my_alloc.h

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/* Copyright (c) 2000, 2025, Oracle and/or its affiliates.
 
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License, version 2.0,
   as published by the Free Software Foundation.
 
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   This program is distributed in the hope that it will be useful,
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   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License, version 2.0, for more details.
 
   You should have received a copy of the GNU General Public License
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   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
/**
 * @file include/my_alloc.h
 *
 * This file follows Google coding style, except for the name MEM_ROOT (which is
 * kept for historical reasons).
 */
 
#ifndef INCLUDE_MY_ALLOC_H_
#define INCLUDE_MY_ALLOC_H_
 
#include <string.h>
 
#include <memory>
#include <new>
#include <type_traits>
#include <utility>
 
#include "memory_debugging.h"
#include "my_compiler.h"
#include "my_dbug.h"
#include "my_inttypes.h"
#include "my_pointer_arithmetic.h"
#include "mysql/psi/psi_memory.h"
 
#if defined(MYSQL_SERVER)
extern "C" void sql_alloc_error_handler();
#endif
 
/**
 * The MEM_ROOT is a simple arena, where allocations are carved out of
 * larger blocks. Using an arena over plain malloc gives you two main
 * advantages:
 *
 *   * Allocation is very cheap (only a few CPU cycles on the fast path).
 *   * You do not need to keep track of which memory you have allocated,
 *     as it will all be freed when the arena is destroyed.
 *
 * Thus, if you need to do many small allocations that all are to have
 * roughly the same lifetime, the MEM_ROOT is probably a good choice.
 * The flip side is that _no_ memory is freed until the arena is destroyed,
 * and no destructors are run (although you can run them manually yourself).
 *
 *
 * This specific implementation works by allocating exponentially larger blocks
 * each time it needs more memory (generally increasing them by 50%), which
 * guarantees O(1) total calls to malloc and free. Only one free block is
 * ever used; as soon as there's an allocation that comes in that doesn't fit,
 * that block is stored away and never allocated from again. (There's an
 * exception for allocations larger than the block size; see #AllocSlow
 * for details.)
 *
 * The MEM_ROOT is thread-compatible but not thread-safe. This means you cannot
 * use the same instance from multiple threads at the same time without external
 * synchronization, but you can use different MEM_ROOTs concurrently in
 * different threads.
 *
 * For C compatibility reasons, MEM_ROOT is a struct, even though it is
 * logically a class and follows the style guide for classes.
 */
struct MEM_ROOT {
 private:
  struct Block {
    Block *prev{nullptr}; /** Previous block; used for freeing. */
    char *end{nullptr};   /** One byte past the end; used for Contains(). */
  };
 
 public:
  MEM_ROOT() : MEM_ROOT(0, 512) {}  // 0 = PSI_NOT_INSTRUMENTED.
 
  MEM_ROOT(PSI_memory_key key, size_t block_size)
      : m_block_size(block_size),
        m_orig_block_size(block_size),
        m_psi_key(key) {
#if defined(MYSQL_SERVER)
    m_error_handler = sql_alloc_error_handler;
#endif
  }
 
  // MEM_ROOT is movable but not copyable.
  MEM_ROOT(const MEM_ROOT &) = delete;
  MEM_ROOT(MEM_ROOT &&other)
  noexcept
      : m_current_block(other.m_current_block),
        m_current_free_start(other.m_current_free_start),
        m_current_free_end(other.m_current_free_end),
        m_block_size(other.m_block_size),
        m_orig_block_size(other.m_orig_block_size),
        m_max_capacity(other.m_max_capacity),
        m_allocated_size(other.m_allocated_size),
        m_error_for_capacity_exceeded(other.m_error_for_capacity_exceeded),
        m_error_handler(other.m_error_handler),
        m_psi_key(other.m_psi_key) {
    other.m_current_block = nullptr;
    other.m_allocated_size = 0;
    other.m_block_size = m_orig_block_size;
    other.m_current_free_start = &s_dummy_target;
    other.m_current_free_end = &s_dummy_target;
  }
 
