Re: [C++0x] contiguous bitfields race implementation

[Aldy Hernandez <aldyh at redhat dot com>]

> > > Sure.  I assume in this case, *bit_offset would be 0, right?
> >
> > It would be DECL_FIELD_BIT_OFFSET of that field.  Oh, and
> > *byte_offset would be
> >
> > *byte_offset = size_binop (MULT_EXPR, TREE_OPERAND (exp, 2),
> >                                        size_int (DECL_OFFSET_ALIGN
> > (field) / BITS_PER_UNIT));
> >
> > see expr.c:component_ref_field_offset () (which you conveniently
> > could use here).
> >
> > Note that both TREE_OPERAND (exp, 2) and compoment_ref_field_offset
> > return offsets relative to the immediate containing struct type, not
> > relative to the base object like get_inner_reference does ...
> > (where it is still unclear to me what we are supposed to return from this
> > function ...)

Ok, I see where your confusion lies.  The function is supposed to return 
a byte offset from the base object, none of this containing object or 
immediate struct, or whatever.  Base object, as in "a" in a.i.j.k, as in 
what you get back from get_base_address().

Originally everything was calculated with get_inner_reference(), which 
is relative to the base object, but now we have this hodge podge of 
get_inner_reference() calls with ad-hoc calculations and optimizations. 
 Gladly, we've agreed to use get_inner_reference() and optimize at a 
later time.

So... base object throughout, anything else is a mistake on my part.

BTW, this whole variable length offset I still can't trigger.  I know 
you want to cater to Ada, but does it even make sense to enable the C++ 
memory model in Ada?  Who would ever do this?  Be that as it may, I'll 
humor you and handle it.

> > Thus, conservative would be using get_inner_reference here, if the
> > offset is supposed to be relative to the base object.
>
> That said, shouldn't *maxbits not at least make sure to cover the field itself?

Is this what you want?

  /* Be as conservative as possible on variable offsets.  */
  if (TREE_OPERAND (exp, 2)
      && !host_integerp (TREE_OPERAND (exp, 2), 1))
    {
      get_inner_reference (build3 (COMPONENT_REF,
				   TREE_TYPE (exp),
				   TREE_OPERAND (exp, 0),
				   field, NULL_TREE),
			   &tbitsize, &start_bitpos, &start_offset,
			   &tmode, &tunsignedp, &tvolatilep, true);

      *byte_offset = start_offset ? start_offset : size_zero_node;
      *bit_offset = start_bitpos;
      *maxbits = tbitsize;
      return;
    }