boost/pool/simple_segregated_storage.hpp
// Copyright (C) 2000, 2001 Stephen Cleary
//
// Distributed under the Boost Software License, Version 1.0. (See
// accompanying file LICENSE_1_0.txt or copy at
// http://www.boost.org/LICENSE_1_0.txt)
//
// See http://www.boost.org for updates, documentation, and revision history.
#ifndef BOOST_SIMPLE_SEGREGATED_STORAGE_HPP
#define BOOST_SIMPLE_SEGREGATED_STORAGE_HPP
/*!
\file
\brief Simple Segregated Storage.
\details A simple segregated storage implementation:
simple segregated storage is the basic idea behind the Boost Pool library.
Simple segregated storage is the simplest, and probably the fastest,
memory allocation/deallocation algorithm.
It begins by partitioning a memory block into fixed-size chunks.
Where the block comes from is not important until implementation time.
A Pool is some object that uses Simple Segregated Storage in this fashion.
*/
// std::greater
#include <functional>
#include <boost/pool/poolfwd.hpp>
#ifdef BOOST_MSVC
#pragma warning(push)
#pragma warning(disable:4127) // Conditional expression is constant
#endif
#ifdef BOOST_POOL_VALIDATE
# define BOOST_POOL_VALIDATE_INTERNALS validate();
#else
# define BOOST_POOL_VALIDATE_INTERNALS
#endif
namespace boost {
/*!
\brief Simple Segregated Storage is the simplest, and probably the fastest,
memory allocation/deallocation algorithm. It is responsible for
partitioning a memory block into fixed-size chunks: where the block comes from
is determined by the client of the class.
\details Template class simple_segregated_storage controls access to a free list of memory chunks.
Please note that this is a very simple class, with preconditions on almost all its functions. It is intended to
be the fastest and smallest possible quick memory allocator - e.g., something to use in embedded systems.
This class delegates many difficult preconditions to the user (i.e., alignment issues).
An object of type simple_segregated_storage<SizeType> is empty if its free list is empty.
If it is not empty, then it is ordered if its free list is ordered. A free list is ordered if repeated calls
to <tt>malloc()</tt> will result in a constantly-increasing sequence of values, as determined by <tt>std::less<void *></tt>.
A member function is <i>order-preserving</i> if the free list maintains its order orientation (that is, an
ordered free list is still ordered after the member function call).
*/
template <typename SizeType>
class simple_segregated_storage
{
public:
typedef SizeType size_type;
private:
simple_segregated_storage(const simple_segregated_storage &);
void operator=(const simple_segregated_storage &);
static void * try_malloc_n(void * & start, size_type n,
size_type partition_size);
protected:
void * first; /*!< This data member is the free list.
It points to the first chunk in the free list,
or is equal to 0 if the free list is empty.
*/
void * find_prev(void * ptr);
// for the sake of code readability :)
static void * & nextof(void * const ptr)
{ //! The return value is just *ptr cast to the appropriate type. ptr must not be 0. (For the sake of code readability :)
//! As an example, let us assume that we want to truncate the free list after the first chunk.
//! That is, we want to set *first to 0; this will result in a free list with only one entry.
//! The normal way to do this is to first cast first to a pointer to a pointer to void,
//! and then dereference and assign (*static_cast<void **>(first) = 0;).
//! This can be done more easily through the use of this convenience function (nextof(first) = 0;).
//! \returns dereferenced pointer.
return *(static_cast<void **>(ptr));
}
public:
// Post: empty()
simple_segregated_storage()
:first(0)
{ //! Construct empty storage area.
//! \post empty()
}
static void * segregate(void * block,
size_type nsz, size_type npartition_sz,
void * end = 0);
// Same preconditions as 'segregate'
// Post: !empty()
void add_block(void * const block,
const size_type nsz, const size_type npartition_sz)
{ //! Add block
//! Segregate this block and merge its free list into the
//! free list referred to by "first".
//! \pre Same as segregate.
