/*

 * Copyright (c) Meta Platforms, Inc. and affiliates.

 *

 * This source code is licensed under the MIT license found in the

 * LICENSE file in the root directory of this source tree.

 */

#ifndef HERMES_VM_SEGMENTINFO_H

#define HERMES_VM_SEGMENTINFO_H

#include "hermes/VM/AlignedStorage.h"

/// Define this macro here because SegmentInfo is necessarily a dependency of

/// anything using compressed pointers.

#if defined(HERMESVM_ALLOW_COMPRESSED_POINTERS) && LLVM_PTR_SIZE == 8 && \

    (defined(HERMESVM_GC_HADES) || defined(HERMESVM_GC_RUNTIME))

/// \macro HERMESVM_COMPRESSED_POINTERS

/// \brief If defined, store pointers as 32 bits in GC-managed Hermes objects.

#define HERMESVM_COMPRESSED_POINTERS

#ifdef HERMESVM_ALLOW_CONTIGUOUS_HEAP

#define HERMESVM_CONTIGUOUS_HEAP

#endif

#endif

namespace hermes {

namespace vm {

/// The very beginning of a segment must contain this small structure, which can

/// contain segment-specific information. See AlignedHeapSegment for details on

/// how it is stored.

struct SegmentInfo {

  /// Returns the index of the segment containing \p lowLim, which is required

  /// to be the start of its containing segment.  (This can allow extra

  /// efficiency, in cases where the segment start has already been computed.)

  static unsigned segmentIndexFromStart(const void *lowLim) {

    assert(lowLim == AlignedStorage::start(lowLim) && "Precondition.");

    return get(lowLim)->index;

  }

  /// Returns the index of the segment containing \p ptr.

  static unsigned segmentIndex(const void *ptr) {

    return segmentIndexFromStart(AlignedStorage::start(ptr));

  }

  /// Requires that \p lowLim is the start address of a segment, and sets

  /// that segment's index to \p index.

  static void setSegmentIndexFromStart(void *lowLim, unsigned index) {

    assert(lowLim == AlignedStorage::start(lowLim) && "Precondition.");

    get(lowLim)->index = index;

  }

 private:

  /// Given the \p lowLim of some valid AlignedStorage's memory region, returns

  /// a pointer to the SegmentInfo laid out in that storage,

  /// assuming it exists.

  static SegmentInfo *get(void *lowLim) {

    assert(lowLim == AlignedStorage::start(lowLim) && "Precondition.");

    return reinterpret_cast<SegmentInfo *>(lowLim);

  }

  static const SegmentInfo *get(const void *lowLim) {

    assert(lowLim == AlignedStorage::start(lowLim) && "Precondition.");

    return reinterpret_cast<const SegmentInfo *>(lowLim);

  }

  unsigned index;

};

} // namespace vm

} // namespace hermes

#endif