linux-kernel - Re: [GIT PULL] string fixes for v6.15-rc1

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Message-ID: <20250407212310.0a934bad@pumpkin>
Date: Mon, 7 Apr 2025 21:23:10 +0100
From: David Laight <david.laight.linux@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Kees Cook <kees@...nel.org>, linux-kernel@...r.kernel.org, Andrey
 Konovalov <andreyknvl@...il.com>, Andy Shevchenko <andy@...nel.org>,
 Catalin Marinas <catalin.marinas@....com>, Nathan Chancellor
 <nathan@...nel.org>, Peter Collingbourne <pcc@...gle.com>, Vincenzo
 Frascino <vincenzo.frascino@....com>, Will Deacon <will@...nel.org>
Subject: Re: [GIT PULL] string fixes for v6.15-rc1

On Sun, 6 Apr 2025 19:04:29 -0700
Linus Torvalds <torvalds@...ux-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