From: DJ Delorie <dj@redhat.com>
To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: libc-alpha@sourceware.org
Subject: Re: [PATCH 6/6] malloc: Ensure mtag code path in checked_request2size is cold
Date: Tue, 23 Mar 2021 16:46:24 -0400 [thread overview]
Message-ID: <xntup1ionz.fsf@rhel8.vm> (raw)
In-Reply-To: <57b96253346fe679c82b7f88e2398ec21553b52c.1616155129.git.szabolcs.nagy@arm.com> (message from Szabolcs Nagy on Fri, 19 Mar 2021 13:27:44 +0000)
Szabolcs Nagy <szabolcs.nagy@arm.com> writes:
> This is a workaround (hack) for a gcc optimization issue (PR 99551).
> Without this the generated code may evaluate the expression in the
> cold path which causes performance regression for small allocations
> in the memory tagging disabled (common) case.
> if (__glibc_unlikely (mtag_enabled))
> - req = (req + (__MTAG_GRANULE_SIZE - 1)) &
> - ~(size_t)(__MTAG_GRANULE_SIZE - 1);
> + {
> + /* Ensure this is not evaluated if !mtag_enabled, see gcc PR 99551. */
> + asm ("");
> +
> + req = (req + (__MTAG_GRANULE_SIZE - 1)) &
> + ~(size_t)(__MTAG_GRANULE_SIZE - 1);
> + }
Hack indeed, but cross-target-safe and shouldn't affect the unused code
removal on non-aarch64 targets.
LGTM.
Reviewed-by: DJ Delorie <dj@redhat.com>
prev parent reply other threads:[~2021-03-23 20:46 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-19 13:25 [PATCH 0/6] malloc: more memory tagging optimizations Szabolcs Nagy
2021-03-19 13:26 ` [PATCH 1/6] malloc: Use memsize instead of CHUNK_AVAILABLE_SIZE Szabolcs Nagy
2021-03-23 20:01 ` DJ Delorie
2021-03-19 13:26 ` [PATCH 2/6] malloc: Use different tag after mremap Szabolcs Nagy
2021-03-23 20:03 ` DJ Delorie
2021-03-19 13:26 ` [PATCH 3/6] malloc: Use chunk2rawmem throughout Szabolcs Nagy
2021-03-23 20:25 ` DJ Delorie
2021-03-19 13:27 ` [PATCH 4/6] malloc: Rename chunk2rawmem Szabolcs Nagy
2021-03-23 20:43 ` DJ Delorie
2021-03-19 13:27 ` [PATCH 5/6] malloc: Remove unnecessary tagging around _mid_memalign Szabolcs Nagy
2021-03-23 20:44 ` DJ Delorie
2021-03-19 13:27 ` [PATCH 6/6] malloc: Ensure mtag code path in checked_request2size is cold Szabolcs Nagy
2021-03-23 20:46 ` DJ Delorie [this message]
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