linux-kernel - Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum

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Message-ID: <CANn89iKkK0eMQkOTSMGbvdtf+G=eRfXJG0Semju5h-C63QytaQ@mail.gmail.com>
Date:   Fri, 26 Nov 2021 12:07:29 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Noah Goldstein <goldstein.w.n@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com,
        Borislav Petkov <bp@...en8.de>, dave.hansen@...ux.intel.com,
        X86 ML <x86@...nel.org>, hpa@...or.com, peterz@...radead.org,
        alexanderduyck@...com, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c

On Fri, Nov 26, 2021 at 11:50 AM Noah Goldstein <goldstein.w.n@...il.com> wrote:
>
> Bright :) but it will need a BMI support check.

Yes, probably not worth the pain.

>
> I actually get better performance in hyperthread benchmarks with 2 accum:
>
> Used:
>
>         u64 res;
>         temp64 = (__force uint64_t)sum;
>         asm("movq 0*8(%[src]),%[res]\n\t"
>             "addq 1*8(%[src]),%[res]\n\t"
>             "adcq 2*8(%[src]),%[res]\n\t"
>             "adcq   $0, %[res]\n"
>             "addq 3*8(%[src]),%[temp64]\n\t"
>             "adcq 4*8(%[src]),%[temp64]\n\t"
>             "adcq   %[res], %[temp64]\n\t"
>             "mov  %k[temp64],%k[res]\n\t"
>             "rorx $32,%[temp64],%[temp64]\n\t"
>             "adcl %k[temp64],%k[res]\n\t"
>             "adcl $0,%k[res]"
>             : [temp64] "+r"(temp64), [res] "=&r"(res)
>             : [src] "r"(buff)
>             : "memory");
>         return (__force __wsum)res;
>
> w/ hyperthread:
> size,    2acc lat,    1acc lat,   2acc tput,   1acc tput
>   40,       6.511,       7.863,       6.177,       6.157
>
> w/o hyperthread:
> size,    2acc lat,    1acc lat,   2acc tput,   1acc tput
>   40,       5.577,       6.764,       3.150,       3.210