[FFmpeg-cvslog] r23391 - trunk/libavcodec/ivi_common.c

[Maxim]

M?ns Rullg?rd schrieb:
> Maxim <max_pole at gmx.de> writes:
>
>   
>> M?ns Rullg?rd schrieb:
>>     
>>> maxim <subversion at mplayerhq.hu> writes:
>>>
>>>   
>>>       
>>>> Author: maxim
>>>> Date: Mon May 31 01:57:51 2010
>>>> New Revision: 23391
>>>>
>>>> Log:
>>>> Make dequantization equation use less registers on some CPUs.
>>>>
>>>> Modified:
>>>>    trunk/libavcodec/ivi_common.c
>>>>
>>>> Modified: trunk/libavcodec/ivi_common.c
>>>> ==============================================================================
>>>> --- trunk/libavcodec/ivi_common.c	Mon May 31 00:25:40 2010	(r23390)
>>>> +++ trunk/libavcodec/ivi_common.c	Mon May 31 01:57:51 2010	(r23391)
>>>> @@ -416,7 +416,7 @@ int ff_ivi_decode_blocks(GetBitContext *
>>>>
>>>>                      q = (base_tab[pos] * scale_tab[quant]) >> 8;
>>>>                      if (q > 1)
>>>> -                        val = val * q + FFSIGN(val) * ((q >> 1) - (q & 1));
>>>> +                        val = val * q + FFSIGN(val) * (((q ^ 1) - 1) >> 1);
>>>>     
>>>>         
>>> What about the CPUs where this sequence is slower?
>>>   
>>>       
>> Which CPU(s) do you mean? On both x86 and PowerPC it should work fine...
>>     
>
> ARM.  The old code can be done like this:
>
>   and  r0, r1, #1
>   rsb  r0, r0, r1, lsr #1
>
> The new code needs three instructions:
>
>   eor  r0, r1, #1
>   sub  r0, r0, #1
>   lsr  r0, r0, #1
>
> Assuming, of course, the compiler doesn't do this transformation.
>   

What do you suggest to do? Introduce a switch like:

#if /ARCH_ARM
/* do it the old way */
#else
/* do it the new way*/

/I personally don't care about such "+/- one instruction"-optimizations
(to be sincerely) because noone is able to notice any performance
difference in this case. Fix me if I'm wrong...

Best regards
Maxim