Re: Sugestions for runtime polymorphism

From	David Brown <david.brown@hesbynett.no>
Newsgroups	comp.lang.c
Subject	Re: Sugestions for runtime polymorphism
Date	2016-09-25 12:43 +0200
Organization	A noiseless patient Spider
Message-ID	<ns89ok$lj2$1@dont-email.me> (permalink)
References	(17 earlier) <58c2a8d6-5609-458e-93f9-0811123a96ff@googlegroups.com> <ns62k1$i6j$1@dont-email.me> <87twd5fitb.fsf@bsb.me.uk> <ns67c3$25n$1@dont-email.me> <87h995f4lm.fsf@bsb.me.uk>

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On 24/09/16 22:03, Ben Bacarisse wrote:
> David Brown <david.brown@hesbynett.no> writes:
<snip>
>> But I could be wrong - and if this really is incorrect behaviour from
>> gcc, then it needs reported and fixed.
>
> And *I* could be wrong, but I've gone though all 9 object accesses and
> every one appears to be explicitly permitted by p6 and p7 of section
> 6.5.
>
> <snip>
>>> My gcc (5.2.1 20151010) gives 2 for all -Ox.  Presumably you have a
>>> newer version?
>>
>> I am testing here with <https://gcc.godbolt.org/> for convenience.  It
>> is targeting 64-bit Linux, and the result is 1 for -O2 and above, on
>> gcc 4.7 onwards.  gcc 4.5 and 4.6 give 2.
>
> Including 5.2?  My system is 64-bit Linux.  (I could not see how to get
> that site to show me the output.)
>
> <snip>
>

My test on gcc.godbolt.org is here:

<https://gcc.godbolt.org/#compilers:!((compiler:g520,options:'-O2+-Wall+-Wextra+-x+c',sourcez:FAAjGIEsDsGMBsCuATApiAPAZwC7JjgHQAWAfKGCFHEmprgE4wDmJ5lVMCK62e8kAEZsKELrV658AexGiQOAJ4AHVGgBmIeNOjMtOvQBUAjAG55S1RpAEAbABYA%2BjhCGATOY7gNMdCYBiALKGIABEAKTw8Mih8t6o6r6uxgBqAIIAMgCqAKIgxgAMBXE%2B0H5u6dl5bkXA8iYggvAAhsQAzAAUAG7SkMggAFTKxgA0ID19g8puAJTyAN7yYA0DOMbKY6vrHkuuboM4bsqeHMkKqAC2x3WnCusgALwgwycch8qPzzu3Wx9PJpVcqZKAB6EEgADqTBw6GaWGSu1WR0%2B7kBOWBYDBkOhsPh7l2MKunyRx1uWIASqhmv04XsCdtPh0TAMZu8MViUpAsEJ4OUFNIzspetAYQwQLAdF1UAxuTpEWtpp9CaSOFioZAYSBaSZdgxUDhEAxoAd1hjVeDKdStXjjPIAL43SgNGG4ToTZBzDiLW7uqafC7NKLSWAdbkAL1Q0k0TOMMxAAGoQOHI9H3DMZq8ncYQHqsIh4C4nk1Wp0NsoM7t1HrUB1y5mwHqDUac6g8wXMw6OAQQAGYN1eh6FrsGrn84Xzq6OhXbsomCL1B1QoZiOhRwWbPDQskgoYxmucNPTo3DcaCmbKPagAA%3D%3D)),filterAsm:(commentOnly:!t,directives:!t,labels:!t),version:3>

I changed the values of T1VALUE and T2VALUE to 100 and 200, to make it 
easier to spot them in the resulting assembly code.

The details of the standards here is getting beyond me.  When you and 
James both think the answer should be 1, and 2 is not allowed, then I am 
far from confident about 2 being allowable - you are both more better at 
"language lawyering" than me.  But I do know that gcc will do this sort 
of transformation in practice, and in real code - you need to be very 
careful in mixing pointers to different integer types even when you know 
they are the same underlying hardware types.

Maybe it's time to pass this on to the gcc mailing lists, and ask how 
they interpret it?

