1 //===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//
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 // Currently this file implements partial alias canonicalization, to
10 // flatten chains of aliases (also done by GlobalOpt, but not on for
11 // O0 compiles). E.g.
12 //  @a = alias i8, i8 *@b
13 //  @b = alias i8, i8 *@g
14 //
15 // will be converted to:
16 //  @a = alias i8, i8 *@g  <-- @a is now an alias to base object @g
17 //  @b = alias i8, i8 *@g
18 //
19 // Eventually this file will implement full alias canonicalization, so that
20 // all aliasees are private anonymous values. E.g.
21 //  @a = alias i8, i8 *@g
22 //  @g = global i8 0
23 //
24 // will be converted to:
25 //  @0 = private global
26 //  @a = alias i8, i8* @0
27 //  @g = alias i8, i8* @0
28 //
29 // This simplifies optimization and ThinLTO linking of the original symbols.
30 //===----------------------------------------------------------------------===//
31 
32 #include "llvm/Transforms/Utils/CanonicalizeAliases.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/InitializePasses.h"
35 #include "llvm/Pass.h"
36 
37 using namespace llvm;
38 
39 namespace {
40 
41 static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
42   if (auto *GA = dyn_cast<GlobalAlias>(C)) {
43     auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);
44     if (NewAliasee != GA->getAliasee()) {
45       GA->setAliasee(NewAliasee);
46       Changed = true;
47     }
48     return NewAliasee;
49   }
50 
51   auto *CE = dyn_cast<ConstantExpr>(C);
52   if (!CE)
53     return C;
54 
55   std::vector<Constant *> Ops;
56   for (Use &U : CE->operands())
57     Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));
58   return CE->getWithOperands(Ops);
59 }
60 
61 /// Convert aliases to canonical form.
62 static bool canonicalizeAliases(Module &M) {
63   bool Changed = false;
64   for (auto &GA : M.aliases())
65     canonicalizeAlias(&GA, Changed);
66   return Changed;
67 }
68 } // anonymous namespace
69 
70 PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
71                                                ModuleAnalysisManager &AM) {
72   if (!canonicalizeAliases(M))
73     return PreservedAnalyses::all();
74 
75   return PreservedAnalyses::none();
76 }
77