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Re: Questions about C as used/implemented in practice
- From: Peter Sewell <Peter dot Sewell at cl dot cam dot ac dot uk>
- To: Paul_Koning at dell dot com
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Fri, 17 Apr 2015 16:01:59 +0100
- Subject: Re: Questions about C as used/implemented in practice
- Authentication-results: sourceware.org; auth=none
- References: <CAHWkzRS5Pfedgre-b93g1yfRT4CeMkEkm+qzok9DFwYGVVZSeg at mail dot gmail dot com> <43F29211-D9EA-48D5-BDED-38C2A7187202 at dell dot com>
- Reply-to: Peter dot Sewell at cl dot cam dot ac dot uk
On 17 April 2015 at 15:19, <Paul_Koning@dell.com> wrote:
>> On Apr 17, 2015, at 9:14 AM, Peter Sewell <Peter.Sewell@cl.cam.ac.uk> wrote:
>> Dear gcc list,
>> we are trying to clarify what behaviour of C implementations is
>> actually relied upon in modern practice, and what behaviour is
>> guaranteed by current mainstream implementations (these are quite
>> different from the ISO standards, and may differ in different
> Iâm not sure what you mean by âguaranteedâ.
> I suspect what the GCC team will say is guaranteed is âwhat the standard saysâ.
If that's really true, that will be interesting, but there may be
areas where (a) current implementation behaviour is stronger than what
the ISO standards require, and (b) important code relies on that
behaviour to such an extent that it becomes pragmatically infeasible
to change it. Such cases are part of what we're trying to discover
here. There are also cases where the ISO standards are unclear or
> If by âguaranteedâ you mean the behavior that happens to be implemented in a particular version of the compiler, that may well be different, as you said. But itâs also not particularly meaningful, because it is subject to change at any time subject to the constraints of the standard, and is likely to be different among different versions, and for that matter among different target architectures and of course optimization settings.
Some amount of variation has to be allowed, of course - in fact, what
we'd like to clarify is really the envelope of allowable variation,
and that will have to be parametric on at least some optimisation