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 at intel.com> >Cc: dev at 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 at dpdk.org] On Behalf Of Daniel Mrzyglod >> Sent: Tuesday, November 24, 2015 4:31 PM >> To: dev at 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 at 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