Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > comp.lang.c > #147258
| From | Philipp Klaus Krause <pkk@spth.de> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Bit field usage |
| Date | 2019-10-26 21:06 +0200 |
| Organization | solani.org |
| Message-ID | <qp25es$o74$1@solani.org> (permalink) |
| References | <qorerp$af6$1@dont-email.me> <qp22e5$lu5$1@solani.org> <qp22g3$lu5$2@solani.org> <qp23p4$gkl$1@dont-email.me> |
Am 26.10.19 um 20:37 schrieb jacobnavia:
> Le 26/10/2019 à 20:16, Philipp Klaus Krause a écrit :
>> Am 26.10.19 um 20:15 schrieb Philipp Klaus Krause:
>>> bitfields are great tool to save data memory. You pay a price in
>>> execution speed and code memory, but often, it is worth it. In small
>>> embedded systems and in retrocomputing, you often have to work with
>>> hardware that has a 32 : 1 ratio of code memory to data memory.
>>>
>>> Philipp
>>>
>>
>> An dof course, what applies on small systems regarding the whole system,
>> also applies on big systems when it comes to performance-optimization by
>> making things fit into caches.
>>
>> Philipp
>>
>
> Hi Philipp
> If you read my message, I demonstrated that in a 64 bit system, and in
> many 32 bit systems, using less than the word size of bit fields doesn't
> lead to ANY space savings compared to unsigned char.
>
> struct foo {
> int field1:2;
> int field2:3;
> char *pData;
> };
>
> This structure has the SAME size than
>
> struct foo {
> unsigned char field1;
> unsigned char field2;
> char *pData;
> };
>
> This is the same for 32 and 64 bit systems. In your message you do not
> address this at all...
Which, however, is a corner case: Large alignment, few bit-field members
in the struct.
However, I have to admit that it becomes important on the big systems
where one wants to optimize for cache usage. Often, one will want to do
some reordering of the members, so the bit-field members are all next to
each other.
The really small embedded systems (and retrogaming) tend to have small
alignment, typically 1 byte. There one might want to reorder to avoid
having bit-fields unnecessarily cross byte boundaries (in which case the
compiler sometimes will sacrifice a few bits and align to byte boundaries).
Philipp
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-24 08:03 +0200
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-24 10:15 -0600
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-24 18:55 +0200
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-24 21:28 -0600
Re: Bit field usage scott@slp53.sl.home (Scott Lurndal) - 2019-10-24 17:22 +0000
Re: Bit field usage Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-24 19:45 +0200
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-24 19:59 +0200
Re: Bit field usage Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-24 20:02 +0200
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-24 21:29 -0600
Re: Bit field usage Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-25 07:51 +0200
Re: Bit field usage scott@slp53.sl.home (Scott Lurndal) - 2019-10-25 15:05 +0000
Re: Bit field usage Bart <bc@freeuk.com> - 2019-10-24 19:13 +0100
Re: Bit field usage Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-24 20:54 +0200
Re: Bit field usage Bart <bc@freeuk.com> - 2019-10-24 20:12 +0100
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-10-24 23:03 +0200
Re: Bit field usage Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-10-24 22:50 +0100
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-10-25 09:18 +0200
Re: Bit field usage Reinhardt Behm <rbehm@hushmail.com> - 2019-10-25 15:46 +0800
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-26 12:36 -0500
Re: Bit field usage scott@slp53.sl.home (Scott Lurndal) - 2019-10-25 15:04 +0000
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-26 12:40 -0500
Re: Bit field usage Philipp Klaus Krause <pkk@spth.de> - 2019-10-26 21:12 +0200
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-25 09:25 -0600
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-26 12:52 -0500
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-24 19:56 +0200
Re: Bit field usage already5chosen@yahoo.com - 2019-10-24 12:03 -0700
Re: Bit field usage scott@slp53.sl.home (Scott Lurndal) - 2019-10-24 19:40 +0000
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-24 22:15 +0200
Re: Bit field usage already5chosen@yahoo.com - 2019-10-24 14:16 -0700
Re: Bit field usage Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-24 17:50 -0700
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-25 09:36 -0600
Re: Bit field usage Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-10-27 04:54 -0700
Re: Bit field usage Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-10-27 10:05 -0600
Re: Bit field usage Philipp Klaus Krause <pkk@spth.de> - 2019-10-26 20:15 +0200
Re: Bit field usage Philipp Klaus Krause <pkk@spth.de> - 2019-10-26 20:16 +0200
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-26 20:37 +0200
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-26 13:41 -0500
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-26 21:54 +0200
Re: Bit field usage already5chosen@yahoo.com - 2019-10-27 03:46 -0700
Re: Bit field usage jacobnavia <jacob@jacob.remcomp.fr> - 2019-10-27 12:50 +0100
Re: Bit field usage already5chosen@yahoo.com - 2019-10-27 06:37 -0700
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-10-27 13:00 +0100
Re: Bit field usage already5chosen@yahoo.com - 2019-10-27 07:19 -0700
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-10-27 22:30 +0100
Re: Bit field usage Christopher Collins <ccollins476ad@gmail.com> - 2019-11-07 12:36 -0800
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-11-08 08:18 +0100
Re: Bit field usage Christopher Collins <ccollins476ad@gmail.com> - 2019-11-08 10:19 -0800
Re: Bit field usage Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-08 21:47 +0000
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-11-10 14:08 +0100
Re: Bit field usage Christopher Collins <ccollins476ad@gmail.com> - 2019-11-11 09:26 -0800
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-11-11 20:31 +0100
Re: Bit field usage Christopher Collins <ccollins476ad@gmail.com> - 2019-11-11 23:01 -0800
Re: Bit field usage Christopher Collins <ccollins476ad@gmail.com> - 2019-11-11 23:25 -0800
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-11-12 10:54 +0100
Re: Bit field usage Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-11-12 14:43 +0000
Re: Bit field usage James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-11-12 10:54 -0500
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-11-12 19:37 +0100
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-27 08:36 -0500
Re: Bit field usage already5chosen@yahoo.com - 2019-10-27 06:49 -0700
Re: Bit field usage Bonita Montero <Bonita.Montero@gmail.com> - 2019-10-27 14:23 +0100
Re: Bit field usage Robert Wessel <robertwessel2@yahoo.com> - 2019-10-27 08:42 -0500
Re: Bit field usage Philipp Klaus Krause <pkk@spth.de> - 2019-10-26 21:06 +0200
Re: Bit field usage Bart <bc@freeuk.com> - 2019-10-26 20:41 +0100
Re: Bit field usage Philipp Klaus Krause <pkk@spth.de> - 2019-10-27 08:29 +0100
Re: Bit field usage David Brown <david.brown@hesbynett.no> - 2019-10-27 13:05 +0100
csiph-web