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Re: RFA and RFC: tweak -fstrict-aliasing docs, provide pointer-cast example
- From: Andrew Haley <aph at redhat dot com>
- To: Hans-Peter Nilsson <hans-peter dot nilsson at axis dot com>
- Cc: gcc-patches at gcc dot gnu dot org, gcc at gcc dot gnu dot org
- Date: Wed, 25 Jun 2008 15:04:40 +0100
- Subject: Re: RFA and RFC: tweak -fstrict-aliasing docs, provide pointer-cast example
- References: <200806241019.m5OAJb9j015429@ignucius.se.axis.com>
Hans-Peter Nilsson wrote:
>> Date: Tue, 24 Jun 2008 10:36:15 +0100
>> From: Andrew Haley <aph@redhat.com>
>
>> I thought cast-through-pointer-to-union didn't work and was already
>> disallowed; we've been around all this already.
>
> We also bless assignments through unions, and this could be
> argued as assigning through a union, albeit casted.
>
>> This patch of yours
>> already documents uncontroversial behaviour.
>
> That's what I hope, but the existence of that code together in
> an *else* clause of #ifdef YES_ALIAS by a well-known author
> makes it de-facto controversial IMHO. Note also that another
> maintainer thought the code to be valid; see the PR.
So I see. I'm pretty sure that the compiler's alias analysis won't
think it's valid, but I haven't checked.
Do we actually document anywhere that a C++-style type pun along the lines
of
reinterpret_cast<T&>(x)
will not work? I'm guessing it probably won't, and that the union trick
is the only thing we do support.
Andrew.