linux-kernel - Re: [RFC] fbdev/riva:change to use generice function to implement reverse

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Date:	Sat, 22 Aug 2015 13:23:10 +0530
From:	Afzal Mohammed <afzal.mohd.ma@...il.com>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	yalin wang <yalin.wang2010@...il.com>, adaplas@...il.com,
	plagnioj@...osoft.com, linux-fbdev@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] fbdev/riva:change to use generice function to implement
 reverse_order()

Hi,

On Fri, Aug 21, 2015 at 11:01:41AM +0300, Tomi Valkeinen wrote:

> >> Possibly the patches are still good for x86 also, but that needs to be
> >> proven.
> >>
> > not exactly, because x86_64 don’t have hardware instruction to do rbit OP,
> > i compile by test :
> 
> For old drivers i386 may be more relevant than x86_64.

It seems asm bit reversal is supported in Kernel on arm & arm64 only,
not sure whether any other arch even provide asm bit reversal
instruction.

> These kind of optimizations should have some real world measurements,

Not for this case, but once measured on ARM, iirc, a 32-bit asm bit
reversal as compared to doing it in C was taking 1 cycle as opposed to
~225 cycles!, of course writing optimized C could have made it fare
better, but still would reach no-way near asm bit reversal.

Regards
Afzal
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