On Wed, Aug 31, 2011 at 6:53 PM, Aldy Hernandez <al...@redhat.com> wrote:
>
>>>> 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;
> }
Yes, exactly.
Richard.
