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

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

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?

Thanks,
Michael
>
> /Bruce
>
>> Need to add CFLAG '-Wno-strict-aliasing' to avoid this issue.
>>
>> Signed-off-by: Michael Qiu <michael.qiu@intel.com>
>> ---
>>  app/test-pmd/Makefile | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/app/test-pmd/Makefile b/app/test-pmd/Makefile
>> index 97dc2e6..995c874 100644
>> --- a/app/test-pmd/Makefile
>> +++ b/app/test-pmd/Makefile
>> @@ -38,7 +38,7 @@ ifeq ($(CONFIG_RTE_TEST_PMD),y)
>>  #
>>  APP = testpmd
>>  
>> -CFLAGS += -O3
>> +CFLAGS += -O3 -Wno-strict-aliasing
>>  CFLAGS += $(WERROR_FLAGS)
>>  
>>  ifeq ($(CONFIG_RTE_LIBRTE_PMD_PCAP),y)
>> -- 
>> 1.9.3
>>