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

[Måns Rullgård]

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.

-- 
M?ns Rullg?rd
mans at mansr.com