tagged-payload.h

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// Copyright 2024 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
 
#ifndef V8_SANDBOX_TAGGED_PAYLOAD_H_
#define V8_SANDBOX_TAGGED_PAYLOAD_H_
 
#include "src/common/globals.h"
#include "src/sandbox/indirect-pointer-tag.h"
 
namespace v8 {
namespace internal {
 
// Struct providing common utilities for pointer tagging.
template <typename PayloadTaggingScheme>
struct TaggedPayload {
  static_assert(PayloadTaggingScheme::kMarkBit != 0,
                "Invalid kMarkBit specified in tagging scheme.");
  // The mark bit does not overlap with the TagMask.
  static_assert((PayloadTaggingScheme::kMarkBit &
                 PayloadTaggingScheme::kTagMask) == 0);
 
  TaggedPayload(Address pointer, typename PayloadTaggingScheme::TagType tag)
      : encoded_word_(Tag(pointer, tag)) {}
 
  Address Untag(typename PayloadTaggingScheme::TagType tag) const {
    return encoded_word_ & ~(tag | PayloadTaggingScheme::kMarkBit);
  }
 
  static Address Tag(Address pointer,
                     typename PayloadTaggingScheme::TagType tag) {
    return pointer | tag;
  }
 
  bool IsTaggedWith(typename PayloadTaggingScheme::TagType tag) const {
    return (encoded_word_ & PayloadTaggingScheme::kTagMask) == tag;
  }
 
  void SetTag(typename PayloadTaggingScheme::TagType new_tag) {
    encoded_word_ = (encoded_word_ & ~PayloadTaggingScheme::kTagMask) | new_tag;
  }
 
  void SetMarkBit() { encoded_word_ |= PayloadTaggingScheme::kMarkBit; }
 
  void ClearMarkBit() { encoded_word_ &= ~PayloadTaggingScheme::kMarkBit; }
 
  bool HasMarkBitSet() const {
    return (encoded_word_ & PayloadTaggingScheme::kMarkBit) != 0;
  }
 
  uint32_t ExtractFreelistLink() const {
    return static_cast<uint32_t>(encoded_word_);
  }
 
  typename PayloadTaggingScheme::TagType ExtractTag() const {
    return static_cast<typename PayloadTaggingScheme::TagType>(
        (encoded_word_ & PayloadTaggingScheme::kTagMask) |
        PayloadTaggingScheme::kMarkBit);
  }
 
  bool ContainsFreelistLink() const {
      return IsTaggedWith(PayloadTaggingScheme::kFreeEntryTag);
  }
 
  bool ContainsEvacuationEntry() const {
    if constexpr (PayloadTaggingScheme::kSupportsEvacuation) {
      return IsTaggedWith(PayloadTaggingScheme::kEvacuationEntryTag);
    } else {
      return false;
    }
  }
 
  bool IsZapped() const {
    if constexpr (PayloadTaggingScheme::kSupportsZapping) {
      return IsTaggedWith(PayloadTaggingScheme::kZappedEntryTag);
    } else {
      return false;
    }
  }
 
  Address ExtractEvacuationEntryHandleLocation() const {
    if constexpr (PayloadTaggingScheme::kSupportsEvacuation) {
      return Untag(PayloadTaggingScheme::kEvacuationEntryTag);
    } else {
      UNREACHABLE();
    }
  }
 
  bool ContainsPointer() const {
    return !ContainsFreelistLink() && !ContainsEvacuationEntry() && !IsZapped();
  }
 
  bool operator==(TaggedPayload other) const {
    return encoded_word_ == other.encoded_word_;
  }
 
  bool operator!=(TaggedPayload other) const {
    return encoded_word_ != other.encoded_word_;
  }
 
 private:
  Address encoded_word_;
};
 
}  // namespace internal
}  // namespace v8
 
#endif  // V8_SANDBOX_TAGGED_PAYLOAD_H_