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

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

On 12/3/2014 11:36 PM, Richardson, Bruce wrote:
> On Wed, Dec 03, 2014 at 03:19:34PM +0000, Qiu, Michael wrote:
>> On 2014/12/3 22:51, Richardson, Bruce wrote:
>>> On Wed, Dec 03, 2014 at 01:59:58PM +0000, Qiu, Michael wrote:
>>>> On 2014/12/3 19:43, Richardson, Bruce wrote:
>>>>> On Wed, Dec 03, 2014 at 07:28:19PM +0800, 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 *'.
>>>>>>
>>>>> What compiler version is this with? Is there any other way to fix this
>>>>> other than disabling the compiler warnings. Turning off strict aliasing may
>>>>> affect performance as it reduces the number of optimizations that the compiler
>>>>> can perform.
>>>> The compile version is:
>>>>
>>>> $ gcc -v
>>>> Using built-in specs.
>>>> Target: x86_64-redhat-linux
>>>> ...
>>>> gcc version 4.4.7 20120313 (Red Hat 4.4.7-4) (GCC)
>>>>
>>>>
>>>> The OS is centos6.5 x86_64
>>>>
>>>>
>>>> Actually, another possible solution is, as gcc manual shows, use union.
>>>> In function rte_raw_cksum() of lib/librte_net/rte_ip.h:
>>>>
>>>> static inline uint16_t
>>>> rte_raw_cksum(const char *buf, size_t len){
>>>>     union {
>>>>         const char *ubuf;
>>>>         const uint16_t *uu16;
>>>>     } convert;
>>>>
>>>>     convert.ubuf = buf;
>>>>     const uint16_t *u16 = convert.uu16;
>>>>     ...
>>>> }
>>>>
>>>> This may be work, but not test yet.
>>>>
>>>> Any comments about this solution?
>>> what happens if you make rte_raw_cksum take a void * (or const void *) parameter
>>> instead of "const char *"?
>> "size_t len" is for type char, it is the the array size(for char array
>> is byte numbers), if we use void *, the meaning maybe confuse I think.
> That shouldn't be a big issue. We can rename it to "size" instead of "len" if
> needed.

As I tested in my env just, void * is still not work, the same errors
post when compile.

Then the solution for DPDK should be:
-  As Olivier suggest, add the -Wno-strict-aliasing only for gcc 4.4 in
the Makefile
- As gcc manual suggest, use "union" to work around this issue.

Thanks,
Michael
>> But it should work with other code change.
>>
>> Thanks,
>> Michael
>>> /Bruce
>>>