Re: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and strict aliasing

["Mrzyglod, DanielX T" <danielx.t.mrzyglod@intel.com>]

If we change input buf:
xapp-gcc/include/rte_ip.h:211: error: dereferencing pointer 'u16' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:204: note: initialized from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:212: error: dereferencing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:212: note: initialized from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:213: error: dereferencing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:213: note: initialized from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:214: error: dereferencing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:214: note: initialized from here

The change was pointed  out by Michael Qiu in patch: 2b039d5f20a34016ecaf9b26f8f8b6c4a81bf4b6

We can only cast void to uint8 but it would involve big endian/ little endian macros:

static inline uint32_t __attribute__((__may_alias__))
__rte_raw_cksum(const void *buf, size_t len, uint32_t sum)
{
	/* workaround gcc strict-aliasing warning */
	const uint8_t *u8 = ((const uint8_t *)buf);
	while (len >= (sizeof(*u8) * 8)) {
		sum += *(u8+1) << 8 | *(u8);
		sum += *(u8+3) << 8 | *(u8+2);
		sum += *(u8+5) << 8 | *(u8+4);
		sum += *(u8+7) << 8 | *(u8+6);

		len -= sizeof(*u8) * 8;
		u8 += 8;
	}

	while (len >= 2*sizeof(*u8)) {
		sum += *u8 << 8 | *(u8+1);
		len -= 2*sizeof(*u8);
		u8 += 2;
	}

	/* if length is in odd bytes */
	if (len == 1)
		sum += *((const uint8_t *)u8);

	return sum;
}

I will research about this union usage suggested by Stephen, but for this moment  local typedef & __attribute__((__may_alias__)) is the most clean solution which work under gcc(4.4.7), clang, icc.

>-----Original Message-----
>From: Ananyev, Konstantin
>Sent: Tuesday, November 24, 2015 6:58 PM
>To: Mrzyglod, DanielX T <danielx.t.mrzyglod@intel.com>
>Cc: dev@dpdk.org
>Subject: RE: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and strict aliasing
>
>Hi Daniel,
>So for my own curiosity: what went wrong here?
>Did compiler avoid to generate a code for while {} loop?
>Or something else?
>BTW, it is an internal function, so instead of introducing new typedef,
>we can just change the type of buf?
>Let say to uint8_t *?
>From gcc manual page: "  A character type may alias any other type."
>Would that work?
>Konstantin
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Daniel Mrzyglod
>> Sent: Tuesday, November 24, 2015 4:31 PM
>> To: dev@dpdk.org
>> Subject: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and strict aliasing
>>
>> This fix is for IPv6 checksum offload error on RHEL65.
>> Any optimalisation above -O0 provide error in IPv6 checksum
>> flag "-fstrict-aliasing" is default for optimalisation above -O0.
>>
>> Fixes: 2b039d5f20a3 ("net: fix build with gcc 4.4.7 and strict aliasing")
>>
>> Signed-off-by: Daniel Mrzyglod <danielx.t.mrzyglod@intel.com>
>> ---
>>  lib/librte_net/rte_ip.h | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/lib/librte_net/rte_ip.h b/lib/librte_net/rte_ip.h
>> index 71c519a..5b7554a 100644
>> --- a/lib/librte_net/rte_ip.h
>> +++ b/lib/librte_net/rte_ip.h
>> @@ -169,7 +169,8 @@ __rte_raw_cksum(const void *buf, size_t len, uint32_t
>sum)
>>  {
>>  	/* workaround gcc strict-aliasing warning */
>>  	uintptr_t ptr = (uintptr_t)buf;
>> -	const uint16_t *u16 = (const uint16_t *)ptr;
>> +	typedef uint16_t __attribute__((__may_alias__)) u16_p;
>> +	const u16_p *u16 = (const u16_p *)ptr;
>>
>>  	while (len >= (sizeof(*u16) * 4)) {
>>  		sum += u16[0];
>> --
>> 2.5.0