[LLVMdev] MI bundle liveness attributes

[Sergei Larin]

Arnold,

  Interesting point. This fake use would also need to be probably marked as
isUndef(), but I could not foresee all possible corner cases from that. 
Could it be overly conservative? Would I lose the ability of some sort
"predicate value propagation" that I seem to gain from introduction of an
explicit flag? Can someone comment?

Thanks.

Sergei

--
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.


> -----Original Message-----
> From: Arnold Schwaighofer [mailto:arnolds at codeaurora.org]
> Sent: Thursday, August 09, 2012 1:54 PM
> To: Sergei Larin
> Cc: 'LLVM Developers Mailing List'
> Subject: Re: [LLVMdev] MI bundle liveness attributes
> 
> Hi Sergei,
> 
> It seems to me that you can represent the semantics of a conditional
> instruction by adding a use of the conditionally defined register to
> the instruction.
> 
> The value of the output register of an instruction is either the value
> of the instruction if it was conditionally executed or the value of the
> output register before the instruction.
> 
> The Bundle would be:
> 
> BUNDLE %PC<imp-def>, %R0<imp-def>, %P0<imp-use,kill>, %R16<imp-use>,
>     %R0<imp-use,kill>
>       * %R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0,
>                           %R0<imp-use,kill>
>       * %P0<def> = CMPEQri %R16, 0
> 
> The individual instruction would be:
> 
>    %R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0, %R0<imp-use,
>                                                             kill>;
> 
> How would you use cond-def/uses? How would they change liveness?
> 
> Best,
> Arnold
> 
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.
> 
> On 8/9/2012 11:48 AM, Sergei Larin wrote:
> >
> > Hello everyone,
> >
> >    Let me (re)present a question that might have previously been
> > discussed, but did not result in any code (AFIK).
> >
> >    How do we represent a _conditional_ assignment (def) in a bundle
> MI?
> >
> >    More contents - currently we expose internal def/use/kill
> > information to a bundle header - something like this:
> >
> >
> > BUNDLE %PC<imp-def>, %R0<imp-def>, %P0<imp-use,kill>, %R16<imp-use>
> >     * %R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;
> >     * %P0<def> = CMPEQri %R16, 0;
> >
> >    Here CMPEQri is a compare to a predicate register instruction, and
> > LDriuh_cdnNotPt is a _conditional_ load, which might or might not
> Take
> > place based on the outcome of the compare... As such R0 might or
> might
> > not be defined in this bundle, which obviously changes the liveness
> > update process.
> >
> >    My question, do we need another attribute along with isImplicit
> and
> > isEarlyClobber etc. to designate a conditional def? Furthermore,
> > depending on architectural details we well might have a conditional
> > use as well... and what about the individual (unbundled) def/use?
> Should this:
> >
> > %R0<def> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;
> >
> > ...become this:
> >
> > %R0<def-cond> = LDriuh_cdnNotPt %P0<kill,internal>, %R16, 0;
> >
> > or even:
> >
> > %R0<def-cond> = LDriuh_cdnNotPt %P0<kill,internal>, %R16<use-cond>,
> 0;
> >
> >    So, if I am missing something in current implementation or an
> > ongoing discussions (and that is entirely possible since I am just
> > back after vacation), please let me know how to achieve this
> > functionality, but if this is something missing in implementation,
> > let's discuss how do we want to realize it.
> >
> >    Thanks.
> >
> > Sergei Larin
> >
> > --
> > Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.
> >
> >
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> >
> 
> 
> 
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum.