Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
Groups > comp.lang.c > #145818
| From | James Kuyper <jameskuyper@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: in place swap of float types |
| Date | 2019-08-27 20:48 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <qk4j0p$bpg$1@dont-email.me> (permalink) |
| References | <slrnqmaoja.4b2.3queens@cpe-65-25-200-198.new.res.rr.com> <9ddbme52b398c5lrr1c653lvdo2oe7qbv5@4ax.com> |
On 8/27/19 6:59 PM, Robert Wessel wrote: > On Tue, 27 Aug 2019 17:03:38 -0000 (UTC), 3queens@shabbyroad.com > wrote: > >> Some time ago, there was a thread concerning using "a^=b; b^=a; a^=b" >> to do an in place swap of two variables of the same size. It was >> pointed out that this can only be done with integer types. I found a >> way to do it with floating point types as well. >> >> Declare a union with the floating point variable and an equally long >> integer variable. Then use the integer member to do the exclusive-or >> process. >> >> Naturally, this would not be efficient if the data had to be copied into >> and out of the union, but if the variables were defined as unions to >> begin with, this would work. >> >> Maybe not that useful, but if space is really tight, perhaps useful in >> some situations. > > > AFAIK, this invokes undefined behavior. You cannot reliably read from > a different member of a union than was written to. > The behavior of such code was never undefined. In C89, it was implementation-defined. In C99, a footnote was added saying: "If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’). This might be a trap representation." Footnotes are non-normative, and are intended to merely summarize and make clear the meaning of normative text. I can't find any normative text that clearly specifies that such reinterpretation takes place - the committee apparently feels otherwise, and some people on this group have claimed that it's an obvious consequence of the referenced section, 6.2.6. Whether or not there's actual normative text backing up that footnote, it definitely reflects the intent of the committee, as well as describing the way that virtually every implementation of C has ever actually handled unions, and the way that a very large fraction of C programmers have always expected C unions to behave. Therefore, I think it's quite safe to assume that this footnote is correct, even if there is in fact no normative text that clearly supports it. The representation of types is almost entirely implementation-defined - in most cases, so code that relies on this footnote is inherently non-portable. But if you read the relevant sections of an implementation's documentation, you should be able to figure out precisely which values of one type have representations that, if reinterpreted as a different type, are also valid representations of that type; you should also be able to determine which values they represent in the new type. However, the ^= trick should work in a union with an integer type of the required size on most implementations. It won't work if the integer type has padding bits. The standard allows each signed type to have one bit pattern that is a trap representation (6.2.6.2p2), which would also cause problems for such code. The unsigned exact-width integer types (7.20.1.1) are guaranteed to have neither of those features, and that is also likely to be the case for almost all integer types on almost all implementations. But the concepts of "padding bits" and trap representations weren't invented just for the fun of it - there are real implementations that have those features.
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
in place swap of float types 3queens@shabbyroad.com - 2019-08-27 17:03 +0000
Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-08-27 19:22 +0000
Re: in place swap of float types Andrew Smallshaw <andrews@sdf.org> - 2019-09-02 15:25 +0000
Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-09-02 19:37 +0000
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-03 06:37 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-03 16:13 +0200
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-03 16:54 +0100
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 11:19 +0200
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-04 02:35 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 12:31 +0200
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 12:12 +0100
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 15:53 +0200
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-04 07:17 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:12 +0200
Re: in place swap of float types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-09-05 21:11 +0100
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 16:30 +0100
Re: in place swap of float types jadill33@gmail.com - 2019-09-04 10:26 -0700
Re: in place swap of float types Ike Naar <ike@iceland.freeshell.org> - 2019-09-05 06:00 +0000
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-05 11:14 +0100
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 14:54 +0200
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-05 15:48 +0100
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:35 +0200
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-06 02:52 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 13:18 +0200
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-06 12:14 +0100
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 15:08 +0200
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-06 06:23 -0700
Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-09-04 14:16 -0700
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-09-04 22:42 +0100
Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-09-04 15:21 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:15 +0200
Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-04 08:05 -0600
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:17 +0200
Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-05 20:28 -0600
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:39 +0200
Re: in place swap of float types Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2019-09-06 08:11 -0600
Re: in place swap of float types scott@slp53.sl.home (Scott Lurndal) - 2019-09-06 18:31 +0000
Re: in place swap of float types Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-09-04 19:02 +0000
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-02 22:16 +0200
Re: in place swap of float types Robert Wessel <robertwessel2@yahoo.com> - 2019-08-27 17:59 -0500
Re: in place swap of float types Richard Damon <Richard@Damon-Family.org> - 2019-08-27 17:45 -0700
Re: in place swap of float types James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-08-27 20:48 -0400
Re: in place swap of float types Tim Rentsch <tr.17687@z991.linuxsc.com> - 2019-09-03 06:17 -0700
Re: in place swap of float types Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-08-28 02:09 +0100
Re: in place swap of float types Siri Cruise <chine.bleu@yahoo.com> - 2019-08-27 21:26 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 09:22 +0200
Re: in place swap of float types Andrey Tarasevich <andreytarasevich@hotmail.com> - 2019-08-28 16:36 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 09:36 +0200
Re: in place swap of float types Bart <bc@freeuk.com> - 2019-08-28 12:02 +0100
Re: in place swap of float types James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-08-28 05:08 -0700
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-08-28 05:26 -0700
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-08-28 14:40 +0200
Re: in place swap of float types Keith Thompson <kst-u@mib.org> - 2019-08-28 13:01 -0700
Re: in place swap of float types Philipp Klaus Krause <pkk@spth.de> - 2019-09-04 11:18 +0200
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-04 12:57 +0200
Re: in place swap of float types Philipp Klaus Krause <pkk@spth.de> - 2019-09-04 13:24 +0200
Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 08:18 +0200
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-05 15:19 +0200
Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 16:18 +0200
Re: in place swap of float types Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2019-09-05 07:32 -0700
Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-05 17:32 +0200
Re: in place swap of float types David Brown <david.brown@hesbynett.no> - 2019-09-06 10:54 +0200
Re: in place swap of float types Bonita Montero <Bonita.Montero@gmail.com> - 2019-09-06 14:23 +0200
csiph-web