//! \post !empty()
BOOST_POOL_VALIDATE_INTERNALS
first = segregate(block, nsz, npartition_sz, first);
BOOST_POOL_VALIDATE_INTERNALS
}
// Same preconditions as 'segregate'
// Post: !empty()
void add_ordered_block(void * const block,
const size_type nsz, const size_type npartition_sz)
{ //! add block (ordered into list)
//! This (slower) version of add_block segregates the
//! block and merges its free list into our free list
//! in the proper order.
BOOST_POOL_VALIDATE_INTERNALS
// Find where "block" would go in the free list
void * const loc = find_prev(block);
// Place either at beginning or in middle/end
if (loc == 0)
add_block(block, nsz, npartition_sz);
else
nextof(loc) = segregate(block, nsz, npartition_sz, nextof(loc));
BOOST_POOL_VALIDATE_INTERNALS
}
// default destructor.
bool empty() const
{ //! \returns true only if simple_segregated_storage is empty.
return (first == 0);
}
void * malloc BOOST_PREVENT_MACRO_SUBSTITUTION()
{ //! Create a chunk.
//! \pre !empty()
//! Increment the "first" pointer to point to the next chunk.
BOOST_POOL_VALIDATE_INTERNALS
void * const ret = first;
// Increment the "first" pointer to point to the next chunk.
first = nextof(first);
BOOST_POOL_VALIDATE_INTERNALS
return ret;
}
void free BOOST_PREVENT_MACRO_SUBSTITUTION(void * const chunk)
{ //! Free a chunk.
//! \pre chunk was previously returned from a malloc() referring to the same free list.
//! \post !empty()
BOOST_POOL_VALIDATE_INTERNALS
nextof(chunk) = first;
first = chunk;
BOOST_POOL_VALIDATE_INTERNALS
}
void ordered_free(void * const chunk)
{ //! This (slower) implementation of 'free' places the memory
//! back in the list in its proper order.
//! \pre chunk was previously returned from a malloc() referring to the same free list
//! \post !empty().
// Find where "chunk" goes in the free list
BOOST_POOL_VALIDATE_INTERNALS
void * const loc = find_prev(chunk);
// Place either at beginning or in middle/end.
if (loc == 0)
(free)(chunk);
else
{
nextof(chunk) = nextof(loc);
nextof(loc) = chunk;
}
BOOST_POOL_VALIDATE_INTERNALS
}
void * malloc_n(size_type n, size_type partition_size);
//! \pre chunks was previously allocated from *this with the same
//! values for n and partition_size.
//! \post !empty()
//! \note If you're allocating/deallocating n a lot, you should
//! be using an ordered pool.
void free_n(void * const chunks, const size_type n,
const size_type partition_size)
{
BOOST_POOL_VALIDATE_INTERNALS
if(n != 0)
add_block(chunks, n * partition_size, partition_size);
BOOST_POOL_VALIDATE_INTERNALS
}
// pre: chunks was previously allocated from *this with the same
// values for n and partition_size.
// post: !empty()
void ordered_free_n(void * const chunks, const size_type n,
const size_type partition_size)
{ //! Free n chunks from order list.
//! \pre chunks was previously allocated from *this with the same
//! values for n and partition_size.
//! \pre n should not be zero (n == 0 has no effect).
BOOST_POOL_VALIDATE_INTERNALS
if(n != 0)
add_ordered_block(chunks, n * partition_size, partition_size);
BOOST_POOL_VALIDATE_INTERNALS
}
#ifdef BOOST_POOL_VALIDATE
void validate()
{
int index = 0;
void* old = 0;
void* ptr = first;
while(ptr)
{
void* pt = nextof(ptr); // trigger possible segfault *before* we update variables
++index;
old = ptr;
ptr = nextof(ptr);
}
}
#endif
};
//! Traverses the free list referred to by "first",
//! and returns the iterator previous to where
//! "ptr" would go if it was in the free list.
//! Returns 0 if "ptr" would go at the beginning
//! of the free list (i.e., before "first").
//! \note Note that this function finds the location previous to where ptr would go
//! if it was in the free list.
//! It does not find the entry in the free list before ptr
//! (unless ptr is already in the free list).
