[llvm-dev] [RFC] New pass: LoopExitValues

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Steve King" <steve at metrokings.com>
> To: "Wei Mi" <wmi at google.com>, "Hal Finkel" <hfinkel at anl.gov>, "Philip Reames" <listmail at philipreames.com>,
> "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, September 23, 2015 1:25:21 PM
> Subject: Re: [llvm-dev] [RFC] New pass: LoopExitValues
> 
> On Mon, Sep 21, 2015 at 11:13 AM, Wei Mi <wmi at google.com> wrote:
> > Maybe it can follow the "Delete dead loops" pass which is after
> > Induction Variable Simplification pass, so it will not affect
> > existing
> > exitvalue rewriting optimization in Induction Variable
> > Simplification
> > to find out and delete dead loops?
> >
> > Existing pass pipeline:
> >         Loop-Closed SSA Form Pass
> >         Loop Pass Manager
> >           Induction Variable Simplification
> >           Recognize loop idioms
> >           Delete dead loops
> >                                                            <=== New
> > LoopExitValues
> >         Function TargetTransformInfo
> >         Loop Pass Manager
> >           Unroll loops
> >         Memory Dependence Analysis
> >
> 
> I tried Wei's suggested location for the LEV pass shown above.
> Placing the pass here fails to find any SCEV redundancy in the test
> case because none exists yet.  The reason is that the relevant SCEV's
> are off-by-1.  Some nitty-gritty:
> In Pass N of the outer loop, the inner loop exits with %add = (N *
> %Size) - 1
> To be useful, Pass N+1 of the outer loop needs some inner loop exit
> value = (N * %Size).
> 
> This mismatch does not jibe with my previous IR hacking experiment.
>  I
> tried the hack again and the assembly was not equivalent.  Somehow I
> fooled myself before.
> 
> In the current patch, LEV runs after LSR and does find redundant
> SCEVs.  Running the pass here would also mop up hypothetical SCEV
> redundancy from any previous pass, but I understand the goal is to
> avoid such redundancy in the first place.
> 
> Should we try the patch in it's current location, namely after LSR?

Sure; post the patch as you have it so we can look at what's going on.

 -Hal

> 
> Regards,
> -steve
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory