Re: Aliasing brokenness (Was: Re: [patch RFC] SH: Use FRAME_GROWS_DOWNWARD)

[Ian Lance Taylor <ian at airs dot com>]

Mark Mitchell <mark@codesourcery.com> writes:

> >>>That's the rule we've tried to use in GCC, as well; that if "p" and
> >>>"q" pointer to distinct types (not part of the same alias set) then
> >>>modifying "*p" cannot change the value of "*q", unless both accesses
> >>>are directly through a union.  I don't know that the front ends and
> >>>optimizers are careful about preserving the user's access through
> >>>unions, but they could in theory be appropriately careful.
> >
> > That is, I think 6.5.2.3 is an explicit statement of a restricted form
> > of the rule you describe in the first paragraph.  It does not go
> > beyond what you described already.
> 
> How do you read the "visibility" language?  The idea that the
> visibility of the union is all that is required seemed to imply a
> stronger statement to me.

I'm sorry, you're right.  I thought I understood, but I did not.  The
effect is that if you have any pointer to a struct which appears in a
union, you may have a pointer to the initial parts of any other struct
in the union.  Which is what you said.  Sorry for the noise.

Ian