From: "H.J. Lu" <hjl.tools@gmail.com>
To: Noah Goldstein <goldstein.w.n@gmail.com>
Cc: GNU C Library <libc-alpha@sourceware.org>,
"Carlos O'Donell" <carlos@systemhalted.org>
Subject: Re: [PATCH v7 2/3] x86: Add bounds `x86_non_temporal_threshold`
Date: Wed, 15 Jun 2022 13:48:46 -0700 [thread overview]
Message-ID: <CAMe9rOoPWjC_rN+ikxEbGwhq5e2YQ+RFoY2CnuKkThJ7Sd4Z-A@mail.gmail.com> (raw)
In-Reply-To: <20220615203436.3686803-1-goldstein.w.n@gmail.com>
On Wed, Jun 15, 2022 at 1:34 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> The lower-bound (16448) and upper-bound (SIZE_MAX / 16) are assumed
> by memmove-vec-unaligned-erms.
>
> The lower-bound is needed because memmove-vec-unaligned-erms unrolls
> the loop aggressively in the L(large_memset_4x) case.
>
> The upper-bound is needed because memmove-vec-unaligned-erms
> right-shifts the value of `x86_non_temporal_threshold` by
> LOG_4X_MEMCPY_THRESH (4) which without a bound may overflow.
>
> The lack of lower-bound can be a correctness issue. The lack of
> upper-bound cannot.
> ---
> manual/tunables.texi | 2 +-
> sysdeps/x86/dl-cacheinfo.h | 8 +++++++-
> 2 files changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/manual/tunables.texi b/manual/tunables.texi
> index 1482412078..2c076019ae 100644
> --- a/manual/tunables.texi
> +++ b/manual/tunables.texi
> @@ -47,7 +47,7 @@ glibc.malloc.mxfast: 0x0 (min: 0x0, max: 0xffffffffffffffff)
> glibc.elision.skip_lock_busy: 3 (min: -2147483648, max: 2147483647)
> glibc.malloc.top_pad: 0x0 (min: 0x0, max: 0xffffffffffffffff)
> glibc.cpu.x86_rep_stosb_threshold: 0x800 (min: 0x1, max: 0xffffffffffffffff)
> -glibc.cpu.x86_non_temporal_threshold: 0xc0000 (min: 0x0, max: 0xffffffffffffffff)
> +glibc.cpu.x86_non_temporal_threshold: 0xc0000 (min: 0x4040, max: 0x0fffffffffffffff)
> glibc.cpu.x86_shstk:
> glibc.cpu.hwcap_mask: 0x6 (min: 0x0, max: 0xffffffffffffffff)
> glibc.malloc.mmap_max: 0 (min: -2147483648, max: 2147483647)
> diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> index cc3b840f9c..e9f3382108 100644
> --- a/sysdeps/x86/dl-cacheinfo.h
> +++ b/sysdeps/x86/dl-cacheinfo.h
> @@ -931,8 +931,14 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>
> TUNABLE_SET_WITH_BOUNDS (x86_data_cache_size, data, 0, SIZE_MAX);
> TUNABLE_SET_WITH_BOUNDS (x86_shared_cache_size, shared, 0, SIZE_MAX);
> + /* SIZE_MAX >> 4 because memmove-vec-unaligned-erms right-shifts the value of
> + 'x86_non_temporal_threshold' by `LOG_4X_MEMCPY_THRESH` (4) and it is best
> + if that operation cannot overflow. Minimum of 0x4040 (16448) because the
> + L(large_memset_4x) loops need 64-byte to cache align and enough space for
> + at least 1 iteration of 4x PAGE_SIZE unrolled loop. Both values are
> + reflected in the manual. */
> TUNABLE_SET_WITH_BOUNDS (x86_non_temporal_threshold, non_temporal_threshold,
> - 0, SIZE_MAX);
> + 0x4040, SIZE_MAX >> 4);
> TUNABLE_SET_WITH_BOUNDS (x86_rep_movsb_threshold, rep_movsb_threshold,
> minimum_rep_movsb_threshold, SIZE_MAX);
> TUNABLE_SET_WITH_BOUNDS (x86_rep_stosb_threshold, rep_stosb_threshold, 1,
> --
> 2.34.1
>
LGTM.
Thanks.
--
H.J.
next prev parent reply other threads:[~2022-06-15 20:49 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-15 0:25 [PATCH v1 1/3] x86: Fix misordered logic for setting `rep_movsb_stop_threshold` Noah Goldstein
2022-06-15 0:25 ` [PATCH v1 2/3] x86: Cleanup bounds checking in large memcpy case Noah Goldstein
2022-06-15 1:07 ` H.J. Lu
2022-06-15 3:57 ` Noah Goldstein
2022-06-15 3:57 ` [PATCH v2] " Noah Goldstein
2022-06-15 14:52 ` H.J. Lu
2022-06-15 15:13 ` Noah Goldstein
2022-06-15 15:12 ` [PATCH v3] " Noah Goldstein
2022-06-15 16:48 ` H.J. Lu
2022-06-15 17:44 ` Noah Goldstein
2022-06-15 17:41 ` [PATCH v4 1/2] " Noah Goldstein
2022-06-15 17:41 ` [PATCH v4 2/2] x86: Add bounds `x86_non_temporal_threshold` Noah Goldstein
2022-06-15 18:22 ` H.J. Lu
2022-06-15 18:33 ` Noah Goldstein
2022-06-15 18:32 ` [PATCH v5 " Noah Goldstein
2022-06-15 18:43 ` H.J. Lu
2022-06-15 19:52 ` [PATCH v6 2/3] " Noah Goldstein
2022-06-15 20:27 ` H.J. Lu
2022-06-15 20:35 ` Noah Goldstein
2022-06-15 20:34 ` [PATCH v7 " Noah Goldstein
2022-06-15 20:48 ` H.J. Lu [this message]
2022-07-14 2:55 ` Sunil Pandey
2022-06-15 18:22 ` [PATCH v4 1/2] x86: Cleanup bounds checking in large memcpy case H.J. Lu
2022-07-14 2:57 ` Sunil Pandey
2022-06-15 0:25 ` [PATCH v1 3/3] x86: Add sse42 implementation to strcmp's ifunc Noah Goldstein
2022-06-15 1:08 ` H.J. Lu
2022-07-14 2:54 ` Sunil Pandey
2022-06-15 1:02 ` [PATCH v1 1/3] x86: Fix misordered logic for setting `rep_movsb_stop_threshold` H.J. Lu
2022-07-14 2:53 ` Sunil Pandey
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