CanonicalizeAliases.cpp revision 360784 
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 canonicalation, 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/Operator.h"
34#include "llvm/IR/ValueHandle.h"
35#include "llvm/InitializePasses.h"
36#include "llvm/Pass.h"
37
38using namespace llvm;
39
40namespace {
41
42static 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.
63static 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.
71class CanonicalizeAliasesLegacyPass : public ModulePass {
72
73public:
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};
84char CanonicalizeAliasesLegacyPass::ID = 0;
85
86} // anonymous namespace
87
88PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
89                                               ModuleAnalysisManager &AM) {
90  if (!canonicalizeAliases(M))
91    return PreservedAnalyses::all();
92
93  return PreservedAnalyses::none();
94}
95
96INITIALIZE_PASS_BEGIN(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
97                      "Canonicalize aliases", false, false)
98INITIALIZE_PASS_END(CanonicalizeAliasesLegacyPass, "canonicalize-aliases",
99                    "Canonicalize aliases", false, false)
100
101namespace llvm {
102ModulePass *createCanonicalizeAliasesPass() {
103  return new CanonicalizeAliasesLegacyPass();
104}
105} // namespace llvm
106