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Date:   Thu, 9 Sep 2021 12:27:06 +0200
From:   Matteo Croce <mcroce@linux.microsoft.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        David Laight <David.Laight@aculab.com>, drew@beagleboard.org,
        Guo Ren <guoren@kernel.org>,
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        Linux-MM <linux-mm@kvack.org>, mcroce@microsoft.com,
        mick@ics.forth.gr, mm-commits@vger.kernel.org,
        Nick Desaulniers <ndesaulniers@google.com>,
        Palmer Dabbelt <palmer@dabbelt.com>
Subject: Re: [patch 103/147] lib/string: optimized memset
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On Wed, 8 Sep 2021 11:34:27 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> I'm dropping this one just to be consistent, although for memset()
> it's possibly a bit more reasonable to fall back on some default.
> 
> But probably not. memcpy and memset really are *so* special that these
> generic versions should be considered to be "stupid placeholders for
> bringup, and nothing more".
> 
> On Tue, Sep 7, 2021 at 7:58 PM Andrew Morton
> <akpm@linux-foundation.org> wrote:
> >
> > On a RISC-V machine the speed goes from 140 Mb/s to 241 Mb/s, and
> > this the binary size increase according to bloat-o-meter:
> 
> I also react to the benchmark numbers: RISC-V already has
> 
>   #define __HAVE_ARCH_MEMSET
>   #define __HAVE_ARCH_MEMCPY
>   #define __HAVE_ARCH_MEMMOVE
> 
> in its <asm/string.h> file, so these are just odd.
> 
> Did you benchmark these generic functions on their own, rather than
> the ones that actually get *used*?
> 
>            Linus

I benchmarked against the generic routines. The RISC-V specific are
even slower than the generic ones, because generates lot of unaligned
accesses.

That was the whole point of the series initially. These C routines
should have replaced the risc-v specific assembly ones, but then it was
proposed to use them as generic:

https://lore.kernel.org/linux-riscv/YNChl0tkofSGzvIX@infradead.org/

-- 
per aspera ad upstream