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Date: Mon, 23 Jan 2012 20:16:42 +0100
From: Stefan Weil <weil@mail.berlios.de>
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Subject: Re: [Qemu-devel] [RFC][PATCH] signrom: Speed up checksum calculation
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To: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Christoph Egger <Christoph.Egger@amd.com>, Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>

Am 23.01.2012 19:38, schrieb Jan Kiszka:
> Forking an expr process for every byte of the input data slows down the
> checksum calculation massively. Fix this while still remaining portable
> by implementing the algorithm in awk.
>
> Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
> ---
>
> That "remaining portable" is an unproven claim. So please check that
> problematic NetBSD and also mingw. Thanks!
>
>   scripts/signrom.sh |   18 ++++++++----------
>   1 files changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/scripts/signrom.sh b/scripts/signrom.sh
> index 9dc5c63..f0f460e 100755
> --- a/scripts/signrom.sh
> +++ b/scripts/signrom.sh
> @@ -23,22 +23,20 @@
>   # did we get proper arguments?
>   test "$1" -a "$2" || exit 1
>
> -sum=0
> -
>   # find out the file size
>   x=`dd if="$1" bs=1 count=1 skip=2 2>/dev/null | od -t u1 -A n`
> -#size=`expr $x \* 512 - 1`
>   size=$(( $x * 512 - 1 ))
>
>   # now get the checksum
>   nums=`od -A n -t u1 -v -N $size "$1"`
> -for i in ${nums}; do
> -    # add each byte's value to sum
> -    sum=`expr \( $sum + $i \) % 256`
> -done
> -
> -sum=$(( (256 - $sum) % 256 ))
> -sum_octal=$( printf "%o" $sum )
> +sum_octal=`echo $nums | awk 'BEGIN {
> +    getline data_str;
> +    sum = 0;
> +    n = split(data_str, data, " ");
> +    for (i = 1; i<= n; i++)
> +        sum = ( sum + data[i] ) % 256;
> +    printf "%o", (256 - sum) % 256;
> +}'`
>
>   # and write the output file
>   cp "$1" "$2"
>    

What about replacing the whole script by a python script?
That would save about 6 more forks :-)

I'd prefer if we could get rid of all AWK dependencies in QEMU
and focus on as few scripting languages as reasonable.

Regards,
Stefan W.