[Dwarf-Discuss] Some DWARFv5 draft feedback
arnez at linux.vnet.ibm.com
Thu Dec 1 06:17:16 PST 2016
On Thu, Dec 01 2016, Mark Wielaard wrote:
> BTW. It would be handy if there were sources for the spec so one can
> create patches for simple typos. Also it is somewhat opaque how Issues
> are handled. Could they and any comments from the committee be sent to
> the mailinglist to make tracking changes to the draft easier.
While we're at it, DWARF5 should improve the description of DW_OP_piece
and DW_OP_bit_piece. AFAIK, their handling is fairly broken in all
existing DWARF producers and consumers (certainly in GDB -- in multiple
ways!), so even incompatible changes may not cause much harm. See my
previous mails on this topic:
* DW_OP_bit_piece: [...] "If the location is a register, the offset is
from the least significant bit end of the register."
Is it intentional that this differs from the definition of
DW_OP_piece, where the "placement of the piece within that register is
defined by the ABI"? Or can it be assumed (like all current
producers/consumers do, AFAIK) that DW_OP_piece shall behave as if it
was a DW_OP_bit_piece with offset 0? What does the least significant
bit end even mean, say, for a vector register? And is this really a
useful definition for FP registers, where the natural alignment is
from the *most* significant bit end?
* DW_OP_piece: Some existing producers may emit DW_OP_piece operations
that exceed the size of a single register, supposedly referring to
multiple ("consecutive") registers.
This usage is not covered by the current description of DW_OP_piece.
Should it be?
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