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malloc: Use global flag instead of function pointer dispatch for mtag

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A flag check can be faster than function pointers because of how
branch prediction and speculation works and it can also remove a layer
of indirection when there is a mismatch between the malloc internal
tag_* api and __libc_mtag_* target hooks.

Memory tagging wrapper functions are moved to malloc.c from arena.c and
the logic now checks mmap_enabled.  The definition of tag_new_usable is
moved after chunk related definitions.

This refactoring also allows using mtag_enabled checks instead of
USE_MTAG ifdefs when memory tagging support only changes code logic
when memory tagging is enabled at runtime. Note: an "if (false)" code
block is optimized away even at -O0 by gcc.

Reviewed-by: DJ Delorie <dj@redhat.com>
This commit is contained in:
Szabolcs Nagy
2021-01-27 15:45:43 +00:00
parent 0c719cf42c
commit 42bac88a21
2 changed files with 43 additions and 56 deletions
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33
malloc/arena.c
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@ -287,34 +287,6 @@ extern struct dl_open_hook *_dl_open_hook;
libc_hidden_proto (_dl_open_hook);
#endif
#ifdef USE_MTAG
/* Generate a new (random) tag value for PTR and tag the memory it
points to upto the end of the usable size for the chunk containing
it. Return the newly tagged pointer. */
static void *
__mtag_tag_new_usable (void *ptr)
{
if (ptr)
{
mchunkptr cp = mem2chunk(ptr);
ptr = __libc_mtag_tag_region (__libc_mtag_new_tag (ptr),
CHUNK_AVAILABLE_SIZE (cp) - CHUNK_HDR_SZ);
}
return ptr;
}
/* Generate a new (random) tag value for PTR, set the tags for the
memory to the new tag and initialize the memory contents to VAL.
In practice this function will only be called with VAL=0, but we
keep this parameter to maintain the same prototype as memset. */
static void *
__mtag_tag_new_memset (void *ptr, int val, size_t size)
{
return __libc_mtag_memset_with_tag (__libc_mtag_new_tag (ptr), val, size);
}
#endif
static void
ptmalloc_init (void)
{
@ -332,11 +304,8 @@ ptmalloc_init (void)
if (__MTAG_SBRK_UNTAGGED)
__morecore = __failing_morecore;
mtag_enabled = true;
mtag_mmap_flags = __MTAG_MMAP_FLAGS;
tag_new_memset = __mtag_tag_new_memset;
tag_region = __libc_mtag_tag_region;
tag_new_usable = __mtag_tag_new_usable;
tag_at = __libc_mtag_address_get_tag;
mtag_granule_mask = ~(size_t)(__MTAG_GRANULE_SIZE - 1);
}
#endif