//===- CanonicalizeAliases.cpp - ThinLTO Support: Canonicalize Aliases ----===//

//

// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.

// See https://llvm.org/LICENSE.txt for license information.

// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

//

//===----------------------------------------------------------------------===//

//

// Currently this file implements partial alias canonicalization, to

// flatten chains of aliases (also done by GlobalOpt, but not on for

// O0 compiles). E.g.

//  @a = alias i8, i8 *@b

//  @b = alias i8, i8 *@g

//

// will be converted to:

//  @a = alias i8, i8 *@g  <-- @a is now an alias to base object @g

//  @b = alias i8, i8 *@g

//

// Eventually this file will implement full alias canonicalization, so that

// all aliasees are private anonymous values. E.g.

//  @a = alias i8, i8 *@g

//  @g = global i8 0

//

// will be converted to:

//  @0 = private global

//  @a = alias i8, i8* @0

//  @g = alias i8, i8* @0

//

// This simplifies optimization and ThinLTO linking of the original symbols.

//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Utils/CanonicalizeAliases.h"

#include "llvm/IR/Constants.h"

using namespace llvm;

namespace {

static Constant *canonicalizeAlias(Constant *C, bool &Changed) {

  if (auto *GA = dyn_cast<GlobalAlias>(C)) {

    auto *NewAliasee = canonicalizeAlias(GA->getAliasee(), Changed);

    if (NewAliasee != GA->getAliasee()) {

      GA->setAliasee(NewAliasee);

      Changed = true;

    }

    return NewAliasee;

  }

  auto *CE = dyn_cast<ConstantExpr>(C);

  if (!CE)

    return C;

  std::vector<Constant *> Ops;

  for (Use &U : CE->operands())

    Ops.push_back(canonicalizeAlias(cast<Constant>(U), Changed));

  return CE->getWithOperands(Ops);

}

/// Convert aliases to canonical form.

static bool canonicalizeAliases(Module &M) {

  bool Changed = false;

  for (auto &GA : M.aliases())

    canonicalizeAlias(&GA, Changed);

  return Changed;

}

} // anonymous namespace

PreservedAnalyses CanonicalizeAliasesPass::run(Module &M,

                                               ModuleAnalysisManager &AM) {

  if (!canonicalizeAliases(M))

    return PreservedAnalyses::all();

  return PreservedAnalyses::none();

}