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Re: Adding UNION/MAP -- Feedback and tips pls!

From: "N.M. Maclaren" <nmm1 at cam dot ac dot uk>
To: gcc at gcc dot gnu dot org, "fortran at gcc dot gnu dot org" <fortran at gcc dot gnu dot org>
Date: 06 Mar 2013 19:15:00 +0000
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> <CAAKgPaEVESzYUh3FAuYx76=jtjBkaY5OwdztcNU=k4X+x5a8Ww@mail.gmail.com>

On Mar 6 2013, Russell Brennan wrote:

Ouch.
This seems to be at odds with C's unions, where it is not allowed to do
type punning.
As of gcc 4.4.6, the description above seems to match the C behavior:

Er, no. One simple test does not prove that it will always work; this sort of thing is most likely to fail because it interacts in very nasty ways with optimisation. C99 introduced a horribly ill-defined concept called "effective types", which specifically allows type-dependent optimisations. I have no idea whether gcc uses it at present, but it might well do so in the future.

Regards,
Nick Maclaren.

Follow-Ups:
- Re: Adding UNION/MAP -- Feedback and tips pls!
  - From: Russell Brennan

References:
- Re: Adding UNION/MAP -- Feedback and tips pls!
  - From: Thomas Koenig
- Re: Adding UNION/MAP -- Feedback and tips pls!
  - From: Russell Brennan

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