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

[Jan Kiszka]

On 2012-01-23 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<address@hidden>
>> ---
>>
>> 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 think we have no python dependency in the build system yet - while we
have for awk. That was also the original reason to go for a shell script.

If the situation changed, I would not vote against python, for sure.

Jan

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