[PATCH v3 2/2] net: have checksum routines accept unaligned data

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

__rte_raw_cksum() (used by rte_raw_cksum() among others) accessed its
data through an uint16_t pointer, which allowed the compiler to assume
the data was 16-bit aligned. This in turn would, with certain
architectures and compiler flag combinations, result in code with SIMD
load or store instructions with restrictions on data alignment.

This patch keeps the old algorithm, but data is read using memcpy()
instead of direct pointer access, forcing the compiler to always
generate code that handles unaligned input. The __may_alias__ GCC
attribute is no longer needed.

The data on which the Internet checksum functions operates are almost
always 16-bit aligned, but there are exceptions. In particular, the
PDCP protocol header may (literally) have an odd size.

Performance impact seems to range from none to a very slight
regression.

Bugzilla ID: 1035
Cc: stable@dpdk.org

---

v3:
  * Use RTE_ALIGN_FLOOR() in the pointer arithmetic (Olivier Matz).
v2:
  * Simplified the odd-length conditional (Morten Brørup).

Reviewed-by: Morten Brørup <mb@smartsharesystems.com>

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
---
 lib/net/rte_ip.h | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/lib/net/rte_ip.h b/lib/net/rte_ip.h
index b502481670..ecd250e9be 100644
--- a/lib/net/rte_ip.h
+++ b/lib/net/rte_ip.h
@@ -160,18 +160,21 @@ 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)
 {
-	/* extend strict-aliasing rules */
-	typedef uint16_t __attribute__((__may_alias__)) u16_p;
-	const u16_p *u16_buf = (const u16_p *)buf;
-	const u16_p *end = u16_buf + len / sizeof(*u16_buf);
+	const void *end;
 
-	for (; u16_buf != end; ++u16_buf)
-		sum += *u16_buf;
+	for (end = RTE_PTR_ADD(buf, RTE_ALIGN_FLOOR(len, sizeof(uint16_t)));
+	     buf != end; buf = RTE_PTR_ADD(buf, sizeof(uint16_t))) {
+		uint16_t v;
+
+		memcpy(&v, buf, sizeof(uint16_t));
+		sum += v;
+	}
 
 	/* if length is odd, keeping it byte order independent */
 	if (unlikely(len % 2)) {
 		uint16_t left = 0;
-		*(unsigned char *)&left = *(const unsigned char *)end;
+
+		memcpy(&left, end, 1);
 		sum += left;
 	}
 
-- 
2.25.1