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| From | Tim Rentsch <txr@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Casts on lvalues |
| Date | 2012-12-17 03:03 -0800 |
| Organization | A noiseless patient Spider |
| Message-ID | <kfnpq29hrff.fsf@x-alumni2.alumni.caltech.edu> (permalink) |
| References | (13 earlier) <50C216B3.60103@verizon.net> <3zqws.788609$vW7.180152@fx19.am4> <50C24019.1020505@verizon.net> <Z8sws.791951$A%.338357@fx26.am4> <50C2547E.30609@verizon.net> |
James Kuyper <jameskuyper@verizon.net> writes:
> On 12/07/2012 03:16 PM, BartC wrote:
>> "James Kuyper" <jameskuyper@verizon.net> wrote in message
>> news:50C24019.1020505@verizon.net...
> ...
>>> I'm not talking about all optimizations, just the ones that depend upon
>>> C's anti-aliasing guarantees. Consider the following function:
>>>
>>> void func(int n, float array[static n], long *l)
>>> {
>>> for(int i=0; i<n; i++)
>>> array[i] = *l;
>>> *l = n;
>>> }
>>
>>> An implementation is permitted, because of 6.5p7, to compile that
>>> function as if it had been written:
>>
>>> void func(int n, float array[static n], long *l)
>>> {
>>> long temp = *l;
>>> for(int i=0; i<n; i++)
>>> array[i] = temp;
>>> *l = n;
>>> }
>>>
>>> The optimized version has exactly the same behavior as the original, so
>>> long as 6.5p7 is not violated. If C were changed to allow a long integer
>>> to be equivalenced with a element of a float array, the value of *l
>>> might change during the loop.
>>
>> (Seems like it might just be copied to itself? In this example).
>
> I think you're assuming that sizeof(long)==sizeof(float), which is
> commonplace, but not required. However, even on systems where that is
> true, the result is not a simple copy. The conversion to float will
> produce a bit pattern that's pretty unlikely to be the same as the
> original (unless the original represents 0).
>
>> >If so, these two versions of the code
>>> would no longer be equivalent, and there's an important question to be
>>> asked about which one actually matches the user's intent. If the initial
>>> value of *l was carefully chosen with the intent that it would, in fact,
>>> be changed by this code inside that loop, the developer is going to be a
>>> little upset if that didn't actually happen.
>>
>> I can't see that the problems are going to be that different from the ones
>> you might get when using pointers. (And the solutions might be similar.)
>
> In C, such code using pointer casts violates 6.5p7, and therefore has
> undefined behavior, so the solution is simply not to write such code.
>
>> Because even without equivalencing, 'l' might anyway point into the array,
>> via a cast.
>
> Not with defined behavior.
>
>> If this was part of the language, at least the aliasing would be done in an
>> overt manner, not hidden away in a pointer cast, in an operation done at
>> runtime so you don't know at any time what is aliasing what.
>
> As I said, the C language does provide a way to explicitly alias objects
> of two different types, and the optimizations I've described above are
> therefore prohibited when they occur within the scope of a union
> declaration that connects the relevant types [snip unimportant exception].
I think you'd get some disagreement on this last point. It
is not enough that the types in question just co-reside in a
visible union type. If that were true there would be no reason
for the "special rule" about struct's and accessing their
common prefix.
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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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