netdev - Re: [PATCH v2] net: ppp: use {get,put}_unaligned

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Message-ID: <4D283D6A.4050502@gmail.com>
Date:	Sat, 08 Jan 2011 11:33:14 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, xiaosuo@...il.com,
	paulus@...ba.org, harvey.harrison@...il.com,
	linux-ppp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: ppp: use {get,put}_unaligned_be{16,32}

Eric Dumazet wrote:
> Le samedi 08 janvier 2011 à 11:04 +0100, Jarek Poplawski a écrit :
> 
>> Just for the record: I agree with Paul that current code is more readable.
>> This code still requires thinking about specific bytes and the patch mixes
>> it only with word access.
>>
>> Jarek P.
>>
>>> @@ -395,16 +396,14 @@ mppe_compress(void *arg, unsigned char *ibuf, unsigned char *obuf,
>>>  	 */
>>>  	obuf[0] = PPP_ADDRESS(ibuf);
>>>  	obuf[1] = PPP_CONTROL(ibuf);
>>> -	obuf[2] = PPP_COMP >> 8;	/* isize + MPPE_OVHD + 1 */
>>> -	obuf[3] = PPP_COMP;	/* isize + MPPE_OVHD + 2 */
>>> +	put_unaligned_be16(PPP_COMP, obuf + 2);
>>>  	obuf += PPP_HDRLEN;
> 
> Compilers are stupid not generating optimal code, so we should help them
> a bit.
> 
> Yes, I agree this is ugly Jarek and makes reading of this code a bit
> more complex, but this is a move we cannot stop. Number of functions,
> macros, etc... is exploding and we must follow the trend ;)
> 
> 41 c6 44 24 02 00       movb   $0x0,0x2(%r12)
> 41 c6 44 24 03 fd       movb   $0xfd,0x3(%r12)
> 
> After patch :
> 
> 66 41 c7 44 24 02 00 fd   movw   $0xfd00,0x2(%r12)

And that's why Paul wanted more justification, because readability
gain is questionable.

Jarek P.
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