Re: [RFC] fbdev/riva:change to use generice function to implement reverse_order()

yalin wang
2015-08-24T08:31:22Z

> On Aug 22, 2015, at 15:53, Afzal Mohammed <afzal.mohd.ma@gmail.com> wrote:
>
> 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.

i only submit the bit reverse patch for arm / arm64 arch,
i am not sure if there are some other arch also have hard ware bit reverse
instructions, need arch maintainers to submit if their arch also have these hard
ware instructions . :)

>
>> 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.
>