netdev - Re: [PATCH net-next 01/14] etherdevice: Add eth_<foo>_addr CONFIG_HAVE_EFFICIENT_UNALIGNED

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Date:	Tue, 03 Mar 2015 05:19:12 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 01/14] etherdevice: Add eth_<foo>_addr
 CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS code

On Mon, 2015-03-02 at 19:54 -0800, Joe Perches wrote:
> Make the memset possibly more efficient with the appropriate
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS #ifdef
> 
> Signed-off-by: Joe Perches <joe@...ches.com>
> ---
>  include/linux/etherdevice.h | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/etherdevice.h b/include/linux/etherdevice.h
> index 606563e..22ad94a 100644
> --- a/include/linux/etherdevice.h
> +++ b/include/linux/etherdevice.h
> @@ -192,7 +192,12 @@ static inline void eth_random_addr(u8 *addr)
>   */
>  static inline void eth_broadcast_addr(u8 *addr)
>  {
> +#if defined(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)
> +	*(u32 *)addr = 0xffffffff;
> +	*(u16 *)(addr + 4) = 0xffff;
> +#else
>  	memset(addr, 0xff, ETH_ALEN);
> +#endif
>  }
>  

I am pretty sure compiler already does an inline, and might chose to 
emit equivalent instructions for :
 *(u16 *) addr = 0xffff;
 *(u32 *)(addr + 2) = 0xffffffff;

if (addr % 4) is known to be 2


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