[PATCH 1/3] riscv: optimized memcpy
bmeng.cn at gmail.com
Tue Jun 15 06:08:50 PDT 2021
On Tue, Jun 15, 2021 at 4:57 PM David Laight <David.Laight at 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.
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