From: David Laight <david.laight.linux@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Kees Cook <kees@kernel.org>,
linux-kernel@vger.kernel.org,
Andrey Konovalov <andreyknvl@gmail.com>,
Andy Shevchenko <andy@kernel.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Nathan Chancellor <nathan@kernel.org>,
Peter Collingbourne <pcc@google.com>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>
Subject: Re: [GIT PULL] string fixes for v6.15-rc1
Date: Mon, 7 Apr 2025 21:23:10 +0100 [thread overview]
Message-ID: <20250407212310.0a934bad@pumpkin> (raw)
In-Reply-To: <CAHk-=wijG2dSOOFr8CCYygwxZQxdTUj73rfB8=tyZP-3G-8-og@mail.gmail.com>
On Sun, 6 Apr 2025 19:04:29 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:
...
> For something like wcslen() the answer is "DON'T DO THIS". Because
> there is absolutely zero upside to trying to recognize this pattern,
> and there is real downside.
gcc also has a nasty habit of converting:
for (i = 0; i < len; i++)
dst[i] = src[i];
into a call to memcpy().
If I wanted a memcpy() call I'd write one - so will most people.
But if 'len' is very small (may even known to be less than, say, 4)
you really want the loop - which is why it was written.
I've even seen (not gcc) it converted to a 'rep movsw' 'rep movsb'
pair at a time when a P4 might have been a likely target cpu.
The 0 to 3 byte 'rep movsb' had a setup cost of IIRC 150 clocks.
David
prev parent reply other threads:[~2025-04-07 20:23 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-04-06 17:54 [GIT PULL] string fixes for v6.15-rc1 Kees Cook
2025-04-06 19:04 ` Linus Torvalds
2025-04-06 19:31 ` Linus Torvalds
2025-04-07 1:32 ` Kees Cook
2025-04-07 2:04 ` Linus Torvalds
2025-04-07 17:37 ` Nathan Chancellor
2025-04-07 19:02 ` Linus Torvalds
2025-04-07 19:25 ` Nathan Chancellor
2025-04-07 20:25 ` Linus Torvalds
2025-04-07 21:01 ` Nathan Chancellor
2025-04-07 20:23 ` David Laight [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20250407212310.0a934bad@pumpkin \
--to=david.laight.linux@gmail.com \
--cc=andreyknvl@gmail.com \
--cc=andy@kernel.org \
--cc=catalin.marinas@arm.com \
--cc=kees@kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=nathan@kernel.org \
--cc=pcc@google.com \
--cc=torvalds@linux-foundation.org \
--cc=vincenzo.frascino@arm.com \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox