[llvm] r268452 - PM: Port LoopRotation to the new loop pass manager

Tue May 10 14:59:37 PDT 2016

----- Original Message -----

> From: "Sean Silva via llvm-commits" <llvm-commits at lists.llvm.org>
> To: "Justin Bogner" <mail at justinbogner.com>
> Cc: "llvm-commits" <llvm-commits at lists.llvm.org>
> Sent: Tuesday, May 10, 2016 4:55:08 PM
> Subject: Re: [llvm] r268452 - PM: Port LoopRotation to the new loop
> pass manager

> On Tue, May 10, 2016 at 12:46 PM, Justin Bogner <
> mail at justinbogner.com > wrote:

> > Sean Silva < chisophugis at gmail.com > writes:
> 
> > > On Tue, May 10, 2016 at 11:27 AM, Justin Bogner <
> > > mail at justinbogner.com >
> 
> > > wrote:
> 
> > >
> 
> > >> Sean Silva < chisophugis at gmail.com > writes:
> 
> > >> > On Tue, May 3, 2016 at 3:02 PM, Justin Bogner via llvm-commits
> > >> > <
> 
> > >> > llvm-commits at lists.llvm.org > wrote:
> 
> > >> >
> 
> > >> >> Author: bogner
> 
> > >> >> Date: Tue May 3 17:02:31 2016
> 
> > >> >> New Revision: 268452
> 
> > >> >>
> 
> > >> >> URL: http://llvm.org/viewvc/llvm-project?rev=268452&view=rev
> 
> > >> >> Log:
> 
> > >> >> PM: Port LoopRotation to the new loop pass manager
> 
> > >> ...
> 
> > >> >> --- llvm/trunk/test/Transforms/LoopRotate/basic.ll (original)
> 
> > >> >> +++ llvm/trunk/test/Transforms/LoopRotate/basic.ll Tue May 3
> > >> >> 17:02:31
> 
> > >> 2016
> 
> > >> >> @@ -1,4 +1,6 @@
> 
> > >> >> ; RUN: opt -S -loop-rotate < %s | FileCheck %s
> 
> > >> >> +; RUN: opt -S
> 
> > >> >>
> 
> > >> -passes='require<loops>,require<targetir>,require<assumptions>,loop(rotate)'
> 
> > >> >> < %s | FileCheck %s
> 
> > >> >>
> 
> > >> >
> 
> > >> > Sorry if this is a stupid question, but why do we need to
> > >> > explicitly
> 
> > >> > "require" the passes when the loop-rotate already declares the
> 
> > >> dependency?
> 
> > >> > (I feel like there's some part of the bigger picture that I'm
> > >> > missing
> 
> > >> here)
> 
> > >>
> 
> > >> To be clear, there are no dependencies in the new pass manager -
> > >> passes
> 
> > >> request analysis results from an analysis manager and they're
> > >> calculated
> 
> > >> or returned from the cache as appropriate. Naturally, the next
> > >> question
> 
> > >> one would ask is "okay then, why aren't these calculated on
> > >> demand
> > >> when
> 
> > >> loop-rotate asks for them?".
> 
> > >>
> 
> > >
> 
> > > Ah, yeah. The whole "schedule vs. cache" is one of the main
> > > differences of
> 
> > > the new PM.
> 
> > >
> 
> > >
> 
> > >>
> 
> > >> The mechanical answer is that loop-rotate uses the
> > >> `getCachedResult`
> 
> > >> API, not the `getResult` one - the difference being that
> 
> > >> `getCachedResult` only returns results if they're already
> > >> calculated,
> 
> > >> and null otherwise. Hence, without the require<> in the test,
> > >> there are
> 
> > >> no results.
> 
> > >
> 
> > >
> 
> > > Ah, makes perfect sense!
> 
> > >
> 
> > >
> 
> > >> This of course leads to "why does loop-rotate use
> 
> > >> getCachedResult?"
> 
> > >>
> 
> > >> It has to. These analyses are function analyses, and a loop pass
> > >> isn't
> 
> > >> allowed to cause a function analysis to run, much like a
> > >> function
> > >> pass
> 
> > >> isn't allowed to cause a module analysis to be run. They have to
> > >> stick
> 
> > >> to their own level. This enforces correct layering and acts as a
> 
> > >> safeguard against accidentally doing extra work.
> 
> > >>
> 
> > >> That said, the "require<loops>" is redundant, since a
> > >> LoopPassManager
> 
> > >> can't operate without having calculated that. There may also be
> > >> an
> 
> > >> argument to be made that the LPM should implicitly calculate
> > >> some
> > >> other
> 
> > >> analyses, but we'd have to be careful not to do too much.
> 
> > >>
> 
> > >
> 
> > > Makes sense. But then `+ assert((LI && TTI && AC) && "Analyses
> > > for
> > > loop
> 
> > > rotation not available");` is not really appropriate, right?
> > > Can't
> > > we hit
> 
> > > that assert just by passing the right arguments to opt? (by
> > > dropping the
> 
> > > `require<*>` analyses from the passes= specification?)
> 
> > >
> 
> > > (sorry, I'm away from the office and don't have an easy way to
> > > test
> > > locally)
> 

> > Yes it does. The other option is to just bail on the transform in
> > that
> 
> > case, which would be a better option for things like fuzzers and
> 
> > bisection scripts, but makes it harder to ensure we're doing
> > everything
> 
> > correctly while we're bringing this up. For now I think the assert
> > is
> 
> > best, but we may want to revisit this at some point.
> 

> I'm uncomfortable with the assert because it won't be there in
> Release and I don't want to be running into UB in my release build
> from a mistyped command line. I think report_fatal_error is more
> appropriate.
+1 

Also, we might want to consider moving the error-reporting logic into the pass manager itself. I could see having a 'Required' parameter to getCachedResult such that it would be a fatal error should no cached result be available. 

-Hal 

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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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