net: fix aliasing in checksum computation

That means a superfluous cast is removed and aliasing through a uint8_t
pointer is eliminated. NB: The C standard specifies that a unsigned char
pointer may alias while the C standard doesn't include such requirement
for uint8_t pointers.

Also simplified the loop since a modern C compiler can speed up (i.e.
auto-vectorize) it in a similar way. For example, GCC auto-vectorizes it
for Haswell using AVX registers while halving the number of instructions
in the generated code.

Fixes: 6006818cfb26 ("net: new checksum functions")
Fixes: e079655c41fb ("net: fix build with gcc 4.4.7 and strict aliasing")
Cc: stable@dpdk.org
Signed-off-by: Georg Sauthoff <mail@gms.tf>
Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>

diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
index 0ef2430..41d90ca 100644 (file)
--- a/lib/net/rte_ip.h
+++ b/lib/net/rte_ip.h
@@ -154,29 +154,18 @@ rte_ipv4_hdr_len(const struct rte_ipv4_hdr *ipv4_hdr)
 static inline uint32_t
 __rte_raw_cksum(const void *buf, size_t len, uint32_t sum)
 {
-       /* workaround gcc strict-aliasing warning */
-       uintptr_t ptr = (uintptr_t)buf;
+       /* extend strict-aliasing rules */
        typedef uint16_t __attribute__((__may_alias__)) u16_p;
-       const u16_p *u16_buf = (const u16_p *)ptr;
-
-       while (len >= (sizeof(*u16_buf) * 4)) {
-               sum += u16_buf[0];
-               sum += u16_buf[1];
-               sum += u16_buf[2];
-               sum += u16_buf[3];
-               len -= sizeof(*u16_buf) * 4;
-               u16_buf += 4;
-       }
-       while (len >= sizeof(*u16_buf)) {
+       const u16_p *u16_buf = (const u16_p *)buf;
+       const u16_p *end = u16_buf + len / sizeof(*u16_buf);
+
+       for (; u16_buf != end; ++u16_buf)
                sum += *u16_buf;
-               len -= sizeof(*u16_buf);
-               u16_buf += 1;
-       }
 
-       /* if length is in odd bytes */
-       if (len == 1) {
+       /* if length is odd, keeping it byte order independent */
+       if (unlikely(len % 2)) {
                uint16_t left = 0;
-               *(uint8_t *)&left = *(const uint8_t *)u16_buf;
+               *(unsigned char *)&left = *(const unsigned char *)end;
                sum += left;
        }