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

[Zaric, Zoran (Zare)]

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