[08/16] malloc: Use global flag instead of function pointer dispatch for mtag
Commit Message
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.
---
malloc/arena.c | 33 +---------------------------
malloc/malloc.c | 58 ++++++++++++++++++++++++++++++++-----------------
2 files changed, 39 insertions(+), 52 deletions(-)
Comments
The 03/04/2021 19:46, DJ Delorie wrote:
> Szabolcs Nagy <szabolcs.nagy@arm.com> writes:
> > The definition of tag_new_usable is moved after chunk related
> > definitions.
>
> A couple thousand lines after, that is. Perhaps it could go at the end
> of the chunk related definitions, closer to the other tag functions?
ok. i'm happy to move it around. (i moved it down
close to its first use).
> > 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.
>
> Are you relying on the compiler to do the work of the old #ifdefs for
> platforms without USE_MTAG set? I.e. are we sure that the compiled
> binaries will have the mtag-related code optimized out?
yes as described in my patch 6 response, i attempted to
remove ifdefs.
i think at -O1 we can rely on 'if (0) {...}' to be
optimized out completely (but that code is still type
checked and has to reference defined variables etc.)
with -O0 i'd expect the code to be there in the
compiled libc, just unreachable.
i can add back the ifdefs if this seems too intrusive
(it is now less clear that mtag related code does not
affect the tagging disabled configuration).
>
> Otherwise LGTM with the above caveats.
>
> Reviewed-by: DJ Delorie <dj@redhat.com>
@@ -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
@@ -441,35 +441,41 @@ void *(*__morecore)(ptrdiff_t) = __default_morecore;
*/
#ifdef USE_MTAG
+static bool mtag_enabled = false;
+static int mtag_mmap_flags = 0;
+static size_t mtag_granule_mask = ~(size_t)0;
+#else
+# define mtag_enabled false
+# define mtag_mmap_flags 0
+#endif
-/* Default implementaions when memory tagging is supported, but disabled. */
-static void *
-__default_tag_region (void *ptr, size_t size)
+static __always_inline void *
+tag_region (void *ptr, size_t size)
{
+ if (__glibc_unlikely (mtag_enabled))
+ return __libc_mtag_tag_region (ptr, size);
return ptr;
}
-static void *
-__default_tag_nop (void *ptr)
+static __always_inline void *
+tag_new_memset (void *ptr, int val, size_t size)
{
- return ptr;
+ if (__glibc_unlikely (mtag_enabled))
+ return __libc_mtag_memset_with_tag (__libc_mtag_new_tag (ptr), val, size);
+ return memset (ptr, val, size);
}
-static int mtag_mmap_flags = 0;
-static size_t mtag_granule_mask = ~(size_t)0;
-
-static void *(*tag_new_memset)(void *, int, size_t) = memset;
-static void *(*tag_region)(void *, size_t) = __default_tag_region;
-static void *(*tag_new_usable)(void *) = __default_tag_nop;
-static void *(*tag_at)(void *) = __default_tag_nop;
+/* Defined later. */
+static void *
+tag_new_usable (void *ptr);
-#else
-# define mtag_mmap_flags 0
-# define tag_new_memset(ptr, val, size) memset (ptr, val, size)
-# define tag_region(ptr, size) (ptr)
-# define tag_new_usable(ptr) (ptr)
-# define tag_at(ptr) (ptr)
-#endif
+static __always_inline void *
+tag_at (void *ptr)
+{
+ if (__glibc_unlikely (mtag_enabled))
+ return __libc_mtag_address_get_tag (ptr);
+ return ptr;
+}
#include <string.h>
@@ -3185,6 +3191,18 @@ tcache_thread_shutdown (void)
#endif /* !USE_TCACHE */
+static __always_inline void *
+tag_new_usable (void *ptr)
+{
+ if (__glibc_unlikely (mtag_enabled) && ptr)
+ {
+ mchunkptr cp = mem2chunk(ptr);
+ ptr = __libc_mtag_tag_region (__libc_mtag_new_tag (ptr),
+ CHUNK_AVAILABLE_SIZE (cp) - CHUNK_HDR_SZ);
+ }
+ return ptr;
+}
+
void *
__libc_malloc (size_t bytes)
{