On Tue, Jun 15, 2021 at 4:57 PM David Laight [off-list ref] wrote:

From: Matteo Croce

quoted

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:

quoted

+             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