> From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Georg Sauthoff
> Sent: Sunday, 17 October 2021 22.37
+Ferruh, as delegate to v1 in Patchwork.
>
> 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.
>
> Signed-off-by: Georg Sauthoff <m...@gms.tf>
> ---
> lib/net/rte_ip.h | 27 ++++++++-------------------
> 1 file changed, 8 insertions(+), 19 deletions(-)
>
> diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
> index 05948b69b7..1b8c6519a9 100644
> --- a/lib/net/rte_ip.h
> +++ b/lib/net/rte_ip.h
> @@ -141,29 +141,18 @@ 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;
> }
>
> --
> 2.31.1
>
Great work documenting your thoughts behind this patch, Georg! I, for one,
didn't know about the aliasing difference between uint8_t and unsigned char. :-)
After taking a good look at v2 and the Godbolt reference to confirm the claimed
benefits, there can be no doubts about this patch.
Reviewed-by: Morten Brørup <m...@smartsharesystems.com>
