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Groups > comp.lang.c > #145968
| From | Philipp Klaus Krause <pkk@spth.de> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: in place swap of float types |
| Date | 2019-09-04 11:18 +0200 |
| Organization | solani.org |
| Message-ID | <qknvgq$rgm$1@solani.org> (permalink) |
| References | <slrnqmaoja.4b2.3queens@cpe-65-25-200-198.new.res.rr.com> <qk5auh$iso$1@dont-email.me> |
Am 28.08.19 um 09:36 schrieb David Brown:
> I challenge you to show a sequence of code, for whatever target
> processor you like, where you think such an xor swap was ever a useful
> trick. Then we can see if it would be at all useful on any realistic
> cpu and compiler now.
Couldn't resist doing a quick experiment swapping two global floats,
using three versions of the float swap:
1) temporary variable
2) loop over bytes
3) Manually unrolled loop
I used current SDCC from svn and compiled using -mpdk13 --stack-auto for
the pdk13 target (the architecture supports a maximum of 1 KW ROM (each
word is 13 bits) and 64 B RAM) as reentrant code.
Code sizes:
1) 42 W
2) 131 W
3) 84 W
Stack space used for local variables:
1) 4 B
2) 4 B
3) 0 B
Still, probably not a very realistic or useful use case.
But let's look at swapping to global unsigned long long (which might be
a more realistic example for such a tiny µC than float). There the
temporary solution uses 75 W code space and 8 B of stack space. But the
xor solution uses only 48 W and 0 B of stack space. That one looks
useful to me.
Philipp
Code used:
float:
float f1, f2;
void f_tmp(void)
{
float tmp = f1;
f1 = f2;
f2 = tmp;
}
void f_xor(void)
{
for(unsigned char i = 0; i < sizeof(float); i++)
{
*((unsigned char*)(&f1) + i) ^= *((unsigned char*)(&f2) + i);
*((unsigned char*)(&f2) + i) ^= *((unsigned char*)(&f1) + i);
*((unsigned char*)(&f1) + i) ^= *((unsigned char*)(&f2) + i);
}
}
#define SWAPBYTE(f1, f2, i) \
{ \
*((unsigned char*)(&f1) + i) ^= *((unsigned char*)(&f2) + i); \
*((unsigned char*)(&f2) + i) ^= *((unsigned char*)(&f1) + i); \
*((unsigned char*)(&f1) + i) ^= *((unsigned char*)(&f2) + i); \
}
void f_xor_macro(void)
{
SWAPBYTE(f1, f2, 0);
SWAPBYTE(f1, f2, 1);
SWAPBYTE(f1, f2, 2);
SWAPBYTE(f1, f2, 3);
}
unsigned long long:
unsigned long long f1, f2;
void f_tmp(void)
{
unsigned long long tmp = f1;
f1 = f2;
f2 = tmp;
}
void f_xor(void)
{
f1 ^= f2;
f2 ^= f1;
f1 ^= f2;
}
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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
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