Subject: Re: [PATCH] powerpc/32: Don't use lmw/stmw for saving/restoring non volatile regs

From: Segher Boessenkool <[email-masked]>
Date: Mon, 23 Aug 2021 13:46:48 -0500

On Mon, Aug 23, 2021 at 03:29:12PM +0000, Christophe Leroy wrote:
> Instructions lmw/stmw are interesting for functions that are rarely
> used and not in the cache, because only one instruction is to be
> copied into the instruction cache instead of 19. However those
> instruction are less performant than 19x raw lwz/stw as they require
> synchronisation plus one additional cycle.

lmw takes N+2 cycles for loading N words on 603/604/750/7400, and N+3 on
7450.  stmw takes N+1 cycles for storing N words on 603, N+2 on 604/750/
7400, and N+3 on 7450 (load latency is 3 instead of 2 on 7450).

There is no synchronisation needed, although there is some serialisation,
which of course doesn't mean much since there can be only 6 or 8 or so
insns executing at once anyway.

So, these insns are almost never slower, they can easily win cycles back
because of the smaller code, too.

What 32-bit core do you see where load/store multiple are more than a
fraction of a cycle (per memory access) slower?

> SAVE_NVGPRS / REST_NVGPRS are used in only a few places which are
> mostly in interrupts entries/exits and in task switch so they are
> likely already in the cache.

Nothing is likely in the cache on the older cores (except in
microbenchmarks), the caches are not big enough for that!

> Using standard lwz improves null_syscall selftest by:
> - 10 cycles on mpc832x.
> - 2 cycles on mpc8xx.

And in real benchmarks?

On mpccore both lmw and stmw are only N+1 btw.  But the serialization
might cost another cycle here?

Segher