net: optimize checksum computation

Very simple loop optimization with a significant performance impact.

Microbenchmark results, modern x86-64:

buffer size | speed up
------------+---------
1500        | 1.7x
64          | 1.5x
8           | 1.15x

Microbenchmark results, POWER7:

buffer size | speed up
------------+---------
1500        | 5x
64          | 3.3x
8           | 1.13x

There is a lot of room for further improvement at the expense of
code complexity - aligned multibyte reads, LE/BE considerations,
architecture-specific optimizations, etc. This patch still keeps
things simple and readable.

Signed-off-by: Ladi Prosek <[email protected]>
Reviewed-by: Dmitry Fleytman <[email protected]>
Signed-off-by: Jason Wang <[email protected]>

net/checksum.c

@@ -22,17 +22,22 @@
 
 uint32_t net_checksum_add_cont(int len, uint8_t *buf, int seq)
 {
-    uint32_t sum = 0;
+    uint32_t sum1 = 0, sum2 = 0;
     int i;
 
-    for (i = seq; i < seq + len; i++) {
-        if (i & 1) {
-            sum += (uint32_t)buf[i - seq];
-        } else {
-            sum += (uint32_t)buf[i - seq] << 8;
-        }
+    for (i = 0; i < len - 1; i += 2) {
+        sum1 += (uint32_t)buf[i];
+        sum2 += (uint32_t)buf[i + 1];
+    }
+    if (i < len) {
+        sum1 += (uint32_t)buf[i];
+    }
+
+    if (seq & 1) {
+        return sum1 + (sum2 << 8);
+    } else {
+        return sum2 + (sum1 << 8);
     }
-    return sum;
 }
 
 uint16_t net_checksum_finish(uint32_t sum)