RE: [RFC] DW_OP_piece vs. DW_OP_bit_piece on a Register

Matthew Fortune
2016-01-25T22:01:53Z

Andreas Arnez <arnez@linux.vnet.ibm.com> writes:
> 6 Summary of Open Questions
> ===========================
>
> 1. Out of the standard interpretations discussed under "options"
> (section 4) above, which do we want to settle on? Or is the
> "preferred" interpretation missing from that list?
> 2. Should pieces fully or partially outside their underlying objects
> be considered valid or invalid? If valid, how should they be
> aligned and padded? In any case, what is the suggested treatment
> by a DWARF consumer?

My dwarf knowledge is not brilliant but I have had to recently consider
it for MIPS floating point ABI changes aka FPXX and friends. I don't know
exactly where this fits in to your whole description but in case it has
a bearing on this we now have the following uses of DW_OP_piece:

1) double precision data split over two 32-bit FPRs
Uses a pair of 32-bit DW_OP_piece (ordered depending on endianness).

2) double precision data in one 64-bit FPR
No need for DW_OP_piece.

3) double precision data that may be in two 32-bit FPRs or may be in
one 64-bit FPR depending on hardware mode
Uses a single 64-bit DW_OP_piece on the even numbered register.

I'm guilty of not actually finishing this off and doing the GDB side but
the theory seemed OK when I did it! From your description this behaviour
best matches DW_OP_piece having ABI dependent behaviour which would make
it acceptable. These three variations can potentially exist in the same
program albeit that (1) and (3) can't appear in a single shared library
or executable. It's all a little strange but the whole floating point
MIPS o32 ABI is pretty complex now.

Matthew