From: vaibhav kurhe <vaibhav.kurhe@gmail.com>
To: "Zaric, Zoran (Zare)" <Zoran.Zaric@amd.com>
Cc: "gdb@sourceware.org" <gdb@sourceware.org>
Subject: Re: Re: GDB | DWARF expression | Extracting a range of bits from an 'xmm' register
Date: Sat, 22 May 2021 02:39:23 +0530 [thread overview]
Message-ID: <CAHKv1qscHZdAY1X7y6y1UdP6TMrV4=bSBdEMffe1T57fvX2WUw@mail.gmail.com> (raw)
In-Reply-To: <4319e88e-4f22-fce6-8678-07e27fb043d4@amd.com>
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
next prev parent reply other threads:[~2021-05-21 21:09 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-21 8:57 vaibhav kurhe
2021-05-21 14:03 ` Andrew Burgess
2021-05-21 16:02 ` Zaric, Zoran (Zare)
2021-05-21 21:09 ` vaibhav kurhe [this message]
2021-05-24 10:00 ` Zaric, Zoran (Zare)
2021-05-21 20:37 ` vaibhav kurhe
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