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From: Georg Sauthoff <mail@gms.tf>
To: Olivier Matz <olivier.matz@6wind.com>,
	Thomas Monjalon <thomas@monjalon.net>,
	David Marchand <david.marchand@redhat.com>
Cc: dev@dpdk.org, Georg Sauthoff <mail@gms.tf>
Subject: [dpdk-dev] [PATCH 0/1] net: fix aliasing issue in checksum computation
Date: Sat, 18 Sep 2021 13:49:29 +0200	[thread overview]
Message-ID: <20210918114930.245387-1-mail@gms.tf> (raw)

The current __rte_raw_cksum() function violates the C strict-aliasing
rules since it uses a uint8_t pointer to access a trailing byte.

This patch also fixes a superfluous cast, i.e.:

	uintptr_t ptr = (uintptr_t)buf;
	typedef uint16_t __attribute__((__may_alias__)) u16_p;
	const u16_p *u16_buf = (const u16_p *)ptr;

Transitive casting involving uintptr_t doesn't solve anything here.
It also doesn't help with fixing a strict-aliasing issue here.

The patch also simplifies the main loop, i.e. it eliminates the manually
unrolled loop

	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;
	}

since modern C compilers are in a better position to decide which level of
unrolling is optimal for the target architecture.

See also https://godbolt.org/z/6rYbYGnj7 which shows how GCC auto-vectorizes
the simplified loop using AVX instructions, when compiling for Haswell. When
looking at the number of instructions in the compiled code, the new version
is half as big as the existing one.

Georg Sauthoff (1):
  net: fix aliasing issue in checksum computation

 lib/net/rte_ip.h | 27 ++++++++-------------------
 1 file changed, 8 insertions(+), 19 deletions(-)

-- 
2.31.1


             reply	other threads:[~2021-09-18 11:49 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-18 11:49 Georg Sauthoff [this message]
2021-09-18 11:49 ` [dpdk-dev] [PATCH 1/1] " Georg Sauthoff
2021-10-14 17:20   ` Ferruh Yigit
2021-10-14 20:22     ` Morten Brørup
2021-10-16  8:21     ` Morten Brørup
2021-10-16 17:17       ` Georg Sauthoff
2021-10-16  8:24     ` Morten Brørup
2021-10-15 14:39   ` Olivier Matz
2021-10-16 17:02     ` Georg Sauthoff

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