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

[David Laight]

On Mon, 30 Mar 2026 20:21:41 +0300
Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:

> On Mon, Mar 30, 2026 at 7:58 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >
> > 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.  
> 
> I have Intel Quark at hand to test. But I need to know the
> step-by-step instructions on what to do.
> 

I can run my test on a few 'older' systems, but I don't have anything
Intel before Sandy bridge and only an AMD 'Excavator' (or similar).

I do remember (a long time ago) getting my Athlon 700 to run a copy
loop as fast as 'rep movl' - but the setup time was a lot worse.
So I suspect that generation of cpu didn't have a large overhead.
If I've read Agner's tables he gives a 40 clock setup to P-II onwards.

I can give you the source of the test I've been using.

	David