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