[llvm-dev] Ok with mismatch between dead-markings in BUNDLE and bundled instructions?

Wed Jun 28 02:28:10 PDT 2017

Not sure if I could follow everything in this discussion regarding subregisters. But I think the problem posted by Mikael just happened to involve subregisters, and the discussions about subregisters is confusing when it comes to Mikaels original question/problem.

I think that the bundle could look something like this just as well:

     BUNDLE %vreg1<def,dead>
       * %vreg1<def> = add %vreg2, %vreg3
       * call @foo, %vreg1<internal-use>

No subregisters involved.
%vreg1 is dead after the bundle.
%vreg1 is not dead when defined at the "add", because it is used later in the same bundle.

Should perhaps the %vreg1 not be included in the BUNDLE head at all here?
(but shouldn't the BUNDLE head be a summary of what is going on inside the bundle, so leaving out information about %vreg1 being defined seems wrong)

To me it seems wrong to add "dead" to the def of %vreg1 at the add (considering the internal-use).
Maybe that even answers the question that the "mismatch" between dead-markings should be OK.
Or would it be OK to mark %vreg1 as dead at the add, even though we have a later internal-use?

Regards,
Björn

> -----Original Message-----
> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of
> Quentin Colombet via llvm-dev
> Sent: den 28 juni 2017 00:02
> To: Matthias Braun <mbraun at apple.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] Ok with mismatch between dead-markings in
> BUNDLE and bundled instructions?
> 
> 
> > On Jun 27, 2017, at 2:51 PM, Matthias Braun via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> >
> >
> >> On Jun 27, 2017, at 2:44 PM, Krzysztof Parzyszek via llvm-dev <llvm-
> dev at lists.llvm.org> wrote:
> >>
> >> On 6/27/2017 4:35 PM, Quentin Colombet via llvm-dev wrote:
> >>> Yeah I was reading this as “only the non-touched part are dead”, and
> that’s what I’d like to see in the representation longer. Obviously, the
> register is not dead as a whole here :)
> >>
> >> I think that having two defs for the same register, one dead and one not
> dead simply doesn't make sense. We already assume that a register is live if
> at least a part of it is live, so if it's "dead", it should mean that the whole thing
> is dead.
> > Without subregister I would agree. However with subregisters and aliases
> in play you can express more situations. Like for example:
> >
> >    %rax<dead>, %eax = ...
> >
> > could mean the instruction writes the full rax register but we are only
> gonna read eax later.
> 
> That sounds like an alias to:
> %rax<def-undef, subeax> = …
> 
> Which sounds fine. Though I am not suggesting we want to move to this
> dead model for such situation.
> 
> > That said I am not sure whether we actually need it, and if llvm works that
> way today. Given how subtle all of this is there is also a high danger that we
> won't get the bahviour consistent.
> 
> I agree that consistent behavior is important and I also think we probably
> cannot model what we want with the current representation. What I would
> like to see if that we don’t sit on potentially useful information, like this part
> of the register is dead, because it is convenient implementation-wise. I am
> not saying that’s what you're suggesting!
> I agree that at the end of the day we want something that works and that is
> understandable. To me having the semantic of dead being this can be killed if
> the instruction does not have side effects sounded easy enough to
> understand.
> 
> What is your proposal for the semantic?
> 
> (IIRC the dead flag is required for values that are never used and the
> proposed fix somehow goes against that.)
> 
> >
> > - Matthias
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev