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#include <sys/mman.h>
#include <assert.h>
#include <errno.h>
#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include "alloc.h"
#include "common.h"
#include "errors.h"
/* TODO: Support malloc() backend for systems without MAP_ANON */
#ifndef MAP_ANON
# error "System not supported (missing MAP_ANON)"
#endif
#define MAX(x, y) ((x) > (y) ? (x) : (y))
#define IS_POW_2(n) ((n) != 0 && ((n) & ((n)-1)) == 0)
struct _arena {
/* DATA points to the start of the block’s memory while FREE points
to the beginning of the unused data in the block */
void *data, *free;
size_t cap;
struct _arena *next;
};
/* Return a new arena block of size SZ */
static struct _arena *mkblk(size_t sz)
__attribute__((returns_nonnull));
/* Return the padding required to properly align an allocation with
alignment ALIGN at offset OFF */
static inline size_t pad(size_t off, size_t align)
__attribute__((const, always_inline));
void *
arena_alloc(struct _arena **a, size_t nmemb, size_t size, size_t align)
{
assert(IS_POW_2(align));
assert(nmemb * size != 0);
if (unlikely(size > SIZE_MAX / nmemb)) {
errno = ENOMEM;
err("%s:", __func__);
}
size *= nmemb;
for (struct _arena *p = *a; p != NULL; p = p->next) {
size_t padding = pad((char *)p->free - (char *)p->data, align);
size_t freespc = p->cap - ((char *)p->free - (char *)p->data);
size_t nsize = size + padding;
if (nsize <= freespc) {
void *ret = p->free;
p->free = (char *)p->free + nsize;
return ret;
}
}
/* No page exists with enough space */
struct _arena *p = mkblk(MAX(size, _ARENA_DFLT_CAP));
p->next = *a;
*a = p;
p->free = (char *)p->data + size;
return p->data;
}
void *
_arena_grow(arena_t *a, void *ptr, size_t old_nmemb, size_t new_nmemb,
size_t size, size_t align)
{
assert(IS_POW_2(align));
assert(new_nmemb * size != 0);
assert(old_nmemb < new_nmemb);
if (unlikely(size > SIZE_MAX / new_nmemb)) {
errno = ENOMEM;
err("%s:", __func__);
}
for (struct _arena *p = *a; p != NULL; p = p->next) {
if (ptr < p->data || ptr > p->free)
continue;
/* If we need to grow the given allocation, but it was the last
allocation made in a region, then we first see if we can just eat
more trailing free space in the region to avoid a memcpy(). */
size_t oldsz = old_nmemb * size;
if ((char *)ptr == (char *)p->free - oldsz) {
size_t rem = p->cap - ((char *)p->free - (char *)p->data);
size_t need = (new_nmemb - old_nmemb) * size;
if (need <= rem) {
p->free = (char *)p->free + need;
return ptr;
}
/* NOTE: After benchmarking, (albiet rather naïvely) it seems
that the closer to the original mapping size we are, the
faster mremap() runs. With really large mappings it
converges with mmap() + memcpy() on performance though, so
it seems this is always the better solution on systems
with mremap() since it’s also a bit more memory efficient.
We can’t pass MREMAP_MAYMOVE though, because it would
invalidate existing pointers to data in the mapping. */
#if __linux__
size_t ncap = p->cap + need - rem;
void *nptr = mremap(p->data, p->cap, ncap, 0);
if (nptr != MAP_FAILED) {
p->cap = ncap;
return ptr;
}
if (errno != ENOMEM)
err("%s:", __func__);
#endif
}
void *dst = arena_alloc(a, new_nmemb, size, align);
return memcpy(dst, ptr, old_nmemb * size);
}
#if DEBUG
err("%s:%d: tried to resize pointer that wasn’t allocated", __func__,
__LINE__);
#else
__builtin_unreachable();
#endif
}
void
arena_free(struct _arena **a)
{
struct _arena *cur, *next;
for (cur = *a; cur != NULL; cur = next) {
next = cur->next;
munmap(cur->data, cur->cap);
free(cur);
}
*a = NULL;
}
snapshot_t
arena_snapshot_create(struct _arena *a)
{
return a == NULL ? NULL : a->free;
}
void
arena_snapshot_restore(struct _arena **a, snapshot_t snp)
{
if (snp == NULL) {
arena_free(a);
return;
}
struct _arena *cur, *next;
for (cur = *a; cur != NULL; cur = next) {
next = cur->next;
if (snp < cur->data || snp > cur->free) {
munmap(cur->data, cur->cap);
free(cur);
} else {
cur->free = snp;
*a = cur;
return;
}
}
}
struct _arena *
mkblk(size_t cap)
{
struct _arena *a = malloc(sizeof(*a));
if (a == NULL)
err("malloc:");
a->cap = cap;
a->data = a->free = mmap(NULL, cap, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANON, -1, 0);
if (a->data == MAP_FAILED)
err("mmap:");
return a;
}
size_t
pad(size_t len, size_t align)
{
return (len + align - 1) & ~(align - 1);
}
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