Re: Casts on lvalues

From	glen herrmannsfeldt <gah@ugcs.caltech.edu>
Newsgroups	comp.lang.c
Subject	Re: Casts on lvalues
Date	2012-12-08 00:04 +0000
Organization	Aioe.org NNTP Server
Message-ID	<k9u06f$odh$1@speranza.aioe.org> (permalink)
References	(7 earlier) <lnboe6psew.fsf@nuthaus.mib.org> <7216db87-28bd-41a0-9fd9-6a0fe23158d5@googlegroups.com> <ln7gotptyq.fsf@nuthaus.mib.org> <k9tone$6ca$1@speranza.aioe.org> <0.282d4b9e5dee28d06a1d.20121207222942GMT.87d2ylo5rd.fsf@bsb.me.uk>

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Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:

(snip, I wrote)
>> Some time ago, I was trying to figure out if it would be
>> possible for a C compiler to generate JVM code.

>> JVM has no operation that stores different types in the same memory.
>> I believe it isn't so hard to make a memcpy() that can do the
>> appropriate conversions and copies, though.

>> It seemed to me at the time that implementing union the same as
>> struct was one way that would work, and would also follow the 
>> standard. (Though maybe less memory efficient.)

>> I asked here at the time, and many seemed to agree, though as you
>> say, it might break existing code.

> I don't think that would work out.  In particular, 6.5.8 p5 says "All
> pointers to members of the same union object compare equal".  Since you
> are talking about a compiler here, that alone is not the end of the
> matter because the compiler might be able to arrange for this to appear
> to be true.  For example, a pointer might be made to consist of two
> parts, one that is used for comparison while the other part is an offset
> used for union access, 

Well, most pointers have to have an Object reference to an array 
and an offset into the array. Any scalar that can be pointed to 
has to instead be an array of length 1.

> but I fear that this won't fly in the long run.
> (For example, this particular ruse will go wrong implementing the
> special provision for "common initial sequences" in 6.5.2.3 p5.  Again,
> a fix-up is possible but I am sceptical about far this can go on.)

As well as I know, all that fun stuff only has to happen for
(unsigned char *), or (void *), so the compiler only has to do that
extra stuff when those occur.

But yes, getting that one right will be tricky.

> It would be an interesting exercise to see if would work, but I'm not
> hopeful.

It already gets tricky for struct. If you make a struct a Java class,
then you can have an object reference to the (instance of the) class, 
which is different from a reference to a class member. So, extra
work to get that right.

-- glen

Thread

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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