Re: [dpdk-dev] [PATCH v3] test-pmd: Fix pointer aliasing error

["Qiu, Michael" <michael.qiu@intel.com>]

Hi Thomas,

What's going on with this patch?

I really do not have other better solution.

Thanks,
Michael
On 12/8/2014 9:30 AM, Qiu, Michael wrote:
> On 12/4/2014 9:35 PM, Michael Qiu wrote:
>> app/test-pmd/csumonly.c: In function ‘get_psd_sum’:
>> build/include/rte_ip.h:161: error: dereferencing pointer ‘u16’
>>         does break strict-aliasing rules
>> build/include/rte_ip.h:157: note: initialized from here
>>         ...
>>
>> The root cause is that, compile enable strict aliasing by default,
>> while in function rte_raw_cksum() try to convert 'const char *'
>> to 'const uint16_t *'.
>>
>> This patch is one workaround fix.
>>
>> Signed-off-by: Michael Qiu <michael.qiu@intel.com>
>> ---
>> v3 --> v2:
>> 	use uintptr_t instead of unsigned long to
>> 	save pointer.
>>
>> v2 --> v1:
>> 	Workaround solution instead of shut off the
>> 	gcc params.
>>
>>  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 61e4457..cda3436 100644
>> --- a/lib/librte_net/rte_ip.h
>> +++ b/lib/librte_net/rte_ip.h
>> @@ -154,7 +154,8 @@ struct ipv4_hdr {
>>  static inline uint16_t
>>  rte_raw_cksum(const char *buf, size_t len)
>>  {
>> -	const uint16_t *u16 = (const uint16_t *)buf;
>> +	uintptr_t ptr = (uintptr_t)buf;
>> +	const uint16_t *u16 = (const uint16_t *)ptr;
>>  	uint32_t sum = 0;
>>  
>>  	while (len >= (sizeof(*u16) * 4)) {
> This workaround is to solve the compile issue of GCC strict-aliasing(Two
> different type pointers should not be point to the same memory address).
>
> For GCC 4.4.7 it will definitely occurs if  flags "-fstrict-aliasing"
> and "-Wall" used.
>
> Thanks,
> Michael
>