[Bug c/63645] Incorrect code generation

[joseph at codesourcery dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63645

--- Comment #24 from joseph at codesourcery dot com <joseph at codesourcery dot com> ---
On Mon, 27 Oct 2014, terra at gnome dot org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63645
> 
> --- Comment #22 from M Welinder <terra at gnome dot org> ---
> >Given
> >
> >  GnmExprBinary res;
> >  GnmExpr const *expr = (GnmExpr *)&res;
> >
> >the C standard does not define where the result of the conversion points; 
> 
> How do you read C99's Section 6.5 #7, then?

It only applies if you have validly obtained lvalues of different types 
referring to overlapping objects.

Sometimes pointer conversions will obtain those; sometimes they won't.  
The C model of pointer conversions is *not* "pointers point to a 
particular byte address, and conversions just change the type but not the 
byte address".  It's much more restricted that that.  Unless there are 
particular statements in the C standard that say what the resulting 
pointer points to (e.g. the statements about conversion from a pointer to 
a structure to a pointer to its first member, and back, *when you have a 
structure object in the first place*, or about conversions to pointers to 
character type), the result is not specified.

But if you have a union between float and int, take a pointer to the first 
member, and cast that pointer to type float *, there is no guarantee that 
the result is a pointer to the float member, absent a specific statement 
that defines the result of such a conversion (whereas if you cast 
indirectly, first to a pointer to the union then to float *, you would get 
a pointer to the float member).  And even when you have pointers to the 
two members of the union, you are still restricted in using them together 
(see DR#236) - that is the point at which 6.5#7 comes in.  But a simple 
cast from int * to float * doesn't even get that far - the value of the 
result is unspecified beyond what you get on converting it back to int *.