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From: "Mrzyglod, DanielX T" <danielx.t.mrzyglod@intel.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
Thread-Topic: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and strict
aliasing
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Cc: "dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and
strict aliasing
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If we change input buf:
xapp-gcc/include/rte_ip.h:211: error: dereferencing pointer 'u16' does brea=
k strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:204: note: initializ=
ed from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:212: error: derefere=
ncing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:212: note: initializ=
ed from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:213: error: derefere=
ncing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:213: note: initializ=
ed from here
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:214: error: derefere=
ncing pointer '({anonymous})' does break strict-aliasing rules
/root/dpdk/x86_64-native-linuxapp-gcc/include/rte_ip.h:214: note: initializ=
ed from here
The change was pointed out by Michael Qiu in patch: 2b039d5f20a34016ecaf9b=
26f8f8b6c4a81bf4b6
We can only cast void to uint8 but it would involve big endian/ little endi=
an 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 =3D ((const uint8_t *)buf);
while (len >=3D (sizeof(*u8) * 8)) {
sum +=3D *(u8+1) << 8 | *(u8);
sum +=3D *(u8+3) << 8 | *(u8+2);
sum +=3D *(u8+5) << 8 | *(u8+4);
sum +=3D *(u8+7) << 8 | *(u8+6);
len -=3D sizeof(*u8) * 8;
u8 +=3D 8;
}
while (len >=3D 2*sizeof(*u8)) {
sum +=3D *u8 << 8 | *(u8+1);
len -=3D 2*sizeof(*u8);
u8 +=3D 2;
}
/* if length is in odd bytes */
if (len =3D=3D 1)
sum +=3D *((const uint8_t *)u8);
return sum;
}
I will research about this union usage suggested by Stephen, but for this m=
oment local typedef & __attribute__((__may_alias__)) is the most clean sol=
ution 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 stric=
t 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 =3D (uintptr_t)buf;
>> - const uint16_t *u16 =3D (const uint16_t *)ptr;
>> + typedef uint16_t __attribute__((__may_alias__)) u16_p;
>> + const u16_p *u16 =3D (const u16_p *)ptr;
>>
>> while (len >=3D (sizeof(*u16) * 4)) {
>> sum +=3D u16[0];
>> --
>> 2.5.0