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#include <assert.h>
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include "pool.h"
/* the actual pool of memory */
static void *pool;
/* size of one block in the pool */
static size_t block_size;
/* number of blocks in the pool */
static size_t blocks;
/* bitmap to track allocated blocks */
static uint64_t *used;
/* number of bits in each slot of `used` */
static const size_t BITS = sizeof(*used) * CHAR_BIT;
/* Allocate a bitmap with at least `blocks` bits. */
static uint64_t *
bitmap(size_t blocks)
{
assert(blocks > 0);
const size_t slots = (blocks / BITS) + 1;
uint64_t *p = calloc(1, sizeof(*p) * slots);
return p; /* this is fine */
}
/* Mark given `block` as free. */
static void
bitmap_free(size_t block)
{
assert(block < blocks);
const size_t slot = (block / BITS);
const size_t bit = (block % BITS);
used[slot] &= ~(1UL << bit);
}
/* Mark given `block` as used. */
static void
bitmap_used(size_t block)
{
assert(block < blocks);
const size_t slot = (block / BITS);
const size_t bit = (block % BITS);
used[slot] |= (1UL << bit);
}
/*
* Find right-most zero bit. Returns 0 if there is none. Examples:
*
* 00000000 01010011 11111111 bits
*
* 11111111 10101100 00000000 ~bits
* 00000001 01010100 00000000 bits+1
* -------- -------- --------
* 00000001 00000100 00000000 ~bits & (bits+1)
*/
static inline uint64_t
find_zero(const uint64_t bits)
{
return ~bits & (bits + 1);
}
static inline uint64_t
log2(const uint64_t x)
{
return ((BITS - 1) - __builtin_clzl(x)); /* count leading zeros */
}
/* Find a free block. Returns -1 if there is none. */
static int
bitmap_alloc(void)
{
for (size_t i = 0; i < blocks / BITS; i++) {
uint64_t free = find_zero(used[i]);
if (free != 0) {
return i * BITS + log2(free);
}
}
return -1;
}
int
pl_setup(size_t object_size, size_t pool_size)
{
assert(object_size > 0 && pool_size > 0);
void *p = calloc(pool_size, object_size);
if (p == NULL) {
return -1;
}
uint64_t *q = bitmap(pool_size);
if (q == NULL) {
free(p);
return -2;
}
pool = p;
used = q;
block_size = object_size;
blocks = pool_size;
return 0;
}
void *
pl_alloc(void)
{
assert(pool);
int block = bitmap_alloc();
if (block < 0) {
return NULL;
}
bitmap_used(block);
const uintptr_t p = (uintptr_t)pool;
const uintptr_t o = p + block * block_size;
return (void *)o;
}
void
pl_free(void *object)
{
assert(pool);
if (object == NULL) {
/* free(3) also does nothing for NULL */
return;
}
/* memset here to catch dangling references */
memset(object, 0, block_size);
const uintptr_t o = (uintptr_t)object;
const uintptr_t p = (uintptr_t)pool;
const uintptr_t offset = o - p;
const size_t block = offset / block_size;
bitmap_free(block);
}
void
pl_cleanup(void)
{
assert(pool);
free(used);
free(pool);
pool = NULL;
}
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