Re: C++ PATCH to refer to base members using COMPONENT_REF

[Mark Mitchell <mark at codesourcery dot com>]

Jason Merrill wrote:

> On Mon, 03 May 2004 13:23:48 -0700, Mark Mitchell <mark@codesourcery.com> wrote:
>
>
> > Jason Merrill wrote:
> >
> >
> > > 	(layout_class_type): Change base fields to their real type
> > > 	after layout is done.
> >
> > This particular part of your change seems dangerous to me.
>
>
> I wondered about it myself, but concluded that it's safe.
>
>
> > It might tell the alias analysis code that assignment to the base field
> > might alias any of the types of the fields in the complete type when really
> > it can only alias things in the base type.
>
>
> I don't think so.  The base type does not alias other members of the
> derived type.

I don't understand.  The base field, after your change, will have the 
type of the derived class.  So, an assignment to that entire field 
(e.g., x.__base = y.__base) would tell the type-based alias analysis 
that any object have the type of any field *in the derived type* could 
be read/written.  Or, did you do something to prevent that from happening?

> > Also, it makes it hard for us to use the base fields in other places
> > (like assignments generated by copy constructors) where it might be
> > advantageous to the optimizers to see what was really going on.  (We
> > can't use them any more because they'll have types that don't match
> > reality.  Also, DECL_FIELD_SIZE and TYPE_SIZE will probably not match any
> > more.)
>
>
> But we only use the tail padding for non-PODs, which we have to deal with
> on a member-by-member basis anyway.

That's only true until we start recognizing that some constructors are 
trivial.  Then, the front end will start treating some non-PODs as if 
they were PODs for the purpose of copying.

> > I think you should instead leave the base field types as they are, but use
> > FIELD_DECLs from the base type instead of the complete type.
>
>
> The offsets would be wrong for non-primary bases unless we did something
> like make copies of the FIELD_DECLs for use in derived types.

I'm not sure why that would be.  But, I believe you -- I'm probably just 
 not thinking about it right.

Fundamentally, my objection to this patch is that it's causing us to lie 
to the compiler.  From the point of view of the language, these fields 
don't exist, so that's not an issue.  From the point of view of the 
compiler, however, these fields do not have the same type as the derived 
type, or the same size.  The relationship is that the base fields are an 
 initial prefix of the derived class.  After your change, we've busted 
the type system -- there are type-safe assignments that are not actually 
safe, for example.  Historically, whenever we've lied like this, we've 
gotten into trouble.

--
Mark Mitchell
CodeSourcery, LLC
mark@codesourcery.com