Re: Bit field usage

Path	csiph.com!eternal-september.org!feeder.eternal-september.org!reader01.eternal-september.org!.POSTED!not-for-mail
From	Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups	comp.lang.c
Subject	Re: Bit field usage
Date	Thu, 24 Oct 2019 17:50:51 -0700
Organization	A noiseless patient Spider
Lines	25
Message-ID	<86woctfy1g.fsf@linuxsc.com> (permalink)
References	<qorerp$af6$1@dont-email.me> <1bo8y6azlw.fsf@pfeifferfamily.net>
Mime-Version	1.0
Content-Type	text/plain; charset=us-ascii
Injection-Info	reader02.eternal-september.org; posting-host="294289d8fb0afb0e43f75d31b2b6cc13"; logging-data="22943"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX19DRgRgEx6sxDrEs5oUIDw23XXIy73BcF4="
User-Agent	Gnus/5.11 (Gnus v5.11) Emacs/22.4 (gnu/linux)
Cancel-Lock	sha1:dEm9yQWdeSBlIPnLKl8gJ6T7ehk= sha1:anIpLWkIFMd3hiPIm9zxRqt4vtU=
Xref	csiph.com comp.lang.c:147220

Show key headers only | View raw

Joe Pfeiffer <pfeiffer@cs.nmsu.edu> writes:

> jacobnavia <jacob@jacob.remcomp.fr> writes:
>
>> 1) To access a bitfield, the compiler generates a sequence of shifts
>> that increases execution time.
>>
>> 2) The instructions sequence of the shifts increases code size.
>>
>> Obviously, if you have more than 64 flags (in a 64 bit machine) or
>> more than 32 flags in a 32 bit machine, bitfields can save a lot of
>> data space:  you can fit up to 128 bitfields in 16 bytes, instead of
>> 128 bytes if you use a full char for each bit field.
>>
>> It is important to check two things:
>> 1) If a bit field is heavily used, consider transforming it into an
>> unsigned char.
>> 2) If you have a few bit fields with bitsize less than the word size
>> of your machine, consider if the alignment requirements are nullifying
>> any space gains obtained by using bit fields.
>
> I got burned badly enough by bitfields doing unexpected things decades
> ago that I abandoned them and do my own shifts and masks.

Out of curiosity, what kind of problems did you run into?

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