Re: [PATCH next] i386: Remove string functions that use 'rep scasb'

[Dave Hansen]

On 3/27/26 12:57, david.laight.linux@xxxxxxxxx wrote:
> The fixed overhead of all the 'rep xxx' instructions is rather more 
> that might expect. While 'rep movs' is getting better on more recent
> CPU, the same is not true for 'rep scasb'. On my Zen-5 it has a
> fixed overhead of 150 clocks and then takes 3 clocks for each byte. 
> I've not measured any Intel CPU, but the cost might be 'only' 40 +
> 2n.

One measurement on a modern 64-bit CPU isn't super convincing to me.

> Remove the asm versions of strcat() strncat() strlen() memchr()
> and memscan(), the generic C versions will be faster.
> 
> It is quite likely that all these functions are slower than the generic
> code on pretty much all CPU since the 486.

This is rather handwavy for my taste.

There seem to be two valid paths here:

 1. We continue the "nobody cares about 32-bit" refrain. This removes a
    bunch of 32-bit-only code and complexity. If it causes a performance
    regression, we do not care much.
 2. Someone makes _some_ kind of effort to test this on at least *one*
    32-bit-only CPU to see if it does any harm.

In other words, I'm not opposed to the patch, but the justification
doesn't really work for me as written.