netdev - RE: [PATCH v1] x86/csum: rewrite csum

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Message-ID: <3518c78fab894d01b391c764efffbb62@AcuMS.aculab.com>
Date:   Sun, 14 Nov 2021 14:21:20 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Alexander Duyck' <alexander.duyck@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>
Subject: RE: [PATCH v1] x86/csum: rewrite csum_partial()

From: Alexander Duyck
> Sent: 11 November 2021 21:56
...
> It might be worthwhile to beef up the odd check to account for
> anything 7 bytes or less. To address it you could do something along
> the lines of:
>     unaligned = 7 & (unsigned long) buff;
>     if (unaligned) {
>         shift = unaligned * 8;
>         temp64 = (*(unsigned long)buff >> shift) << shift;
>         buff += 8 - unaligned;
>         if (len < 8 - unaligned) {
>             shift = (8 - len - unaligned) * 8;
>             temp64 <<= shift;
>             temp64 >>= shift;
>             len = 0;
>         } else {
>             len -= 8 - unaligned;
>         }
>         result += temp64;
>         result += result < temp64;
>    }

I tried doing that.
Basically it is likely to take longer that just doing the memory reads.
The register dependency chain is just too long.

	David

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