Re: [dpdk-dev] [PATCH v2 1/1] net: fix aliasing issue in checksum computation

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 10/18/2021 8:58 AM, Olivier Matz wrote:
> On Mon, Oct 18, 2021 at 09:35:41AM +0200, Morten Brørup wrote:
>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Georg Sauthoff
>>> Sent: Sunday, 17 October 2021 22.37
>>
>> +Ferruh, as delegate to v1 in Patchwork.
>>
>>>
>>> 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
>>>
>>
>> Great work documenting your thoughts behind this patch, Georg! I, for one, didn't know about the aliasing difference between uint8_t and unsigned char. :-)
>>
>> After taking a good look at v2 and the Godbolt reference to confirm the claimed benefits, there can be no doubts about this patch.
> 
> +1, thanks for the good documentation
> 
>> Reviewed-by: Morten Brørup <mb@smartsharesystems.com>
> 
> Acked-by: Olivier Matz <olivier.matz@6wind.com>
> 

Updated patch title as: "net: fix aliasing in checksum computation"

Added fixes tags:
     Fixes: 6006818cfb26 ("net: new checksum functions")
     Fixes: e079655c41fb ("net: fix build with gcc 4.4.7 and strict aliasing")
     Cc: stable@dpdk.org

Following warning fixed in next-net:
   ERROR:POINTER_LOCATION: "(foo*)" should be "(foo *)"
   #65: FILE: lib/net/rte_ip.h:168:
   +               *(unsigned char*)&left = *(const unsigned char *)end;


Applied to dpdk-next-net/main, thanks.