[RFC] DW_OP_piece vs. DW_OP_bit_piece on a Register

[Matthew Fortune]

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