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pyarena.c
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#include "Python.h"
#include "pycore_pyarena.h" // PyArena
/* A simple arena block structure.
Measurements with standard library modules suggest the average
allocation is about 20 bytes and that most compilers use a single
block.
*/
#define DEFAULT_BLOCK_SIZE 8192
#define ALIGNMENT 8
typedef struct _block {
/* Total number of bytes owned by this block available to pass out.
* Read-only after initialization. The first such byte starts at
* ab_mem.
*/
size_t ab_size;
/* Total number of bytes already passed out. The next byte available
* to pass out starts at ab_mem + ab_offset.
*/
size_t ab_offset;
/* An arena maintains a singly-linked, NULL-terminated list of
* all blocks owned by the arena. These are linked via the
* ab_next member.
*/
struct _block *ab_next;
/* Pointer to the first allocatable byte owned by this block. Read-
* only after initialization.
*/
void *ab_mem;
} block;
/* The arena manages two kinds of memory, blocks of raw memory
and a list of PyObject* pointers. PyObjects are decrefed
when the arena is freed.
*/
struct _arena {
/* Pointer to the first block allocated for the arena, never NULL.
It is used only to find the first block when the arena is
being freed.
*/
block *a_head;
/* Pointer to the block currently used for allocation. Its
ab_next field should be NULL. If it is not-null after a
call to block_alloc(), it means a new block has been allocated
and a_cur should be reset to point it.
*/
block *a_cur;
/* A Python list object containing references to all the PyObject
pointers associated with this arena. They will be DECREFed
when the arena is freed.
*/
PyObject *a_objects;
#if defined(Py_DEBUG)
/* Debug output */
size_t total_allocs;
size_t total_size;
size_t total_blocks;
size_t total_block_size;
size_t total_big_blocks;
#endif
};
static block *
block_new(size_t size)
{
/* Allocate header and block as one unit.
ab_mem points just past header. */
block *b = (block *)PyMem_Malloc(sizeof(block) + size);
if (!b)
return NULL;
b->ab_size = size;
b->ab_mem = (void *)(b + 1);
b->ab_next = NULL;
b->ab_offset = (char *)_Py_ALIGN_UP(b->ab_mem, ALIGNMENT) -
(char *)(b->ab_mem);
return b;
}
static void
block_free(block *b) {
while (b) {
block *next = b->ab_next;
PyMem_Free(b);
b = next;
}
}
static void *
block_alloc(block *b, size_t size)
{
void *p;
assert(b);
size = _Py_SIZE_ROUND_UP(size, ALIGNMENT);
if (b->ab_offset + size > b->ab_size) {
/* If we need to allocate more memory than will fit in
the default block, allocate a one-off block that is
exactly the right size. */
block *newbl = block_new(
size < DEFAULT_BLOCK_SIZE ?
DEFAULT_BLOCK_SIZE : size);
if (!newbl)
return NULL;
assert(!b->ab_next);
b->ab_next = newbl;
b = newbl;
}
assert(b->ab_offset + size <= b->ab_size);
p = (void *)(((char *)b->ab_mem) + b->ab_offset);
b->ab_offset += size;
return p;
}
PyArena *
_PyArena_New(void)
{
PyArena* arena = (PyArena *)PyMem_Malloc(sizeof(PyArena));
if (!arena)
return (PyArena*)PyErr_NoMemory();
arena->a_head = block_new(DEFAULT_BLOCK_SIZE);
arena->a_cur = arena->a_head;
if (!arena->a_head) {
PyMem_Free((void *)arena);
return (PyArena*)PyErr_NoMemory();
}
arena->a_objects = PyList_New(0);
if (!arena->a_objects) {
block_free(arena->a_head);
PyMem_Free((void *)arena);
return (PyArena*)PyErr_NoMemory();
}
#if defined(Py_DEBUG)
arena->total_allocs = 0;
arena->total_size = 0;
arena->total_blocks = 1;
arena->total_block_size = DEFAULT_BLOCK_SIZE;
arena->total_big_blocks = 0;
#endif
return arena;
}
void
_PyArena_Free(PyArena *arena)
{
assert(arena);
#if defined(Py_DEBUG)
/*
fprintf(stderr,
"alloc=%zu size=%zu blocks=%zu block_size=%zu big=%zu objects=%zu\n",
arena->total_allocs, arena->total_size, arena->total_blocks,
arena->total_block_size, arena->total_big_blocks,
PyList_Size(arena->a_objects));
*/
#endif
block_free(arena->a_head);
/* This property normally holds, except when the code being compiled
is sys.getobjects(0), in which case there will be two references.
assert(arena->a_objects->ob_refcnt == 1);
*/
Py_DECREF(arena->a_objects);
PyMem_Free(arena);
}
void *
_PyArena_Malloc(PyArena *arena, size_t size)
{
void *p = block_alloc(arena->a_cur, size);
if (!p)
return PyErr_NoMemory();
#if defined(Py_DEBUG)
arena->total_allocs++;
arena->total_size += size;
#endif
/* Reset cur if we allocated a new block. */
if (arena->a_cur->ab_next) {
arena->a_cur = arena->a_cur->ab_next;
#if defined(Py_DEBUG)
arena->total_blocks++;
arena->total_block_size += arena->a_cur->ab_size;
if (arena->a_cur->ab_size > DEFAULT_BLOCK_SIZE)
++arena->total_big_blocks;
#endif
}
return p;
}
int
_PyArena_AddPyObject(PyArena *arena, PyObject *obj)
{
int r = PyList_Append(arena->a_objects, obj);
if (r >= 0) {
Py_DECREF(obj);
}
return r;
}