mimir/src/arena.cpp

//
//  arena.cpp
//  Kraken Engine
//
//  Copyright 2026 Kearwood Gilbert. All rights reserved.
//
//  Redistribution and use in source and binary forms, with or without modification, are
//  permitted provided that the following conditions are met:
//
//  1. Redistributions of source code must retain the above copyright notice, this list of
//  conditions and the following disclaimer.
//
//  2. Redistributions in binary form must reproduce the above copyright notice, this list
//  of conditions and the following disclaimer in the documentation and/or other materials
//  provided with the distribution.
//
//  THIS SOFTWARE IS PROVIDED BY KEARWOOD GILBERT ''AS IS'' AND ANY EXPRESS OR IMPLIED
//  WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
//  FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL KEARWOOD GILBERT OR
//  CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
//  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
//  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
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//  NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
//  ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
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//  authors and should not be interpreted as representing official policies, either expressed
//  or implied, of Kearwood Gilbert.
//

#include "../include/mimir.h"
#include "mimir_impl.h"

#if defined(__unix__) || defined(__APPLE__) || defined(ANDROID)
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#endif

#if defined(_WIN32) || defined(_WIN64)
#include <Windows.h>
#include <memoryapi.h>
#endif

#include <errno.h>
#include <cassert>
#include <cstring>
#include <bit>

namespace mimir {

struct ArenaControlBlock
{
  size_t allocated;
  size_t comitted;
  size_t next;
};

static_assert(sizeof(ArenaControlBlock) < 4096);

Arena::Arena(size_t minSize, size_t maxSize)
  : m_minSize(minSize)
  , m_maxSize(maxSize)
  , m_data(nullptr)
  , m_usedSize(0)
  , m_committedSize(0)
{
  assert(maxSize >= minSize);

  for (size_t i = 0; i < kWatermarkLen; i++) {
    m_watermark[i] = 0;
  }
}

Arena::~Arena()
{
  if (m_data) {
#if defined(_WIN32) || defined(_WIN64)
    if (!VirtualFree((void*)m_data, 0, MEM_RELEASE)) {
      ReportWindowsLastError("VirtualAlloc");
    }
#elif defined(__unix__) || defined(__APPLE__) || defined(ANDROID)
    munmap(m_data, KRAKEN_MEM_ROUND_UP_PAGE(m_maxSize));
#else
    static_assert(false, "Not Implemented");
#endif
  }
}

std::byte* Arena::alloc(size_t size)
{
  size_t neededSize = m_usedSize + size;

  if (m_data == nullptr) {
#if defined(_WIN32) || defined(_WIN64)
    m_data = (std::byte*)VirtualAlloc(NULL, KRAKEN_MEM_ROUND_UP_PAGE(m_maxSize), MEM_RESERVE, PAGE_NOACCESS);
    if (m_data == nullptr) {
      ReportWindowsLastError("VirtualAlloc");
      return nullptr;
    }
#elif defined(__unix__) || defined(__APPLE__) || defined(ANDROID)
    m_data = (std::byte*)mmap(NULL, KRAKEN_MEM_ROUND_UP_PAGE(m_maxSize), PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (m_data == MAP_FAILED) {
      return nullptr;
    }
#else
    static_assert(false, "Not Implemented");
#endif
  }

  if (neededSize > m_committedSize)
  {
    size_t newSize = std::bit_ceil(neededSize) << 1;
    if (newSize < m_minSize) {
      newSize = m_minSize;
    }
    newSize = KRAKEN_MEM_ROUND_UP_PAGE(newSize);

#if defined(_WIN32) || defined(_WIN64)
    if(VirtualAlloc(m_data + m_committedSize, newSize - m_committedSize, MEM_COMMIT, PAGE_READWRITE) == nullptr) {
      ReportWindowsLastError("VirtualAlloc");
      return nullptr;
    }
#elif defined(__unix__) || defined(__APPLE__) || defined(ANDROID)
    if (mprotect(m_data + m_committedSize, newSize - m_committedSize, PROT_READ | PROT_WRITE) != 0) {
      return nullptr;
    }
#else
    static_assert(false, "Not Implemented");
#endif
    m_committedSize = newSize;
  } // if (neededSize > m_committedSize)

  std::byte* ret = m_data + m_usedSize;
  m_usedSize = neededSize;
  return ret;
}

std::byte* Arena::allocA16(size_t size)
{
  uint32_t nextByte = (m_usedSize + 15) & ~15;
  uint32_t roundedSize = (size + 15) & ~15;
  if (alloc(size) == nullptr)     {
    return nullptr;
  }

  return m_data + nextByte;
}

std::byte* Arena::allocA64(size_t size)
{
  uint32_t nextByte = (m_usedSize + 63) & ~63;
  uint32_t roundedSize = (size + 63) & ~63;
  if (alloc(size) == nullptr) {
    return nullptr;
  }

  return m_data + nextByte;
}

void Arena::reset()
{
  size_t highWatermark = m_usedSize;
  for (size_t i = 0; i < kWatermarkLen - 1; i++) {
    m_watermark[i] = m_watermark[i + 1];
    if (m_watermark[i] > highWatermark) {
      highWatermark = m_watermark[i];
    }
  }
  m_watermark[kWatermarkLen - 1] = m_usedSize;

  size_t targetSize = std::bit_ceil(highWatermark) << 1;
  if (targetSize < m_minSize) {
    targetSize = m_minSize;
  }
  size_t thresholdSize = targetSize << 1; // The threshold for shrinking is greater than the target to implement hysteresis
  if (m_committedSize > thresholdSize) {
    size_t newSize = KRAKEN_MEM_ROUND_UP_PAGE(targetSize);

    std::byte* rangeStart = m_data + newSize;
    size_t rangeLen = m_committedSize - newSize;
#if defined(_WIN32) || defined(_WIN64)
    if (!VirtualAlloc(rangeStart, rangeLen, MEM_RESET, PAGE_NOACCESS)) {
      ReportWindowsLastError("VirtualAlloc");
    } else {
      m_committedSize = newSize;
    }
#elif defined(__unix__) || defined(__APPLE__) || defined(ANDROID)
    if (madvise(rangeStart, rangeLen, MADV_DONTNEED) != 0) {
      // TODO - Log Error or debug build assert
    }
    if (mprotect(rangeStart, rangeLen, PROT_NONE) != 0) {
      // TODO - Log Error or debug build assert
    }
#else
    static_assert(false, "Not Implemented");
#endif
    m_committedSize = newSize;
  }

  m_usedSize = 0;
}

} // namespace mimir