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

[Christoph Egger]

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<address@hidden>
> > ---
> >
> > 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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