1 //===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//
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 // Currently this file implements partial alias canonicalization, to
11 // flatten chains of aliases (also done by GlobalOpt, but not on for
12 // O0 compiles). E.g.
13 //  @a = alias i8, i8 *@b
14 //  @b = alias i8, i8 *@g
15 //
16 // will be converted to:
17 //  @a = alias i8, i8 *@g  <-- @a is now an alias to base object @g
18 //  @b = alias i8, i8 *@g
19 //
20 // Eventually this file will implement full alias canonicalation, so that
21 // all aliasees are private anonymous values. E.g.
22 //  @a = alias i8, i8 *@g
23 //  @g = global i8 0
24 //
25 // will be converted to:
26 //  @0 = private global
27 //  @a = alias i8, i8* @0
28 //  @g = alias i8, i8* @0
29 //
30 // This simplifies optimization and ThinLTO linking of the original symbols.
31 //===----------------------------------------------------------------------===//
32 
33 #include "llvm/Transforms/Utils/CanonicalizeAliases.h"
34 
35 #include "llvm/IR/Operator.h"
36 #include "llvm/IR/ValueHandle.h"
37 
38 using namespace llvm;
39 
40 namespace {
41 
42 static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
43   if (auto *GA = dyn_cast<GlobalAlias>(C)) {
44     auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);
45     if (NewAliasee != GA->getAliasee()) {
46       GA->setAliasee(NewAliasee);
47       Changed = true;
48     }
49     return NewAliasee;
50   }
51 
52   auto *CE = dyn_cast<ConstantExpr>(C);
53   if (!CE)
54     return C;
55 
56   std::vector<Constant *> Ops;
57   for (Use &U : CE->operands())
58     Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));
59   return CE->getWithOperands(Ops);
60 }
61 
62 /// Convert aliases to canonical form.
63 static bool canonicalizeAliases(Module &M) {
64   bool Changed = false;
65   for (auto &GA : M.aliases())
66     canonicalizeAlias(&GA, Changed);
67   return Changed;
68 }
69 
70 // Legacy pass that canonicalizes aliases.
71 class CanonicalizeAliasesLegacyPass : public ModulePass {
72 
73 public:
74   /// Pass identification, replacement for typeid
75   static char ID;
76 
77   /// Specify pass name for debug output
78   StringRef getPassName() const override { return "Canonicalize Aliases"; }
79 
80   explicit CanonicalizeAliasesLegacyPass() : ModulePass(ID) {}
81 
82   bool runOnModule(Module &M) override { return canonicalizeAliases(M); }
83 };
84 char CanonicalizeAliasesLegacyPass::ID = 0;
85 
86 } // anonymous namespace
87 
88 PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
89                                                ModuleAnalysisManager &AM) {
90   if (!canonicalizeAliases(M))
91     return PreservedAnalyses::all();
92 
93   return PreservedAnalyses::none();
94 }
95 
96 INITIALIZE_PASS_BEGIN(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
97                       "Canonicalize aliases", false, false)
98 INITIALIZE_PASS_END(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
99                     "Canonicalize aliases", false, false)
100 
101 namespace llvm {
102 ModulePass *createCanonicalizeAliasesPass() {
103   return new CanonicalizeAliasesLegacyPass();
104 }
105 } // namespace llvm
106