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/IR/Module.h"
35
36using namespace llvm;
37
38namespace {
39
40static Constant *canonicalizeAlias(Constant *C, bool &Changed) {
41 if (auto *GA = dyn_cast<GlobalAlias>(Val: C)) {
42 auto *NewAliasee = canonicalizeAlias(C: GA->getAliasee(), Changed);
43 if (NewAliasee != GA->getAliasee()) {
44 GA->setAliasee(NewAliasee);
45 Changed = true;
46 }
47 return NewAliasee;
48 }
49
50 auto *CE = dyn_cast<ConstantExpr>(Val: C);
51 if (!CE)
52 return C;
53
54 std::vector<Constant *> Ops;
55 for (Use &U : CE->operands())
56 Ops.push_back(x: canonicalizeAlias(C: cast<Constant>(Val&: U), Changed));
57 return CE->getWithOperands(Ops);
58}
59
60/// Convert aliases to canonical form.
61static bool canonicalizeAliases(Module &M) {
62 bool Changed = false;
63 for (auto &GA : M.aliases())
64 canonicalizeAlias(C: &GA, Changed);
65 return Changed;
66}
67} // anonymous namespace
68
69PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,
70 ModuleAnalysisManager &AM) {
71 if (!canonicalizeAliases(M))
72 return PreservedAnalyses::all();
73
74 return PreservedAnalyses::none();
75}
76