linux-kernel - Re: [PATCH] zstd: Fix decompression of large window archives on 32-bit platforms

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Message-ID: <CANr2Dbft5ax3eOc1DGztLWHGqR_GztAQM1n+5HJippD9R_1e=Q@mail.gmail.com>
Date:   Mon, 14 Sep 2020 12:14:48 -0700
From:   Nick Terrell <nickrterrell@...il.com>
To:     Petr Malat <oss@...at.biz>
Cc:     linux-kernel@...r.kernel.org, gustavoars@...nel.org,
        Nick Terrell <terrelln@...com>
Subject: Re: [PATCH] zstd: Fix decompression of large window archives on
 32-bit platforms

On Sun, Sep 13, 2020 at 11:19 PM Petr Malat <oss@...at.biz> wrote:
>
> It seems some optimization has been removed from the code without removing
> the if condition which should activate it only on 64-bit platforms and as
> a result the code responsible for decompression with window larger than
> 8MB was disabled on 32-bit platforms.
>
> Signed-off-by: Petr Malat <oss@...at.biz>

Reviewed-by: Nick Terrell <terrelln@...com>

Thanks for the fix! I looked upstream and this fix corresponds to this
upstream commit:
https://github.com/facebook/zstd/commit/8a5c0c98ae5a7884694589d7a69bc99011add94d

Thanks,
Nick Terrell

> ---
>  lib/zstd/decompress.c | 8 ++------
>  1 file changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/lib/zstd/decompress.c b/lib/zstd/decompress.c
> index db6761ea4deb..509a3b8d51b9 100644
> --- a/lib/zstd/decompress.c
> +++ b/lib/zstd/decompress.c
> @@ -1457,12 +1457,8 @@ static size_t ZSTD_decompressBlock_internal(ZSTD_DCtx *dctx, void *dst, size_t d
>                 ip += litCSize;
>                 srcSize -= litCSize;
>         }
> -       if (sizeof(size_t) > 4) /* do not enable prefetching on 32-bits x86, as it's performance detrimental */
> -                               /* likely because of register pressure */
> -                               /* if that's the correct cause, then 32-bits ARM should be affected differently */
> -                               /* it would be good to test this on ARM real hardware, to see if prefetch version improves speed */
> -               if (dctx->fParams.windowSize > (1 << 23))
> -                       return ZSTD_decompressSequencesLong(dctx, dst, dstCapacity, ip, srcSize);
> +       if (dctx->fParams.windowSize > (1 << 23))
> +               return ZSTD_decompressSequencesLong(dctx, dst, dstCapacity, ip, srcSize);
>         return ZSTD_decompressSequences(dctx, dst, dstCapacity, ip, srcSize);
>  }
>
> --
> 2.20.1
>