On 01/27/2017 05:11 PM, Cary Coutant wrote:
I still haven't understood *why* DWARF insists on trying to establish a
universal register bit numbering scheme, and just for the definition of
DW_OP_bit_piece? I don't know of any other normative source that tries
this; and DWARF usually avoids going into such low-level detail, leaving
it to the ABI instead. The fact that it does in this case also breaks
the link to DW_OP_piece, where the placement *can* be freely defined by
the ABI.
With the exception I mentioned above, DWARF doesn't mention bit
numbering. DWARF makes no mention of bit numbering with regard to
registers, and clearly doesn't establish a universal register bit
numbering scheme.
It does at least implicitly, when defining the placement rule for
DW_OP_bit_piece. This definition implies a "universal" bit numbering
scheme that starts with 0 at a register's "least significant bit".
This is unfortunate, and we shouldn't be referring to an ABI-defined
bit numbering scheme here.
Agreed.
I think the original intent of this wording was to describe the bit
offset of a field within a structure, given the byte address of the
beginning of the structure. Said bit offset would be the total number
of bits occupied by preceding fields plus internal padding, as
demonstrated in the big- and little-endian examples in D.2.8. We
should never have implied that this bit numbering was meant to match
the bit numbering convention used in the ABI specification.
Unfortunately, that's exactly what DWARF says.
This is not the only place we use this bit numbering convention -- we
also use it for the DW_AT_data_bit_offset attribute, and it's defined
in Section 5.7, with a reference to the examples in D.2.8.
Also, not what we want.
I think it's also unfortunate that we allowed the offset field for
DW_OP_bit_piece to apply to register values. That's a very different
use for the value, and just adds to the confusion.
It's the same use when a register contains multiple fields of a struct.
--
Michael Eager ea...@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306 650-325-8077
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