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

[David Laight]

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