Re: what you can do with bitfields

BGB pisze:

On 11/30/2024 9:50 AM, fir wrote:

Lawrence D'Oliveiro pisze:

On Thu, 28 Nov 2024 14:39:43 +0100, fir wrote:
>

i use them but i never fully chcked what i can do with them ...

>
One problem with C is that bitfields can only have static offsets and
sizes. If these were dynamic, you could express much more powerful
manipulations more conveniently than with shifting and masking.
>

what do you mean?
i must check hovever how this static work at all.. i used them
and not so rarely but only simple cases usually for solo bit flags

 Most common way to do bit flags:
   #define WHATEVER_FLAG1  0x0001
   #define WHATEVER_FLAG2  0x0002
   ...
 And:
   if(flags&WHATEVER_FLAG1)
     { ... }
  Though, for larger bit-flags, on some targets it may be more efficient to do:
   if((flags>>WHATEVER_FLAG43_SHR)&1)
     { ... }
 For example, RISC-V, where shift-right and mask will use fewer instructions than constant-load and mask.
 ...
 

and you thing doin it with bitfields would be worse?
i dont tested such things but i use
  union Color
  {
    unsigned u;
    struct { unsigned char b,g,r,a;};
   };
in my per-pixel graphics codes to spare unpacking/packing  unsigned color into r g b a with shifts and ands or ors
anbd in this case its not only much simpler/shortes yet use this
union is more effective (i tested it bac then some hours and
not met a case when this union woudl be slower than shifts by hand)
(speakin on gcc /tdm for windows)