliminate available external global
10 // definitions from the program, turning them into declarations.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Transforms/IPO/ElimAvailExtern.h"
15 #include "llvm/ADT/Statistic.h"
16 #include "llvm/IR/Constant.h"
17 #include "llvm/IR/Function.h"
18 #include "llvm/IR/GlobalValue.h"
19 #include "llvm/IR/GlobalVariable.h"
20 #include "llvm/IR/Module.h"
21 #include "llvm/InitializePasses.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Transforms/IPO.h"
24 #include "llvm/Transforms/Utils/GlobalStatus.h"
25 
26 using namespace llvm;
27 
28 #define DEBUG_TYPE "elim-avail-extern"
29 
30 STATISTIC(NumFunctions, "Number of functions removed");
31 STATISTIC(NumVariables, "Number of global variables removed");
32 
33 static bool eliminateAvailableExternally(Module &M) {
34   bool Changed = false;
35 
36   // Drop initializers of available externally global variables.
37   for (GlobalVariable &GV : M.globals()) {
38     if (!GV.hasAvailableExternallyLinkage())
39       continue;
40     if (GV.hasInitializer()) {
41       Constant *Init = GV.getInitializer();
42       GV.setInitializer(nullptr);
43       if (isSafeToDestroyConstant(Init))
44         Init->destroyConstant();
45     }
46     GV.removeDeadConstantUsers();
47     GV.setLinkage(GlobalValue::ExternalLinkage);
48     NumVariables++;
49     Changed = true;
50   }
51 
52   // Drop the bodies of available externally functions.
53   for (Function &F : M) {
54     if (!F.hasAvailableExternallyLinkage())
55       continue;
56     if (!F.isDeclaration())
57       // This will set the linkage to external
58       F.deleteBody();
59     F.removeDeadConstantUsers();
60     NumFunctions++;
61     Changed = true;
62   }
63 
64   return Changed;
65 }
66 
67 PreservedAnalyses
68 EliminateAvailableExternallyPass::run(Module &M, ModuleAnalysisManager &) {
69   if (!eliminateAvailableExternally(M))
70     return PreservedAnalyses::all();
71   return PreservedAnalyses::none();
72 }
73 
74 namespace {
75 
76 struct EliminateAvailableExternallyLegacyPass : public ModulePass {
77   static char ID; // Pass identification, replacement for typeid
78 
79   EliminateAvailableExternallyLegacyPass() : ModulePass(ID) {
80     initializeEliminateAvailableExternallyLegacyPassPass(
81         *PassRegistry::getPassRegistry());
82   }
83 
84   // run - Do the EliminateAvailableExternally pass on the specified module,
85   // optionally updating the specified callgraph to reflect the changes.
86   bool runOnModule(Module &M) override {
87     if (skipModule(M))
88       return false;
89     return eliminateAvailableExternally(M);
90   }
91 };
92 
93 } // end anonymous namespace
94 
95 char EliminateAvailableExternallyLegacyPass::ID = 0;
96 
97 INITIALIZE_PASS(EliminateAvailableExternallyLegacyPass, "elim-avail-extern",
98                 "Eliminate Available Externally Globals", false, false)
99 
100 ModulePass *llvm::createEliminateAvailableExternallyPass() {
101   return new EliminateAvailableExternallyLegacyPass();
102 }
103