Re: [PATCH 1/3] riscv: optimized memcpy

[Bin Meng <bmeng.cn@gmail.com>]

On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight@aculab.com> wrote:
>
> From: Matteo Croce
> > Sent: 15 June 2021 03:38
> >
> > Write a C version of memcpy() which uses the biggest data size allowed,
> > without generating unaligned accesses.
>
> I'm surprised that the C loop:
>
> > +             for (; count >= bytes_long; count -= bytes_long)
> > +                     *d.ulong++ = *s.ulong++;
>
> ends up being faster than the ASM 'read lots' - 'write lots' loop.

I believe that's because the assembly version has some unaligned
access cases, which end up being trap-n-emulated in the OpenSBI
firmware, and that is a big overhead.

>
> Especially since there was an earlier patch to convert
> copy_to/from_user() to use the ASM 'read lots' - 'write lots' loop
> instead of a tight single register copy loop.
>
> I'd also guess that the performance needs to be measured on
> different classes of riscv cpu.
>
> A simple cpu will behave differently to one that can execute
> multiple instructions per clock.
> Any form of 'out of order' execution also changes things.
> The other big change is whether the cpu can to a memory
> read and write in the same clock.
>
> I'd guess that riscv exist with some/all of those features.

Regards,
Bin