//! Specifically, find_prev(0) will return 0,
//! not the last entry in the free list.
//! \returns location previous to where ptr would go if it was in the free list.
template <typename SizeType>
void * simple_segregated_storage<SizeType>::find_prev(void * const ptr)
{
// Handle border case.
if (first == 0 || std::greater<void *>()(first, ptr))
return 0;
void * iter = first;
while (true)
{
// if we're about to hit the end, or if we've found where "ptr" goes.
if (nextof(iter) == 0 || std::greater<void *>()(nextof(iter), ptr))
return iter;
iter = nextof(iter);
}
}
//! Segregate block into chunks.
//! \pre npartition_sz >= sizeof(void *)
//! \pre npartition_sz = sizeof(void *) * i, for some integer i
//! \pre nsz >= npartition_sz
//! \pre Block is properly aligned for an array of object of
//! size npartition_sz and array of void *.
//! The requirements above guarantee that any pointer to a chunk
//! (which is a pointer to an element in an array of npartition_sz)
//! may be cast to void **.
template <typename SizeType>
void * simple_segregated_storage<SizeType>::segregate(
void * const block,
const size_type sz,
const size_type partition_sz,
void * const end)
{
// Get pointer to last valid chunk, preventing overflow on size calculations
// The division followed by the multiplication just makes sure that
// old == block + partition_sz * i, for some integer i, even if the
// block size (sz) is not a multiple of the partition size.
char * old = static_cast<char *>(block)
+ ((sz - partition_sz) / partition_sz) * partition_sz;
// Set it to point to the end
nextof(old) = end;
// Handle border case where sz == partition_sz (i.e., we're handling an array
// of 1 element)
if (old == block)
return block;
// Iterate backwards, building a singly-linked list of pointers
for (char * iter = old - partition_sz; iter != block;
old = iter, iter -= partition_sz)
nextof(iter) = old;
// Point the first pointer, too
nextof(block) = old;
return block;
}
//! \pre (n > 0), (start != 0), (nextof(start) != 0)
//! \post (start != 0)
//! The function attempts to find n contiguous chunks
//! of size partition_size in the free list, starting at start.
//! If it succeds, it returns the last chunk in that contiguous
//! sequence, so that the sequence is known by [start, {retval}]
//! If it fails, it does do either because it's at the end of the
//! free list or hits a non-contiguous chunk. In either case,
//! it will return 0, and set start to the last considered
//! chunk. You are at the end of the free list if
//! nextof(start) == 0. Otherwise, start points to the last
//! chunk in the contiguous sequence, and nextof(start) points
//! to the first chunk in the next contiguous sequence (assuming
//! an ordered free list).
template <typename SizeType>
void * simple_segregated_storage<SizeType>::try_malloc_n(
void * & start, size_type n, const size_type partition_size)
{
void * iter = nextof(start);
while (--n != 0)
{
void * next = nextof(iter);
if (next != static_cast<char *>(iter) + partition_size)
{
// next == 0 (end-of-list) or non-contiguous chunk found
start = iter;
return 0;
}
iter = next;
}
return iter;
}
//! Attempts to find a contiguous sequence of n partition_sz-sized chunks. If found, removes them
//! all from the free list and returns a pointer to the first. If not found, returns 0. It is strongly
//! recommended (but not required) that the free list be ordered, as this algorithm will fail to find
//! a contiguous sequence unless it is contiguous in the free list as well. Order-preserving.
//! O(N) with respect to the size of the free list.
template <typename SizeType>
void * simple_segregated_storage<SizeType>::malloc_n(const size_type n,
const size_type partition_size)
{
BOOST_POOL_VALIDATE_INTERNALS
if(n == 0)
return 0;
void * start = &first;
void * iter;
do
{
if (nextof(start) == 0)
return 0;
iter = try_malloc_n(start, n, partition_size);
} while (iter == 0);
void * const ret = nextof(start);
nextof(start) = nextof(iter);
BOOST_POOL_VALIDATE_INTERNALS
return ret;
}
} // namespace boost
#ifdef BOOST_MSVC
#pragma warning(pop)
#endif
#endif