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
37using namespace llvm;
38
39namespace {
40
41static 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.
62static 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
70PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
71                                               ModuleAnalysisManager &AM) {
72  if (!canonicalizeAliases(M))
73    return PreservedAnalyses::all();
74
75  return PreservedAnalyses::none();
76}
77