GDB | DWARF expression | Extracting a range of bits from an 'xmm' register

[vaibhav kurhe]

On Fri, May 21, 2021 at 10:36 PM Zaric, Zoran (Zare) via Gdb
<gdb@sourceware.org> wrote:
>
> On 5/21/21 3:03 PM, Andrew Burgess wrote:
> > * vaibhav kurhe via Gdb <gdb@sourceware.org> [2021-05-21 14:27:15 +0530]:
> >
> >> Hello all,
> >> For a use case, I am trying to build a DWARF expression which represents
> >> the value of an arbitrary range of bits (e.g. 96-127 bits) in an *128-bit
> >> xmm register* to be used as a *location attribute value* for a variable DIE.
> >> I am using GDB to consume the debug info and test it.
> >>
> >> Following is the expression I started with to test out a shift operation on
> >> an 128-bit xmm0 register using Typed DWARF stack :-
> >>
> >> *"DW_OP_GNU_regval_type: 21 (xmm0) <0x30>; DW_OP_GNU_const_type: <0x30>  16
> >> byte block: 20 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 ; DW_OP_shl;
> >
> > I'm probably just not understanding correctly, but I'm confused by the
> > use of DW_OP_GNU_const_type.  Isn't this providing the number of bits
> > to shift?  I'd have expected something like 'DW_OP_const1u 96'.
> >
> > Thanks,
> > Andrew
> >
>
> Hi Vaibhav,
>
> Maybe I am missing something, but what is the end goal that you are
> trying to accomplish?
>
> The way how you formed your expression, you can only get a read only
> stack value location description.
>
> Why not use the DW_OP_bit_piece with your register being the only piece
> inside of it and then use that as your end location description?
>
> Thanks,
> Zoran

Hi Zoran,
Thanks for the reply!

Actually I am trying to improve an object file's debug info in case of
a vectorized transformation by the compiler.
e.g. when a source variable, 'sum' = (xmm0[0-31] + xmm0[32-63] +
xmm0[64-95] + xmm0[96-127]).

Thanks for pointing out the DW_OP_bit_piece operation! It worked in a
setting where a source variable resides directly in a 32-bit chunk of
an 128-bit xmm register.
But, I think it won't be possible for the above example(?).
Here, we'll have to do 128-bit operations (such as DW_OP_shl) on the
register to get its 32-bit chunks. Is that correct?

Regards,
Vaibhav