Re: c++/9881: What is an address constant expression?

[Nathan Sidwell <nathan at codesourcery dot com>]

Richard C Bilson wrote:

> Whether the new behavior is in error is a matter of debate it seems.
> I think that 5.19.4 requires the initializer in question to be an
> address constant expression and thus be a static initialization.
yup, I see you are correct, thanks for the reference.


> As far as I can tell, the code to handle &(((T*)0)->x) also works
> just fine in the case that the pointer is not null.  So I removed the
> restriction that the operand of the indirection be a NPC.
>
> This fixes our example code, solves the real problem that prompted us to
> complain in the first place, and doesn't seem to cause any trouble with
> the gcc test suite.  I can't say whether it would cause a recurrence
> of the problems you fixed in the first place, but I do have some SPARC
> machines at my disposal if you have an example of the problem for me
> to try.
I suspect it will break. you need g++.dg/other/packed1.C, I'm curious as to
why it didn't fail for you.

The real problem is gcc does represents a pointer to a misaligned
int as an 'int *' rather than something like 'int __attribute__((misaligned)) *'
which would help the dereferencing machinery out. The current mechanmism 'works'
only when the field decl is visible. The fold behaviour takes that out. Yuck!

nathan

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Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
         The voices in my head said this was stupid too
nathan at codesourcery dot com : http://www.cs.bris.ac.uk/~nathan/ : nathan at acm dot org