Re: subreg question

[Aldy Hernandez <aldyh at redhat dot com>]

On Thu, 2001-12-13 at 13:04, Dale Johannesen wrote:
> 
> On Thursday, December 13, 2001, at 12:43 PM, Daniel Jacobowitz wrote:
> >>>>
> >>>> I don't see why not.  (C89) 3.3.4 says the cast might produce an 
> >>>> invalid
> >>>> pointer if it's not aligned properly, which is not the case here.
> >>>> Other than that I don't find any relevant restriction in the standard.
> >>>> What did you have in mind?
> >>>> (FWIW, gcc doesn't warn about this even with -pedantic.)
> >>>
> >>> It's not an invalid pointer.  It doesn't alias with Z, though.
> >>
> >> Why not?
> >
> > The C standard's aliasing rules.  *((float *)&z) is an object of type
> > float; double is a different type than float; z is an object of type
> > double; *((float *)&z) and z can not alias.
> 
> Can you point me to someplace in the standard that says this?

i am NOT a language lawyer, as most of my patches can show :))

but i think this is it:


    7 An object shall have its stored value accessed only by an lvalue
    expression that has one of the following types:73) [The intent of
    this list is to specify those circumstances in which an object may
    or may not be aliased.]   a type compatible with the effective type
    of the object,   a qualified version of a type compatible with the
    effective type of the object,   a type that is the signed or
    unsigned type corresponding to the effective type of the object,   a
    type that is the signed or unsigned type corresponding to a
    qualified version of the effective type of the object,   an
    aggregate or union type that includes one of the aforementioned
    types among its members (including, recursively, a member of a
    subaggregate or contained union), or   a character type.