pz_gc_layout.h

/*
 * Plasma garbage collector memory layout
 * vim: ts=4 sw=4 et
 *
 * Copyright (C) Plasma Team
 * Distributed under the terms of the MIT license, see ../LICENSE.code
 */

#ifndef PZ_GC_LAYOUT_H
#define PZ_GC_LAYOUT_H

#include "pz_gc.h"
#include "pz_gc.impl.h"

namespace pz {

constexpr uint8_t Poison_Byte = 0xF0;

/*
 * These must be a power-of-two and mmap must align to them. 4K is the
 * default.
 */
static const unsigned GC_Block_Log     = 13;
static const size_t   GC_Block_Size    = 1 << (GC_Block_Log - 1);
static const size_t   GC_Block_Mask    = ~(GC_Block_Size - 1);
static const unsigned GC_Min_Cell_Size = 2;
static const unsigned GC_Cells_Per_Block =
    GC_Block_Size / (GC_Min_Cell_Size * WORDSIZE_BYTES);

/*
 * GC_Chunk_Size is also a power of two and is therefore a multiple of
 * GC_Block_Size.  4MB is the default.
 */
static const unsigned GC_Chunk_Log       = 23;
static const size_t   GC_Chunk_Size      = 1 << (GC_Chunk_Log - 1);
static const size_t   GC_Block_Per_Chunk = (GC_Chunk_Size / GC_Block_Size) - 1;

#if PZ_DEV
// Set this low for testing.
static const size_t GC_Initial_Threshold = 8 * GC_Block_Size;
#else
static const size_t GC_Initial_Threshold = 64 * GC_Block_Size;
#endif
static const float GC_Threshold_Factor = 1.5f;

// The threshold for small allocations in words.  Allocations of less than
// this many words are small allocations.
static const size_t GC_Small_Alloc_Threshold = 64;

static_assert(GC_Chunk_Size > GC_Block_Size,
              "Chunks must be larger than blocks");

/*
 * The heap is made out of blocks and chunks.  A chunk contains multiple
 * blocks, which each contain multiple cells.
 */

enum CellType {
    // Used for Invalid cells or unallocated chunks.
    CT_INVALID,

    CT_BOP,
    CT_FIT
};

/*
 * This class should be used by-value as a reference to a cell.
 */
class CellPtr
{
   protected:
    void **  m_ptr;
    CellType m_type;

    constexpr CellPtr() : m_ptr(nullptr), m_type(CT_INVALID) {}

   public:
    constexpr explicit CellPtr(void * ptr, CellType type)
        : m_ptr(static_cast<void **>(ptr))
        , m_type(type)
    {}

    void ** pointer()
    {
        return m_ptr;
    }

    bool is_valid() const
    {
        return m_ptr != nullptr;
    }
    bool is_bop_cell() const
    {
        return m_type == CT_BOP;
    }
    bool is_fit_cell() const
    {
        return m_type == CT_FIT;
    }
};

/*
 * Chunks
 */
class Chunk
{
   protected:
    Heap * m_heap;
    CellType m_type;

   private:
    Chunk(const Chunk &) = delete;
    void operator=(const Chunk &) = delete;

    Chunk(Heap * heap) : m_heap(heap), m_type(CT_INVALID) {}

   protected:
    Chunk(Heap * heap, CellType type) : m_heap(heap), m_type(type) {}

   public:
    /*
     * True if this pointer lies within the allocated part of this chunk.
     */
    bool contains_pointer(void * ptr) const
    {
        return ptr >= this &&
               ptr < (reinterpret_cast<const uint8_t *>(this) + GC_Chunk_Size);
    };
};

}  // namespace pz

#include "pz_gc_layout_bop.h"
#include "pz_gc_layout_fit.h"

namespace pz {

static_assert(
    GC_Small_Alloc_Threshold <= Block::Max_Cell_Size,
    "The small alloc threshold must be less than the maximum cell size");

}  // namespace pz

#endif  // ! PZ_GC_LAYOUT_H