Re: is there a "single_bit_set" macro somewhere?

[walter harms <wharms@bfs.de>]

Robert P. J. Day schrieb:
> On Thu, 23 Apr 2009, Julia Lawall wrote:
> 
>> On Thu, 23 Apr 2009, Robert P. J. Day wrote:
>>
>>> On Thu, 23 Apr 2009, Julia Lawall wrote:
>>>
>>>> I found 8 occurrences with the following Coccinelle semantic match:
>>>>
>>>> @@ expression n; @@
>>>>
>>>> * (n & (n-1)) = 0
>>>>
>>>>
>>>> The most unpleasant was:
>>>>
>>>> (!(((fp)->ipend & ~0x30) & (((fp)->ipend & ~0x30) - 1)))
>>>>
>>>> in the file arch/blackfin/kernel/traps.c.
>>>>
>>>> At least some of them seemed to have comments nearby that suggested
>>>> that searching for a single 1-bit was indeed the goal.  I haven't
>>>> looked at all of them, though.
>>>   behold, the power of shell and REs, which searches for four
>>> variations of that test:
>> Actually, it seems that it was specifying = 0 that was not really
>> necessary.  Perhaps the possibility of parentheses is useful too.
>>
>> With the following semantic patch, I find 93 occurrences.
>>
>> @@ expression n; @@
>>
>> * (n) & ((n)-1)
>>
>> That could indeed be worth doing something about, if they are indeed all
>> representing a check for a single bit.
> 
>   they're not.  we're actually now discussing this on the main LKML,
> but that construct -- n & (n - 1) -- has two semantic meanings:
> 
>   a) power of two?
>   b) single bit set?
> 
>   mathematically, those two things have the same form but
> *semantically* they're read very differently, which is why i'm
> proposing to add a simple function or two for single-bit testing that
> would make a lot of the code easier to read.
> 
> rday
> --
> 
hi all,

glibc provides in sys/param.h a set a macros (seem to come from BSD):

#define NBBY            CHAR_BIT
...
#define setbit(a,i)     ((a)[(i)/NBBY] |= 1<<((i)%NBBY))
#define clrbit(a,i)     ((a)[(i)/NBBY] &= ~(1<<((i)%NBBY)))
#define isset(a,i)      ((a)[(i)/NBBY] & (1<<((i)%NBBY)))
#define isclr(a,i)      (((a)[(i)/NBBY] & (1<<((i)%NBBY))) = 0)

IMHO this is readable. This could be a nice starting point (or endpoint ?) for a S&R mission.

re,
 wh