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161206.2 Andreas Arnez Clarify relationship between DW_OP_piece and DW_OP_bit_piece Clarification Rejected with clarification Adrian Prantl


Section 2.6.1.2, pg 42
The current specifications of DW_OP_piece and DW_OP_bit_piece do not
make it sufficiently clear whether DW_OP_piece is just a
backward-compatible short-hand notation for special cases of
DW_OP_bit_piece.  All DWARF producers/consumers I know of interpret it
that way, but the current specification says otherwise:

- DW_OP_piece: "If the piece is located in a register, but does not
  occupy the entire register, the placement of the piece within that
  register is defined by the ABI."
- DW_OP_bit_piece: "If the location is a register, the offset is from
  the least significant bit end of the register."

This wording seems to imply that byte- and bit pieces may follow
different rules for the placement of pieces within a register, one
defined by the ABI, and the other being the LSB0 rule defined by
DWARF.

Also, placement rules for other types of objects are only given for
DW_OP_bit_piece and missing for DW_OP_piece.

Thus I propose to clarify the specification of DW_OP_piece in 2.6.1.2
"Composite Location Descriptions":

      1. DW_OP_piece

      The DW_OP_piece operation takes a single operand, which is
      an unsigned LEB128 number.  The number gives the size in
      bytes of the piece of the object referenced by the
      preceding simple location description.  The placement of
      the piece within that object is the same as defined by the
      appropriate DW_OP_bit_piece operation with offset zero.
      Thus the following operations are equivalent:
      - DW_OP_piece <n>
      - DW_OP_bit_piece <8*n> 0


RESOLUTION

We found that multiple big-endian ABIs (including MIPS, and Sparc) are
defining DW_OP_piece as different from DW_OP_Bit_piece with offset 0.
In order to avoid breaking compatibility, we add the following 
non-normative text to Section 2.6.1.2:

   For instance, so that structure variables passed by register can be
   loaded and stored using register-sized transfers, a big-endian ABI may
   specify that the pieces of a structure are "left aligned" within a
   register (located in the most significant bytes of the register). Thus,
   DW_OP_piece <n> is not necessarily equivalent to DW_OP_bit_piece <8*n> 0.

--

2021-03-22 Rejected with clarification:  Add non-normative text:
  Whether or not a DW_OP_piece operation is equivalent to any DW_OP_bit_piece
  operation with an offset of 0 is ABI dependent.
  


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