llvm/lib/Transforms/Utils/CanonicalizeAliases.cpp

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