Re: [dpdk-dev] [PATCH v2 1/1] net: fix aliasing issue in checksum computation

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@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 <mail@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 <mb@smartsharesystems.com>