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From: Georg Sauthoff <mail@gms.tf>
To: "Morten Brørup" <mb@smartsharesystems.com>,
	"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 v2 1/1] net: fix aliasing issue in checksum computation
Date: Sun, 17 Oct 2021 22:37:18 +0200	[thread overview]
Message-ID: <20211017203718.801998-2-mail@gms.tf> (raw)
In-Reply-To: <20211017203718.801998-1-mail@gms.tf>

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


  reply	other threads:[~2021-10-17 20:38 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-17 20:37 [dpdk-dev] [PATCH v2 0/1] " Georg Sauthoff
2021-10-17 20:37 ` Georg Sauthoff [this message]
2021-10-18  7:35   ` [dpdk-dev] [PATCH v2 1/1] " Morten Brørup
2021-10-18  7:58     ` Olivier Matz
2021-10-18 16:14       ` Ferruh Yigit

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