What does multiple DW_OP_piece mean in DWARF?

Jakub Jelinek jakub@redhat.com
Mon Oct 31 13:39:00 GMT 2011

On Mon, Oct 31, 2011 at 12:01:00PM -0000, Dan Towner wrote:
> Could someone tell me whether the following sequence of DWARF information
> is correct please, and if it is, how it should be interpreted? GCC emits
> something like the following [1]:
>         .byte   0x75    # DW_OP_breg5
>         .sleb128 0
>         .byte   0x93    # DW_OP_piece
>         .uleb128 0x4
>         .byte   0x93    # DW_OP_piece
>         .uleb128 0x4
> Is it valid to emit two DW_OP_pieces with no separating location? My copy


> of the spec for DWARF (v4 taken from www.dwarfstd.org) seems to suggest
> that all pieces must have a location preceeding them. There is also a
> comment in dwarf2out.c which says:
>    "DW_OP_piece is only added if the location description expression already
> 		doesn't end with DW_OP_piece"
> so it would seem like two contiguous pieces is wrong, but it seems to
> occur so frequently I wonder if it is a correct output after all.

DW_OP_piece is preceeded by simple location description (DWARF4,
And one of the valid simple location descriptions is empty location
description:  Empty Location Descriptions
An empty location description consists of a DWARF expression containing no
operations. It represents a piece or all of an object that is present in the
source but not in the object code (perhaps due to optimization).

So the above means that the first 4 bytes of the variable live in
memory pointed by register 5 and the second 4 bytes of the variable are
optimized out.


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