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

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

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