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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Mrzyglod, DanielX T
> Sent: Wednesday, November 25, 2015 5:07 PM
> To: Ananyev, Konstantin
> Cc: dev@dpdk.org
> Subject: RE: [dpdk-dev] [PATCH v2] net: fix build with gcc 4.4.7 and strict aliasing
> 
> 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;
> }

So I misinterpreted gcc manual: it only allows conversion from any other type to char, not visa-versa.
Sorry for wrong suggestion.

> 
> 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.

Ok, if there is no other known way - let's stick with your original patch. 
Konstantin

> 
> >-----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