From: Bin Meng <bmeng.cn@gmail.com>
To: David Laight <David.Laight@aculab.com>
Cc: Matteo Croce <mcroce@linux.microsoft.com>,
"linux-riscv@lists.infradead.org"
<linux-riscv@lists.infradead.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
Paul Walmsley <paul.walmsley@sifive.com>,
Palmer Dabbelt <palmer@dabbelt.com>,
Albert Ou <aou@eecs.berkeley.edu>,
Atish Patra <atish.patra@wdc.com>,
Emil Renner Berthing <kernel@esmil.dk>,
Akira Tsukamoto <akira.tsukamoto@gmail.com>,
Drew Fustini <drew@beagleboard.org>
Subject: Re: [PATCH 1/3] riscv: optimized memcpy
Date: Tue, 15 Jun 2021 21:08:50 +0800 [thread overview]
Message-ID: <CAEUhbmV+Vi0Ssyzq1B2RTkbjMpE21xjdj2MSKdLydgW6WuCKtA@mail.gmail.com> (raw)
In-Reply-To: <6cff2a895db94e6fadd4ddffb8906a73@AcuMS.aculab.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
next prev parent reply other threads:[~2021-06-15 13:09 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-15 2:38 [PATCH 0/3] riscv: optimized mem* functions Matteo Croce
2021-06-15 2:38 ` [PATCH 1/3] riscv: optimized memcpy Matteo Croce
2021-06-15 8:57 ` David Laight
2021-06-15 13:08 ` Bin Meng [this message]
2021-06-15 13:18 ` David Laight
2021-06-15 13:28 ` Bin Meng
2021-06-15 16:12 ` Emil Renner Berthing
2021-06-16 0:33 ` Bin Meng
2021-06-16 2:01 ` Matteo Croce
2021-06-16 8:24 ` David Laight
2021-06-16 10:48 ` Akira Tsukamoto
2021-06-16 19:06 ` Matteo Croce
2021-06-15 13:44 ` Matteo Croce
2021-06-16 11:46 ` Guo Ren
2021-06-16 18:52 ` Matteo Croce
2021-06-17 21:30 ` David Laight
2021-06-17 21:48 ` Matteo Croce
2021-06-18 0:32 ` Matteo Croce
2021-06-18 1:05 ` Matteo Croce
2021-06-18 8:32 ` David Laight
2021-06-15 2:38 ` [PATCH 2/3] riscv: optimized memmove Matteo Croce
2021-06-15 2:38 ` [PATCH 3/3] riscv: optimized memset Matteo Croce
2021-06-15 2:43 ` [PATCH 0/3] riscv: optimized mem* functions Bin Meng
2024-01-28 11:10 [PATCH 0/3] riscv: optimize memcpy/memmove/memset Jisheng Zhang
2024-01-28 11:10 ` [PATCH 1/3] riscv: optimized memcpy Jisheng Zhang
2024-01-28 12:35 ` David Laight
2024-01-30 12:11 ` Nick Kossifidis
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