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

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Message-ID: <4A890388.8070709@gmail.com>
Date:	Mon, 17 Aug 2009 09:15:20 +0200
From:	Jiri Slaby <jirislaby@...il.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:	Andi Kleen <andi@...stfloor.org>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Make shr to divide by power of 2 (resend)

On 08/16/2009 11:14 AM, Sergey Senozhatsky wrote:
> There are different shr/shl operations in bitblit.c. Like,
> #1
> cur_height = vc->vc_font.height >> 1
> cur_height = (vc->vc_font.height << 1)/3;
> c |= c >> 1;
> u32 idx = vc->vc_font.width >> 3;
> etc.
> 
> So, should I convert it to something like:

Not at all. I was writing about about (X + 7) / 8 [or >> 3] which is
DIV_ROUND_UP(X, 8).

> #2
> cur_height = DIV_XXX(vc->vc_font.height, 2)
> cur_height = DIV_XXX( MUL_XXX(vc->vc_font.height, 2), 3); or cur_height = DIV_XXX( (vc->vc_font.height * 2), 3);
> c |= DIV_XXX(c, 1);
> u32 idx = DIV_XXX(vc->vc_font.width, 8);
> 
> #1 is better-looking I think.

Agreed. I would write it without shifts though. But we were talking
about the roundup case not about the shifts all over the code.
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