lib/checksum.c: optimize do_csum a bit

Reduce the number of variables modified by the loop in do_csum() by 1,
which seems like a good idea.  On Nios II (a RISC CPU with 3-operand
instruction set) it reduces the loop from 7 to 6 instructions, including
the conditional branch.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 8 deletions

diff --git a/lib/checksum.c b/lib/checksum.c
index 097508732f34..8df2f91e6d98 100644
--- a/lib/checksum.c
+++ b/lib/checksum.c
@@ -49,7 +49,7 @@ static inline unsigned short from32to16(unsigned int x)
 
 static unsigned int do_csum(const unsigned char *buff, int len)
 {
-        int odd, count;
+        int odd;
         unsigned int result = 0;
 
         if (len <= 0)
@@ -64,25 +64,22 @@ static unsigned int do_csum(const unsigned char *buff, int len)
                 len--;
                 buff++;
         }
-        count = len >> 1;               /* nr of 16-bit words.. */
-        if (count) {
+        if (len >= 2) {
                 if (2 & (unsigned long) buff) {
                         result += *(unsigned short *) buff;
-                        count--;
                         len -= 2;
                         buff += 2;
                 }
-                count >>= 1;            /* nr of 32-bit words.. */
-                if (count) {
+                if (len >= 4) {
+                        const unsigned char *end = buff + ((unsigned)len & ~3);
                         unsigned int carry = 0;
                         do {
                                 unsigned int w = *(unsigned int *) buff;
-                                count--;
                                 buff += 4;
                                 result += carry;
                                 result += w;
                                 carry = (w > result);
-                        } while (count);
+                        } while (buff < end);
                         result += carry;
                         result = (result & 0xffff) + (result >> 16);
                 }