What does multiple DW_OP_piece mean in DWARF?

[Dan Towner]

Hi all,

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.

I am working on a symbolic debugger for the Picochip platform, and need to
understand why this sequence is emitted, and what the debugger should do
with it. I can supply test cases if necessary, but hopefully someone may
know if this sequence is intentional or not.

thanks,

dan.

[1] Using `gcc -O1 -dA -S', on versions 4.4.5 and 4.6.2 on x86_64 and
Picochip. There are subtle variations, but the same basic pattern keeps
reappearing.