Thread

Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 12:01 -0700
  Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 13:15 -0700
  Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 13:27 -0700
    Re: Sugestions for runtime polymorphism Richard Heathfield <rjh@cpax.org.uk> - 2016-09-14 21:50 +0100
      Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 14:20 -0700
        Re: Sugestions for runtime polymorphism "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-14 17:32 -0400
          Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-14 16:30 -0700
            Re: Sugestions for runtime polymorphism "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-14 17:54 -0400
          Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 17:34 -0700
            Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 17:58 -0700
              Re: Sugestions for runtime polymorphism Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-14 19:50 -0700
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-15 05:03 -0700
              Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-15 10:43 +0100
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-15 05:29 -0700
                Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-15 19:49 +0100
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-15 11:58 -0700
                Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-15 20:29 +0100
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-15 14:00 -0700
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-16 07:27 -0700
                Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-16 16:34 +0100
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-16 09:36 -0700
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-20 12:59 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-20 14:08 -0700
                Re: Sugestions for runtime polymorphism Ian Collins <ian-news@hotmail.com> - 2016-09-21 15:53 +1200
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-21 07:38 -0700
                Re: Sugestions for runtime polymorphism Ian Collins <ian-news@hotmail.com> - 2016-09-23 09:12 +1200
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-21 05:26 -0700
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-21 05:35 -0700
                Re: Sugestions for runtime polymorphism Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-21 06:01 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 01:44 -0700
                Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-22 13:16 +0100
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-22 08:02 -0700
                Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-22 11:44 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-22 09:52 -0700
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-23 09:49 +0200
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-23 07:55 -0700
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-24 16:29 +0200
                Re: Sugestions for runtime polymorphism Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-24 15:56 +0100
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-24 17:50 +0200
                Re: Sugestions for runtime polymorphism "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-24 13:45 -0400
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-24 11:26 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 10:38 -0700
                Re: Sugestions for runtime polymorphism Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-24 21:03 +0100
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-25 12:43 +0200
                Re: Sugestions for runtime polymorphism "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-25 11:48 -0400
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-25 18:39 +0200
                Re: Sugestions for runtime polymorphism "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-25 13:14 -0400
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-25 13:47 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 10:39 -0700
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-25 23:25 +0200
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-25 13:10 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-25 12:55 -0700
                Re: Sugestions for runtime polymorphism Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-25 20:38 +0100
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-25 23:20 +0200
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 07:15 -0700
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-28 21:29 +0200
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-24 10:52 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-24 10:33 -0700
                Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-25 12:46 +0200
                Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-24 13:58 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-28 04:46 -0700
                Re: Sugestions for runtime polymorphism Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-24 00:07 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 08:52 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-23 09:56 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 12:00 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-22 08:45 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-22 09:36 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-23 12:06 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-23 15:29 -0700
                Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-29 07:56 -0700
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-30 10:54 -0700
                Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-30 11:18 -0700
                Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-30 19:26 +0100
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-30 12:41 -0700
                Re: Sugestions for runtime polymorphism Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-30 21:07 +0100
                Re: Sugestions for runtime polymorphism Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-09-30 23:30 -0700
                Re: Sugestions for runtime polymorphism Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-10-01 11:35 +0100
                Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-10-01 14:44 -0700
        Re: Sugestions for runtime polymorphism Richard Heathfield <rjh@cpax.org.uk> - 2016-09-14 22:56 +0100
          Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-15 13:19 +0200
            Re: Sugestions for runtime polymorphism James Kuyper <jameskuyper@verizon.net> - 2016-09-15 08:07 -0400
              Re: Sugestions for runtime polymorphism David Brown <david.brown@hesbynett.no> - 2016-09-15 14:24 +0200
                Re: Sugestions for runtime polymorphism Richard Heathfield <rjh@cpax.org.uk> - 2016-09-15 15:34 +0100
            Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-15 05:10 -0700
          Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-15 09:12 -0700
        Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-14 23:32 +0100
        Re: Sugestions for runtime polymorphism "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-09-14 17:35 -0700
        Re: Sugestions for runtime polymorphism Ian Collins <ian-news@hotmail.com> - 2016-09-15 16:24 +1200
          Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-15 10:31 -0700
    Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-14 22:53 +0100
      Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-14 16:56 -0700
        Re: Sugestions for runtime polymorphism supercat@casperkitty.com - 2016-09-14 15:58 -0700
          Re: Sugestions for runtime polymorphism Keith Thompson <kst-u@mib.org> - 2016-09-14 18:49 -0700
  Re: Sugestions for runtime polymorphism luser droog <luser.droog@gmail.com> - 2016-09-14 14:17 -0700
    Re: Sugestions for runtime polymorphism Thiago Adams <thiago.adams@gmail.com> - 2016-09-14 14:25 -0700
      Re: Sugestions for runtime polymorphism luser droog <luser.droog@gmail.com> - 2016-09-14 14:40 -0700
      Re: Sugestions for runtime polymorphism luser droog <luser.droog@gmail.com> - 2016-09-14 14:57 -0700
      Re: Sugestions for runtime polymorphism BartC <bc@freeuk.com> - 2016-09-14 23:10 +0100
  Re: Sugestions for runtime polymorphism Tim Rentsch <txr@alumni.caltech.edu> - 2016-09-15 11:23 -0700

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