Re: [Qemu-devel] [RFC][PATCH] signrom: Speed up checksum calculation

[Christoph Egger <Christoph.Egger@amd.com>]

On 01/23/12 20:16, Stefan Weil wrote:
> 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!

The "unproven claim" is actually what SuSv3 spec
(http://pubs.opengroup.org/onlinepubs/009695399/utilities/od.html)
says:

"and the output from each of the transformations shall be separated by
one or more <blank>s."

The actual problem is that the programmer expects the behaviour
of either GNU od(1) or AT&T UNIX od(1) rather the SuSv3 spec.

NetBSD has AT&T UNIX od(1).

Christoph


>>
>> 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.
>
>


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