Author: Heiner Marxen
Date: 15:55:22 01/06/03
Go up one level in this thread
On January 03, 2003 at 20:26:44, Matt Taylor wrote:
>On January 03, 2003 at 14:27:02, Ricardo Gibert wrote:
>
>>On January 03, 2003 at 14:09:56, Uri Blass wrote:
>>
>>>On January 03, 2003 at 14:04:23, Ricardo Gibert wrote:
>>>
>>>>On January 03, 2003 at 12:03:45, Uri Blass wrote:
>>>>
>>>>>code B is slightly faster than code A.
>>>>>I know that side can get only 0 or 1(something that the compiler does not know)
>>>>>and B is eqvivalent to A if you assume that side gets only 0 or 1.
>>>>>
>>>>>Is it possible to write a third code that will be even faster than B?
>>>>>
>>>>>I think that if the compiler can know that side is or 0 or 1 it can do B even
>>>>>faster.
>>>>>
>>>>>code A:
>>>>>
>>>>>if (side==LIGHT)
>>>>>{
>>>>> if (to>=56)
>>>>> {
>>>>> gen_promote(from,to,bits);
>>>>> return;
>>>>> }
>>>>>}
>>>>>else
>>>>>{
>>>>> if (to<=7)
>>>>> {
>>>>> gen_promote(from,to,bits);
>>>>> return;
>>>>> }
>>>>>}
>>>>
>>>>
>>>>Try something like this:
>>>>
>>>>if ((to<=7) || (to>=56))
>>
>>
>>You could also use "if (to-8 >= 48)" instead of the above. This assumes "to" is
>>unsigned. It would be a shame for you to duplicate code just avoid 1 extra
>>operation (the subtraction).
><snip>
>
>That is awesome. I did not even think of that. That is probably the best code
>possible to put there.
>
>-Matt
That is a trick to do a range check, which I first saw in some code generated
from a C compiler for a switch statement (loong ago).
From the sources of Chest:
/*
* in_range() is general purpose range checking macro for integral types.
* Note, that the lower bound is inclusive and the upper bound exclusive !
* Note, that only the lower bound is expanded more than once.
*/
#define in_range(x, lo, hi) \
( (((unsigned long)(x )) - ((unsigned long)(lo))) \
< (((unsigned long)(hi)) - ((unsigned long)(lo))) \
)
Cheers,
Heiner
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