[PATCH v9] net: optimize raw checksum computation

[Stephen Hemminger]

On Thu,  8 Jan 2026 16:47:13 -0500
scott.k.mitch1 at gmail.com wrote:

> diff --git a/lib/net/rte_ip6.h b/lib/net/rte_ip6.h
> index d1abf1f5d5..8a7e5e4b8a 100644
> --- a/lib/net/rte_ip6.h
> +++ b/lib/net/rte_ip6.h
> @@  -560,19 +560,18 @@ rte_ipv6_phdr_cksum(const struct rte_ipv6_hdr *ipv6_hdr, uint64_t ol_flags)
>  static inline uint16_t
>  rte_ipv6_phdr_cksum(const struct rte_ipv6_hdr *ipv6_hdr, uint64_t ol_flags)
>  {
> -	uint32_t sum;
>  	struct {
>  		rte_be32_t len;   /* L4 length. */
>  		rte_be32_t proto; /* L4 protocol - top 3 bytes must be zero */
> -	} psd_hdr;
> -
> -	psd_hdr.proto = (uint32_t)(ipv6_hdr->proto << 24);
> -	if (ol_flags & (RTE_MBUF_F_TX_TCP_SEG | RTE_MBUF_F_TX_UDP_SEG))
> -		psd_hdr.len = 0;
> -	else
> -		psd_hdr.len = ipv6_hdr->payload_len;
> +	} psd_hdr = {
> +		.len = (ol_flags & (RTE_MBUF_F_TX_TCP_SEG | RTE_MBUF_F_TX_UDP_SEG))
> +			? (rte_be32_t)0
> +			: ipv6_hdr->payload_len,
> +		.proto = (uint32_t)(ipv6_hdr->proto << 24)
> +	};
> +	RTE_SUPPRESS_UNINITIALIZED_WARNING(psd_hdr);
>  
> -	sum = __rte_raw_cksum(&ipv6_hdr->src_addr,
> +	uint32_t sum = __rte_raw_cksum(&ipv6_hdr->src_addr,
>  		sizeof(ipv6_hdr->src_addr) + sizeof(ipv6_hdr->dst_addr),
>  		0);
>  	sum = __rte_raw_cksum(&psd_hdr, sizeof(psd_hdr), sum);
> -- 

Seems like this could be unrolled as well.

static inline uint16_t
rte_ipv6_phdr_cksum(const struct rte_ipv6_hdr *ipv6_hdr, uint64_t ol_flags)
{
	union {
		struct {
			struct rte_ipv6_addr src_addr;  /* 16 bytes */
			struct rte_ipv6_addr dst_addr;  /* 16 bytes */
			rte_be32_t len;                 /* 4 bytes  */
			rte_be32_t proto;               /* 4 bytes  */
		} psd;
		uint16_t u16[20];
	} hdr = {
		.psd = {
			.src_addr = ipv6_hdr->src_addr,
			.dst_addr = ipv6_hdr->dst_addr,
			.proto = (uint32_t)(ipv6_hdr->proto << 24),
		}
	};
	uint32_t sum;

	if (!(ol_flags & (RTE_MBUF_F_TX_TCP_SEG | RTE_MBUF_F_TX_UDP_SEG)))
		hdr.psd.len = ipv6_hdr->payload_len;

	/* Unrolled sum of 20 uint16_t words:
	 * [0-7]:   src_addr
	 * [8-15]:  dst_addr
	 * [16-17]: len
	 * [18-19]: proto (3 zero bytes + next header)
	 */
	sum = hdr.u16[0]  + hdr.u16[1]  + hdr.u16[2]  + hdr.u16[3]  +
	      hdr.u16[4]  + hdr.u16[5]  + hdr.u16[6]  + hdr.u16[7]  +
	      hdr.u16[8]  + hdr.u16[9]  + hdr.u16[10] + hdr.u16[11] +
	      hdr.u16[12] + hdr.u16[13] + hdr.u16[14] + hdr.u16[15] +
	      hdr.u16[16] + hdr.u16[17] + hdr.u16[18] + hdr.u16[19];

	sum = (sum & 0xffff) + (sum >> 16);
	sum = (sum & 0xffff) + (sum >> 16);
	return (uint16_t)sum;
}