  MEM_ROOT &operator=(const MEM_ROOT &) = delete;
  MEM_ROOT &operator=(MEM_ROOT &&other) noexcept {
    Clear();
    ::new (this) MEM_ROOT(std::move(other));
    return *this;
  }
 
  ~MEM_ROOT() { Clear(); }
 
  /**
   * Allocate memory. Will return nullptr if there's not enough memory,
   * or if the maximum capacity is reached.
   *
   * Note that a zero-length allocation can return _any_ pointer, including
   * nullptr or a pointer that has been given out before. The current
   * implementation takes some pains to make sure we never return nullptr
   * (although it might return a bogus pointer), since there is code that
   * assumes nullptr always means 'out of memory', but you should not rely on
   * it, as it may change in the future.
   *
   * The returned pointer will always be 8-aligned.
   */
  void *Alloc(size_t length) MY_ATTRIBUTE((malloc)) {
    length = ALIGN_SIZE(length);
 
    // Skip the straight path if simulating OOM; it should always fail.
    DBUG_EXECUTE_IF("simulate_out_of_memory", return AllocSlow(length););
 
    // Fast path, used in the majority of cases. It would be faster here
    // (saving one register due to CSE) to instead test
    //
    //   m_current_free_start + length <= m_current_free_end
    //
    // but it would invoke undefined behavior, and in particular be prone
    // to wraparound on 32-bit platforms.
    if (static_cast<size_t>(m_current_free_end - m_current_free_start) >=
        length) {
      void *ret = m_current_free_start;
      m_current_free_start += length;
      return ret;
    }
 
    return AllocSlow(length);
  }
 
  /**
    Allocate 'num' objects of type T, and initialize them to a default value
    that is created by passing the supplied args to T's constructor. If args
    is empty, value-initialization is used. For primitive types, like int and
    pointers, this means the elements will be set to the equivalent of 0
    (or false or nullptr).
 
    If the constructor throws an exception, behavior is undefined.
 
    We don't use new[], as it can put extra data in front of the array.
   */
  template <class T, class... Args>
  T *ArrayAlloc(size_t num, Args... args) {
    static_assert(alignof(T) <= 8, "MEM_ROOT only returns 8-aligned memory.");
    if (num * sizeof(T) < num) {
      // Overflow.
      return nullptr;
    }
    T *ret = static_cast<T *>(Alloc(num * sizeof(T)));
    if (ret == nullptr) {
      // Out of memory.
      return nullptr;
    }
 
    // Initialize all elements.
    for (size_t i = 0; i < num; ++i) {
      ::new (&ret[i]) T(args...);
    }
 
    return ret;
  }
 
  /**
   * Claim all the allocated memory for the current thread in the performance
   * schema. Use when transferring responsibility for a MEM_ROOT from one thread
   * to another.
   */
  void Claim(bool claim);
 
  /**
   * Deallocate all the RAM used. The MEM_ROOT itself continues to be valid,
   * so you can make new calls to Alloc() afterwards.
 
   * @note
   *   One can call this function either with a MEM_ROOT initialized with the
   *   constructor, or with one that's memset() to all zeros.
   *   It's also safe to call this multiple times with the same mem_root.
   */
  void Clear();
 
  /**
   * Similar to Clear(), but anticipates that the block will be reused for
   * further allocations. This means that even though all the data is gone,
   * one memory block (typically the largest allocated) will be kept and
   * made immediately available for calls to Alloc() without having to go to the
   * OS for new memory. This can yield performance gains if you use the same
   * MEM_ROOT many times. Also, the block size is not reset.
   */
  void ClearForReuse();
 
  /**
    Whether the constructor has run or not.
 
