clang/lib/CodeGenUtils/ItaniumCXXABIUtils.cpp

//===--- ItaniumCXXABIUtils.cpp - Shared Itanium C++ ABI queries ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "clang/CodeGenUtils/ItaniumCXXABIUtils.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/RecordLayout.h"

namespace clang::CodeGenUtils {

CharUnits computeOffsetHint(ASTContext &Ctx, const CXXRecordDecl *Src,
                            const CXXRecordDecl *Dst) {
  CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
                     /*DetectVirtual=*/false);

  // If Dst is not derived from Src we can skip the whole computation below and
  // return that Src is not a public base of Dst.  Record all inheritance paths.
  if (!Dst->isDerivedFrom(Src, Paths))
    return CharUnits::fromQuantity(-2ULL);

  unsigned NumPublicPaths = 0;
  CharUnits Offset;

  // Now walk all possible inheritance paths.
  for (const CXXBasePath &Path : Paths) {
    if (Path.Access != AS_public) // Ignore non-public inheritance.
      continue;

    ++NumPublicPaths;

    for (const CXXBasePathElement &PathElement : Path) {
      // If the path contains a virtual base class we can't give any hint.
      // -1: no hint.
      if (PathElement.Base->isVirtual())
        return CharUnits::fromQuantity(-1ULL);

      if (NumPublicPaths > 1) // Won't use offsets, skip computation.
        continue;

      // Accumulate the base class offsets.
      const ASTRecordLayout &L = Ctx.getASTRecordLayout(PathElement.Class);
      Offset += L.getBaseClassOffset(
          PathElement.Base->getType()->getAsCXXRecordDecl());
    }
  }

  // -2: Src is not a public base of Dst.
  if (NumPublicPaths == 0)
    return CharUnits::fromQuantity(-2ULL);

  // -3: Src is a multiple public base type but never a virtual base type.
  if (NumPublicPaths > 1)
    return CharUnits::fromQuantity(-3ULL);

  // Otherwise, the Src type is a unique public nonvirtual base type of Dst.
  // Return the offset of Src from the origin of Dst.
  return Offset;
}

} // namespace clang::CodeGenUtils