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

vaibhav kurhe vaibhav.kurhe@gmail.com
Fri May 21 20:37:45 GMT 2021


On Fri, May 21, 2021 at 7:33 PM Andrew Burgess
<andrew.burgess@embecosm.com> 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 attri
bute 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 Andrew,
Thanks for the reply!

Yes, the second operand to DW_OP_shl is indeed the number of bits to
shift. But, I saw that the DWARF5 standard mentions that both the
operands should be of the same type.
So, in this case, as the 1st operand is typed (which points to a DIE
for the base type), I think the 2nd operand also has to be typed and
hence used the operation DW_OP_const_type.

Regards,
Vaibhav


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