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

[Ferruh Yigit]

On 9/18/2021 12:49 PM, Georg Sauthoff wrote:
> That means a superfluous cast is removed and aliasing through a uint8_t
> pointer is eliminated. Note that uint8_t doesn't have the same
> strict-aliasing properties as unsigned char.
> 
> 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 at gms.tf>

+ Morten. (Because of past reviews on cksum code)

> ---
>   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..386db94c85 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 (len % 2) {
>   		uint16_t left = 0;
> -		*(uint8_t *)&left = *(const uint8_t *)u16_buf;
> +		*(unsigned char*)&left = *(const unsigned char *)end;
>   		sum += left;
>   	}
>   
>