[Dwarf-Discuss] DWARF piece questions

Michael Eager eager at eagercon.com
Thu Jan 26 17:17:22 PST 2017


On 01/26/2017 11:17 AM, Andreas Arnez wrote:
> Exactly: the current DWARF text*differs*  from the usual "defined by the
> ABI"-principle when it states for DW_OP_bit_piece: "If the location is a
> register, the offset is from the least significant bit end of the
> register".  This definition limits the ABI's freedom such that register
> growth can only be anchored at the "least significant bit".

That's not the case.  The ABI is free to put a value where ever it wishes
in a register.  The DWARF description will be different, depending on where
the ABI puts the value, indexed from the least-significant bit of the register.
DW_OP_bit_piece is designed explicitly to support this.

I have to admit that I'm unclear exactly what you mean by "register growth".
But if you load a 16-bit value into the most-significant half of a 32-bit register
(is this growing a register?) then you would describe the value in the register
with a length of 16 and an offset of 16.  Same applies for 32-bit values in
64-bit registers.

-- 
Michael Eager	 eager at eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077


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