Re: [Tinycc-devel] [PATCH v3] forbid invalid comparison of struct

[Hitoshi Mitake]

On Tue, Nov 6, 2012 at 5:28 AM, Rob <address@hidden> wrote:
> On Mon, Nov 05, 2012 at 11:19:00PM +0900, Hitoshi Mitake wrote:
>> On Mon, Nov 5, 2012 at 11:04 PM, Thomas Preud'homme <address@hidden> wrote:
>> > Le lundi 5 novembre 2012 14:53:16, Hitoshi Mitake a écrit :
>> >> Of course, comparison between different typed structs or structs and
>> >> scalar typed values should cause compile error. But I feel that if we
>> >> can write comparison between same struct typed values with ==
>> >> operator, a compiler will be very useful. Because we can avoid the
>> >> possibility of passing wrong value as 3rd parameter of memcmp().
>> >
>> > memcmp is not what you want because if a structure contains padding you are
>> > not sure of its content. Therefore two structures could be identical with
>> > different content inside the padding. You must compare field by field. 
>> > That's why
>> > assignement is possible but not equality: assignment is ok because you can
>> > copy the padding so memcpy will give a valid copy of the structure. 
>> > Comparing
>> > with memcpy on the other hand will return false for some equal structures.
>>
>> Ah, I missed about the padding... As you say, memcmp() is not a stuff for the
>> above situation.
>
> I had never thought of that, interesting edge case.
>
>
>> >> The standard of C doesn't allow this behaviour. But I think it may be
>> >> worth implementing it on TCC. Can TCC accept this behaviour as
>> >> implementation specific dialect?
>> >
>> > From http://bellard.org/tcc/ :
>> >
>> > UNLIMITED! Any C dynamic library can be used directly. TCC is heading 
>> > torward
>> > full ISOC99 compliance. TCC can of course compile itself.
>> >
>> > I don't think we should not follow C99 on this.
>> >
>>
>> OK, I understand the direction of TCC.
>> I'll implement == for field by field comparison only for my toy if I'll do :)
>>
>> Thanks,
>
> Regardless, I'd be interested in seeing your branch of tcc if you have it
> hosted anywhere?
>

Thanks for your interest :)
I'm hosting my tcc here, and I'll push the change to this branch:
https://github.com/mitake/tinycc/tree/struct-cmp
I've not implemented the feature yet, but my work in progress will appear here.

Thanks,

--
Hitoshi Mitake
address@hidden