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

[Jan Kiszka <jan.kiszka@siemens.com>]

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"
-- 
1.7.3.4