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

Andrew Burgess andrew.burgess@embecosm.com
Fri May 21 14:03:40 GMT 2021


* 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


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