[Dwarf-discuss] Interpretation of FBREG: implied deref of frame_base?

[Ron 603-884-2088]

There seems to be a general feeling that DW_OP_reg<n> should be
a valid frame base location that "computes" a frame base address
by taking the contents of the register <n>.

I am trying figure out how best to express that in the DWARF
document.

I don't think anything belongs in the description of the
DW_OP_reg<n> operators themselves. This implicit dereference does
not, after all, apply to a location expression for a variable
that is allocated in a register.

It appears that the DW_OP_fbreg operator is the only DWARF
construct whose definition uses the frame base value, so it is
perhaps possible to incorporate the implicit dereference there.
However, other parts of a debugger probably also could or do make
use of the frame base address, so it seems better to incorporate
the explanation in the frame base attribute description proper.

The current description of DW_AT_frame_base (Section 3.3.5, page
46 of V3) reads:

    A subroutine or entry point may also have a DW_AT_frame_base
    attribute, whose value is a location description that
    computes the "frame base" for the subroutine or entry point.

I propose the following replacement:

    A subroutine or entry point may also have a DW_AT_frame_base
    attribute, whose value is a location description. Evaluation
    of that location description computes the "frame base"
    address, which can serve as a base pointer for accessing
    locations in the frame of that subroutine or entry. In the
    special case that the location expression is one of the
    register name operators (see Section 2.5.1), the given
    register is implicitly dereferenced to obtain the contents
    of the register.

    <i>For example, for the DW_AT_frame_base attribute (and
    only that attribute) the location expression DW_OP_reg7
    is equivalent to either of the expressions
    <b>DW_OP_reg7 DW_OP_deref</b> or <b>DW_OP_breg7 0</b></i>.

    <i>This special case interpretation confirms established
    practice and provides a useful interpretation to an
    otherwise undefined construction.</i>

Thoughts?

Ron