linux-kernel - RE: [PATCH] riscv: fix memmove and optimise memcpy when misalign

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Message-ID: <17637b10e71b41b89126cbb1b2fa61cf@AcuMS.aculab.com>
Date:   Sun, 23 May 2021 17:12:23 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Palmer Dabbelt' <palmer@...belt.com>,
        "gary@...yguo.net" <gary@...yguo.net>
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        "aou@...s.berkeley.edu" <aou@...s.berkeley.edu>,
        "nickhu@...estech.com" <nickhu@...estech.com>,
        "nylon7@...estech.com" <nylon7@...estech.com>,
        "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] riscv: fix memmove and optimise memcpy when misalign

From: Palmer Dabbelt
> Sent: 23 May 2021 02:47
...
> IMO the right way to go here is to just move to C-based string routines,
> at least until we get to the point where we're seriously optimizing for
> specific processors.  We went with the C-based string rountines in glibc
> as part of the upstreaming process and found only some small performance
> differences when compared to the hand-written assembly, and they're way
> easier to maintain.
> 
> IIRC Linux only has trivial C string routines in lib, I think the best
> way to go about that would be to higher performance versions in there.
> That will allow other ports to use them.

I certainly wonder how much benefit these massively unrolled
loops have on modern superscaler processors - especially those
with any form of 'out of order' execution.

It is often easy to write assembler where all the loop
control instructions happen in parallel with the memory
accesses - which cannot be avoided.
Loop unrolling is so 1970s.

Sometimes you need to unroll once.
And maybe interleave the loads and stores.
But after that you can just be trashing the i-cache.

	David

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