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| From | Willem <willem@turtle.stack.nl> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Casts on lvalues |
| Date | 2012-12-06 18:35 +0000 |
| Organization | Stack Usenet News Service |
| Message-ID | <slrnkc1pbr.rll.willem@turtle.stack.nl> (permalink) |
| References | <xDIus.482873$GX.378393@fx01.am4> <lny5hg8a7r.fsf@nuthaus.mib.org> <4BPus.800532$EF4.173275@fx23.am4> <slrnkbvj42.jen.willem@turtle.stack.nl> <k1Rvs.1060147$ti6.214777@fx20.am4> |
BartC wrote:
)
)
) "Willem" <willem@turtle.stack.nl> wrote in message
) news:slrnkbvj42.jen.willem@turtle.stack.nl...
)> BartC wrote:
)> ) The offsets come from externally generated data, and initially were
)> simple
)> ) counts: +2, -1 etc. The code looked like this:
)> )
)> ) R *p,*q;
)> )
)> ) p=q+n;
)> )
)> ) Very nice. Then I noticed this addition involved internally multiplying
)> n by
)> ) 16 (or a shift as it was), which wasn't so nice! It was just as easy to
)> ) generate these numbers as multiples of 16 anyway (+32, -16 etc) so I did
)> ) that. But then the code wasn't so pretty.
)>
)> That's quite odd. A good compiler should have done that optimization for
)> you, if it's at all possible. Also, on x86 CPU's, there are addressing
)> modes that implicitly multiply by factors of two, with no speed penalty.
)
) How can it be optimised? If the pointer has a 16-byte stride, and you're
) adding an offset N, then N*16 must be added to the pointer.
I was under the impression that you were generating the offsets somewhere.
If you can pre-multiply those by 16, so can the compiler, one would think.
Take, for example:
int step = some_function();
struct struct_16_bytes_long p = start;
while (p->marker != 0) {
frobnicate(p->data);
p += step;
}
In this case, the optimizer should pull the multiply-by-16 out of the loop,
shouldn't it?
) The x86's address scaling feature only goes up to *8. This *16 is
) implemented with a shift.
I wasn't aware the stride was 16 bytes.
SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT
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Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 13:23 +0000
Re: Casts on lvalues SG <sgesemann@gmail.invalid> - 2012-12-02 14:40 +0100
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-02 13:53 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-06 12:26 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-06 15:15 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 12:05 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-02 12:34 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-02 21:18 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-05 22:36 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:46 +0000
Re: Casts on lvalues Willem <willem@turtle.stack.nl> - 2012-12-06 18:35 +0000
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 16:21 -0500
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-02 17:10 -0500
Re: Casts on lvalues pete <pfiland@mindspring.com> - 2012-12-02 18:50 -0500
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-02 19:14 -0500
Re: Casts on lvalues falk@rahul.net (Edward A. Falk) - 2012-12-04 23:57 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 01:35 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-05 16:07 +0100
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 16:47 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 13:04 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 18:39 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-05 14:23 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:11 +0000
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-06 11:51 +1300
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 23:58 +0000
Re: Casts on lvalues Piotr Kalinowski <pitkali@gmail.com> - 2012-12-06 22:00 +0100
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-06 03:05 +0000
Re: Casts on lvalues Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-12-05 14:43 -0500
Re: Casts on lvalues Greg Martin <greg@softsprocket.com> - 2012-12-05 12:25 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 21:57 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:09 -0800
Re: Casts on lvalues Philipp Thomas <kthomas@linux01.gwdg.de> - 2012-12-07 01:24 +0100
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-06 17:22 -0800
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 00:31 -0800
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:18 -0500
Re: Casts on lvalues Öö Tiib <ootiib@hot.ee> - 2012-12-07 09:07 -0800
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-07 11:01 -0800
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 21:57 +0000
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-07 22:29 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-08 00:04 +0000
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-15 16:51 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-15 15:48 -0800
Re: Casts on lvalues lawrence.jones@siemens.com - 2012-12-17 16:37 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:58 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:53 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 02:43 -0800
Re: Casts on lvalues Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-12-05 22:07 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-05 22:20 +0000
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-05 16:10 -0800
Re: Casts on lvalues Philip Lantz <prl@canterey.us> - 2012-12-06 22:49 -0800
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 12:54 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 08:22 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 13:48 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 11:17 -0500
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 17:33 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:28 +0000
Re: Casts on lvalues glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2012-12-07 22:18 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 18:27 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 14:14 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 20:16 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-07 15:41 -0500
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:03 -0800
Re: Casts on lvalues Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-12-09 17:54 +0200
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 12:38 -0500
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-09 18:50 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-09 14:31 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-09 20:12 -0800
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 07:38 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 08:12 -0500
Re: Casts on lvalues Heikki Kallasjoki <fis+usenet@zem.fi> - 2012-12-10 17:46 +0000
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 12:59 -0500
Re: Casts on lvalues Keith Thompson <kst-u@mib.org> - 2012-12-10 12:07 -0800
Re: Casts on lvalues Tim Rentsch <txr@alumni.caltech.edu> - 2012-12-17 03:11 -0800
Re: Casts on lvalues Ian Collins <ian-news@hotmail.com> - 2012-12-10 20:50 +1300
Re: Casts on lvalues James Kuyper <jameskuyper@verizon.net> - 2012-12-10 07:49 -0500
Re: Casts on lvalues Rui Maciel <rui.maciel@gmail.com> - 2012-12-07 15:19 +0000
Re: Casts on lvalues "BartC" <bc@freeuk.com> - 2012-12-07 16:54 +0000
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