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From: fir <profesor.fir@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: what you can do with bitfields
Date: Sat, 30 Nov 2024 23:11:09 +0100
Organization: i2pn2 (i2pn.org)
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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)