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Re: Adding UNION/MAP -- Feedback and tips pls!
- From: Ian Lance Taylor <iant at google dot com>
- To: "N.M. Maclaren" <nmm1 at cam dot ac dot uk>
- Cc: gcc at gcc dot gnu dot org, "fortran at gcc dot gnu dot org" <fortran at gcc dot gnu dot org>
- Date: Wed, 6 Mar 2013 12:36:40 -0800
- Subject: Re: Adding UNION/MAP -- Feedback and tips pls!
- References: <51314549.6080902@earthlink.net> <Prayer.1.3.5.1303020815470.18790@hermes-1.csi.cam.ac.uk> <CAGkQGiL7Qg0RAqJrT5i4LDzjnsr3jQ42Ws2BQFDpPUHyQe3+WQ@mail.gmail.com> <CAKwh3qgyS0nRcGSE5UT+DN+11kjYRE9Vu7SsQ_AGPtuD7VV4dA@mail.gmail.com> <Prayer.1.3.5.1303021519480.2329@hermes-1.csi.cam.ac.uk> <CAAKgPaHZwaoSBVg-hNZxan-uAtnmEBjPxyv9iuh=xdOshFp1uQ@mail.gmail.com> <Prayer.1.3.5.1303041813340.19357@hermes-1.csi.cam.ac.uk> <CAAKgPaGtNQoA=2h+5Z8DmvgsEBKUKX78Ojwi_SY2RG8hUcksOg@mail.gmail.com> <Prayer.1.3.5.1303042034040.18634@hermes-1.csi.cam.ac.uk> <51378C6D.5060609@netcologne.de> <CA+=Sn1kGyBabx+JgrPFJQ6GKU+PbfagygG16=oAhC6qyW1b7Rg@mail.gmail.com> <Prayer.1.3.5.1303061946580.24498@hermes-1.csi.cam.ac.uk>
On Wed, Mar 6, 2013 at 11:46 AM, N.M. Maclaren <nmm1@cam.ac.uk> wrote:
> On Mar 6 2013, Andrew Pinski wrote:
>>
>>
>> Except GCC implements C's unions as allowing to do type punning as an
>> extension and as far as GCC is concerned that is not going to change
>> any time soon.
>> This is a documented exception to the aliasing/type punning rules.
>
>
> The problem is that this is worse than type punning, because it also
> allows the preservation of data. Does the extension guarantee that,
> or would it need a change to at least the documentation?
The documentation is not clear on whether data is preserved. I
believe that as a matter of fact the data is preserved, but I don't
think we need to guarantee it.
Ian