    This exists solely to support legacy code that memset()s the MEM_ROOT to
    all zeros, which wants to distinguish between that state and a properly
    initialized MEM_ROOT. If you do not run the constructor _nor_ do memset(),
    you are invoking undefined behavior.
  */
  bool inited() const { return m_block_size != 0; }
 
  /**
   * Set maximum capacity for this MEM_ROOT. Whenever the MEM_ROOT has
   * allocated more than this (not including overhead), and the free block
   * is empty, future allocations will fail.
   *
   * @param max_capacity        Maximum capacity this mem_root can hold
   */
  void set_max_capacity(size_t max_capacity) { m_max_capacity = max_capacity; }
 
  /**
   * Return maximum capacity for this MEM_ROOT.
   */
  size_t get_max_capacity() const { return m_max_capacity; }
 
  /**
   * Enable/disable error reporting for exceeding the maximum capacity.
   * If error reporting is enabled, an error is flagged to indicate that the
   * capacity is exceeded. However, allocation will still happen for the
   * requested memory.
   *
   * @param report_error    whether the error should be reported
   */
  void set_error_for_capacity_exceeded(bool report_error) {
    m_error_for_capacity_exceeded = report_error;
  }
 
  /**
   * Return whether error is to be reported when
   * maximum capacity exceeds for MEM_ROOT.
   */
  bool get_error_for_capacity_exceeded() const {
    return m_error_for_capacity_exceeded;
  }
 
  /**
   * Set the error handler on memory allocation failure (or nullptr for none).
   * The error handler is called called whenever my_malloc() failed to allocate
   * more memory from the OS (which causes my_alloc() to return nullptr).
   */
  void set_error_handler(void (*error_handler)(void)) {
    m_error_handler = error_handler;
  }
 
  /**
   * Amount of memory we have allocated from the operating system, not including
   * overhead.
   */
  size_t allocated_size() const { return m_allocated_size; }
 
  /**
   * Set the desired size of the next block to be allocated. Note that future
   * allocations
   * will grow in size over this, although a Clear() will reset the size again.
   */
  void set_block_size(size_t block_size) {
    m_block_size = m_orig_block_size = block_size;
  }
 
  /**
   * @name Raw interface
   * Peek(), ForceNewBlock() and RawCommit() together define an
   * alternative interface to MEM_ROOT, for special uses. The raw interface
   * gives direct access to the underlying blocks, allowing a user to bypass the
   * normal alignment requirements and to write data directly into them without
   * knowing beforehand exactly how long said data is going to be, while still
   * retaining the convenience of block management and automatic freeing. It
   * generally cannot be combined with calling Alloc() as normal; see RawCommit.
   *
   * The raw interface, unlike Alloc(), is not affected by running under
   * ASan or Valgrind.
   *
   * @{
   */
 
  /**
   * Get the bounds of the currently allocated memory block. Assuming no other
   * MEM_ROOT calls are made in the meantime, you can start writing into this
   * block and then call RawCommit() once you know how many bytes you actually
   * needed. (This is useful when e.g. packing rows.)
   */
  std::pair<char *, char *> Peek() const {
    return {m_current_free_start, m_current_free_end};
  }
 
  /**
   * Allocate a new block of at least 'minimum_length' bytes; usually more.
   * This holds no matter how many bytes are free in the current block.
   * The new block will always become the current block, ie., the next call
   * to Peek() will return the newly allocated block. (This is different
   * from Alloc(), where it is possible to allocate a new block that is
   * not made into the current block.)
   *
   * @return true Allocation failed (possibly due to size restrictions).
   */
  bool ForceNewBlock(size_t minimum_length);
 
  /**
   * Mark the first N bytes as the current block as used.
   *
   * WARNING: If you use RawCommit() with a length that is not a multiple of 8,
   * you cannot use Alloc() afterwards! The exception is that if EnsureSpace()
   * has just returned, you've got a new block, and can use Alloc() again.
   */
  void RawCommit(size_t length) {
    assert(static_cast<size_t>(m_current_free_end - m_current_free_start) >=
           length);
    m_current_free_start += length;
  }
 
  /**
   * Returns whether this MEM_ROOT contains the given pointer,
   * ie., whether it was given back from Alloc(n) (given n >= 1)
   * at some point. This means it will be legally accessible until
   * the next Clear() or ClearForReuse() call.
   */
  bool Contains(void *ptr) const {
    for (Block *block = m_current_block; block != nullptr;
         block = block->prev) {
      if (ptr >= block && ptr < block->end) {
        return true;
      }
    }
    return false;
  }
 
  bool IsSingleBlock() const {
    return m_current_block != nullptr && m_current_block->prev == nullptr;
  }
  /// @}
 
 private:
  /**
   * Something to point on that exists solely to never return nullptr
   * from Alloc(0).
   */
  static char s_dummy_target;
 
  /**
    Allocate a new block of the given length (plus overhead for the block
    header). If the MEM_ROOT is near capacity, it may allocate less memory
    than wanted_length, but if it cannot allocate at least minimum_length,
    will return nullptr.
  */
  Block *AllocBlock(size_t wanted_length, size_t minimum_length);
 
  /** Allocate memory that doesn't fit into the current free block. */
  void *AllocSlow(size_t length);
 
  /** Free all blocks in a linked list, starting at the given block. */
  static void FreeBlocks(Block *start);
 
  /** The current block we are giving out memory from. nullptr if none. */
  Block *m_current_block = nullptr;
 
  /** Start (inclusive) of the current free block. */
  char *m_current_free_start = &s_dummy_target;
 
  /** End (exclusive) of the current free block. */
  char *m_current_free_end = &s_dummy_target;
 
  /** Size of the _next_ block we intend to allocate. */
  size_t m_block_size;
 
  /** The original block size the user asked for on construction. */
  size_t m_orig_block_size;
 
  /**
    Maximum amount of memory this MEM_ROOT can hold. A value of 0
    implies there is no limit.
  */
  size_t m_max_capacity = 0;
 
  /**
   * Total allocated size for this MEM_ROOT. Does not include overhead
   * for block headers or malloc overhead, since especially the latter
   * is impossible to quantify portably.
   */
  size_t m_allocated_size = 0;
 
  /** If enabled, exceeding the capacity will lead to a my_error() call. */
  bool m_error_for_capacity_exceeded = false;
 
  void (*m_error_handler)(void) = nullptr;
 
  PSI_memory_key m_psi_key = 0;
};
 
/**
 * Allocate an object of the given type. Use like this:
 *
 *   Foo *foo = new (mem_root) Foo();
 *
 * Note that unlike regular operator new, this will not throw exceptions.
 * However, it can return nullptr if the capacity of the MEM_ROOT has been
 * reached. This is allowed since it is not a replacement for global operator
 * new, and thus isn't used automatically by e.g. standard library containers.
 *
 * TODO: This syntax is confusing in that it could look like allocating
 * a MEM_ROOT using regular placement new. We should make a less ambiguous
 * syntax, e.g. new (On(mem_root)) Foo().
 */
inline void *operator new(size_t size, MEM_ROOT *mem_root,
                          const std::nothrow_t &arg
                          [[maybe_unused]] = std::nothrow) noexcept {
  return mem_root->Alloc(size);
}
 
inline void *operator new[](size_t size, MEM_ROOT *mem_root,
                            const std::nothrow_t &arg
                            [[maybe_unused]] = std::nothrow) noexcept {
  return mem_root->Alloc(size);
}
 
inline void operator delete(void *, MEM_ROOT *,
                            const std::nothrow_t &) noexcept {
  /* never called */
}
 
inline void operator delete[](void *, MEM_ROOT *,
                              const std::nothrow_t &) noexcept {
  /* never called */
}
 
template <class T>
inline void destroy_at(T *ptr) {
  assert(ptr != nullptr);
  std::destroy_at(ptr);
  TRASH(const_cast<std::remove_const_t<T> *>(ptr), sizeof(T));
}
 
/*
 * For std::unique_ptr with objects allocated on a MEM_ROOT, you shouldn't use
 * Default_deleter; use this deleter instead.
 */
template <class T>
class Destroy_only {
 public:
  void operator()(T *ptr) const { ::destroy_at(ptr); }
};
 
/** std::unique_ptr, but only destroying. */
template <class T>
using unique_ptr_destroy_only = std::unique_ptr<T, Destroy_only<T>>;
 
template <typename T, typename... Args>
unique_ptr_destroy_only<T> make_unique_destroy_only(MEM_ROOT *mem_root,
                                                    Args &&...args) {
  return unique_ptr_destroy_only<T>(new (mem_root)
                                        T(std::forward<Args>(args)...));
}
 
#endif  // INCLUDE_MY_ALLOC_H_