linux-kernel - Re: [PATCH] Make shr to divide by power of 2 (resend)

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Date:	Sat, 15 Aug 2009 22:42:23 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, andi@...stfloor.org
Subject: Re: [PATCH] Make shr to divide by power of 2 (resend)

On 08/15/2009 04:12 PM, Sergey Senozhatsky wrote:
> On (08/15/09 09:49), Robert P. J. Day wrote:
>> Date: Sat, 15 Aug 2009 09:49:15 -0400 (EDT)
>> From: "Robert P. J. Day" <rpjday@...shcourse.ca>
>> To: Jiri Slaby <jirislaby@...il.com>
>> cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
>> 	Andrew Morton <akpm@...ux-foundation.org>,
>> 	linux-kernel@...r.kernel.org
>> Subject: Re: [PATCH] Make shr to divide by power of 2 (resend)
>> User-Agent: Alpine 2.00 (LFD 1167 2008-08-23)
>>
>> On Sat, 15 Aug 2009, Jiri Slaby wrote:
>>
>>> On 08/15/2009 03:43 PM, Sergey Senozhatsky wrote:
>>>> Make an arithmetic right shift to divide by power of 2.
>>>
>>> Why? Is out there a compiler not doing that?
>>>
>>> Seems like it should be rather converted to DIV_ROUND_UP.
>>>
> 
>                                  
> It was duscussed, so I'll just copy paste my answer.
> On (08/08/09 09:35), Andi Kleen wrote:
>> DIV should be always slower than a SHIFT.
>>
>> But it has nothing really to do with the CPU. The point is that the compiler
>> always selects a suitable one by itself. Rewriting x / 2 to x >> 1 is
>> one of the easiest exercises in compiler optimizations.
>>
>> The only case when the compiler cannot do this easily by itself is
>> when the dividend is not a constant.

I think Andi (Cced again) meant divisor. When a divisor is constant, you
can always transform division to some of subs/adds/muls/shifts.

>         int width = (vc->vc_font.width + 7) >> 3;
> 
>> That said -Os sometimes screws us up on this, but it's still not worth
>> doing this change manually.
>>
> 
> My point is that it should 'look the same'.
> I mean there are 5
>         int width = (vc->vc_font.width + 7) >> 3;
> *not exactly this one, but vc->vc_font.width (+ 7)? >> 3
> 
> and _only_ one
>         int width = (vc->vc_font.width + 7) / 8;
> 
> 
>>   it does seem like a DIV_ROUND_UP would be better but perhaps another
>> solution would be to define a meaningful macro name for that
>> operation.  if that conversion is going to be done regularly, a macro
>> with an informative name might be in order.  if it's a one-shot
>> operation, though, not much point.
>>
> 
> No, it's not a one-shot.
> 
> grep -c '>>' bitblit.c
> 7

Then please use DIV_ROUND_UP macro (or whatever alias) all over the place.
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