Re: [PATCH v2] examples/l3fwd: fix aliasing in port grouping

[Jerin Jacob <jerin.jacob@caviumnetworks.com>]

-----Original Message-----
> Date: Fri,  3 Nov 2017 16:13:51 +0530
> From: Guduri Prathyusha <gprathyusha@caviumnetworks.com>
> To: tomasz.kantecki@intel.com
> CC: Jianbo.Liu@arm.com, guduriprathyusha@gmail.com,
>  konstantin.ananyev@intel.com, dev@dpdk.org, Guduri Prathyusha
>  <gprathyusha@caviumnetworks.com>
> Subject: [dpdk-dev] [PATCH v2] examples/l3fwd: fix aliasing in port grouping
> X-Mailer: git-send-email 2.14.1
> 
> With -f-strict-aliasing enabled by default from -O2, gcc > 5.x gives
> undefined behavior in port_groupx4 in ARM. 'pn' and 'pnum' are
> two different pointers pointing to same chunk of memory and
> with -f-strict-aliasing the pointers are assumed to be pointing to
> different memory and compiler reorders instructions that depend on
> pnum and pn. This breaks port grouping algorithm.
> 
> This patch eliminates the above problem by introducing a compiler
> barrier between the instructions that depend on pnum, pn and lp.
> 
> Fixes: 569b290cdb36 ("examples/l3fwd: add NEON implementation")
> 
> Signed-off-by: Guduri Prathyusha <gprathyusha@caviumnetworks.com>

Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>

> ---
>  examples/l3fwd/l3fwd_neon.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/examples/l3fwd/l3fwd_neon.h b/examples/l3fwd/l3fwd_neon.h
> index 4bc161394..b319b5a92 100644
> --- a/examples/l3fwd/l3fwd_neon.h
> +++ b/examples/l3fwd/l3fwd_neon.h
> @@ -114,6 +114,7 @@ port_groupx4(uint16_t pn[FWDSTEP + 1], uint16_t *lp, uint16x8_t dp1,
>  
>  	/* update last port counter. */
>  	lp[0] += gptbl[v].lpv;
> +	rte_compiler_barrier();
>  
>  	/* if dest port value has changed. */
>  	if (v != GRPMSK) {
> -- 
> 2.14.1
>