[llvm-dev] [PM] I think that the new PM needs to learn about inter-analysis dependencies...

[Hal Finkel via llvm-dev]

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

> From: "Sean Silva" <chisophugis at gmail.com>
> To: "Chandler Carruth" <chandlerc at google.com>
> Cc: "Hal J. Finkel" <hfinkel at anl.gov>, "llvm-dev"
> <llvm-dev at lists.llvm.org>, "Davide Italiano"
> <dccitaliano at gmail.com>, "Xinliang David Li" <davidxl at google.com>
> Sent: Monday, July 25, 2016 8:32:06 PM
> Subject: Re: [llvm-dev] [PM] I think that the new PM needs to learn
> about inter-analysis dependencies...

> On Mon, Jul 25, 2016 at 3:48 PM, Chandler Carruth <
> chandlerc at google.com > wrote:

> > On Mon, Jul 25, 2016 at 3:40 PM Finkel, Hal J. via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> 

> > > Sent from my Verizon Wireless 4G LTE DROID
> > 
> 

> > > On Jul 25, 2016 6:16 PM, Sean Silva < chisophugis at gmail.com >
> > > wrote:
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > On Mon, Jul 25, 2016 at 9:27 AM, Hal Finkel < hfinkel at anl.gov >
> > > > wrote:
> > 
> 
> > > >>
> > 
> 
> > > >>
> > 
> 
> > > >> ________________________________
> > 
> 
> > > >>>
> > 
> 
> > > >>> From: "Sean Silva" < chisophugis at gmail.com >
> > 
> 
> > > >>> To: "Chandler Carruth" < chandlerc at gmail.com >
> > 
> 
> > > >>> Cc: "Xinliang David Li" < davidxl at google.com >, "llvm-dev" <
> > > >>> llvm-dev at lists.llvm.org >, "Davide Italiano" <
> > > >>> dccitaliano at gmail.com >, "Tim Amini Golling" <
> > > >>> mehdi.amini at apple.com >, "Hal Finkel" < hfinkel at anl.gov >,
> > > >>> "Sanjoy Das" < sanjoy at playingwithpointers.com >, "Pete
> > > >>> Cooper"
> > > >>> <
> > > >>> peter_cooper at apple.com >
> > 
> 
> > > >>> Sent: Friday, July 22, 2016 3:55:52 AM
> > 
> 
> > > >>> Subject: Re: [PM] I think that the new PM needs to learn
> > > >>> about
> > > >>> inter-analysis dependencies...
> > 
> 
> > > >>>
> > 
> 
> > > >>> The more closely I look at this, the more it seems like there
> > > >>> may
> > > >>> be a useful incremental step in the transition to the new PM:
> > > >>> use the new PM analysis machinery in the old PM. If this is
> > > >>> possible, it will simplify the old PM and (hopefully) allow
> > > >>> an
> > > >>> incremental transition to the new PM instead of a flag day
> > > >>> transition for the switch.
> > 
> 
> > > >>>
> > 
> 
> > > >>> I.e., AFAICT, the new PM transition is essentially about 2
> > > >>> mostly
> > > >>> orthogonal aspects of running optimization pipelines:
> > 
> 
> > > >>> 1. Analysis computation and analysis result lifetime
> > > >>> management
> > > >>> (including avoiding analysis recomputation)
> > 
> 
> > > >>> 2. Running transformation passes over their respective
> > > >>> IRUnit's
> > > >>> in some order
> > 
> 
> > > >>>
> > 
> 
> > > >>> These are conflated in the old PM. In reality, the only
> > > >>> interaction between them (with the new PM machinery for 1.)
> > > >>> is
> > > >>> a
> > > >>> small number of places within 2. which need to call
> > > >>> 'invalidate'.
> > 
> 
> > > >>>
> > 
> 
> > > >>> I'm pretty sure that 2. is fairly similar in the new PM and
> > > >>> old
> > > >>> PM (the main difference is that the notion of "adapters" is
> > > >>> split out in the new PM). The analysis handling seems to be
> > > >>> what
> > > >>> makes the old PM so difficult to understand (e.g. it is the
> > > >>> cause of the multiple inheritance in the implementation).
> > > >>> Trying
> > > >>> to unify analyses and transformations (and some questionable
> > > >>> (in
> > > >>> hindsight) implementation decisions) seems to be the main
> > > >>> "problem" with the design of the old PM AFAICT (there are
> > > >>> other
> > > >>> issues, but they are more "nice to have").
> > 
> 
> > > >>>
> > 
> 
> > > >>> IMO it is an anti-pattern to think of analyses as "passes".
> > > >>> There
> > > >>> are just "analyses" and "transformations" and they are two
> > > >>> separate things. In fact, the `run` method on analyses should
> > > >>> probably be called `computeResult` or something like that to
> > > >>> avoid confusion.
> > 
> 
> > > >>
> > 
> 
> > > >> This makes sense to me.
> > 
> 
> > > >>
> > 
> 
> > > >> We do currently have some "in between" passes, like LCSSA,
> > > >> which
> > > >> are transformations, but are required by other passes, and
> > > >> transform the IR but whose preservation represents properties
> > > >> of
> > > >> the IR. The particulars of how we handle LCSSA aside (e.g. I
> > > >> think we should preserve it more, perhaps everywhere), how are
> > > >> we
> > > >> planning on handling this class of things?
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > The new PM doesn't currently have a concept like this. As you
> > > > mentioned, it is a weird cross between a transformation and an
> > > > analysis: it can be "invalidated" like an analysis, but
> > > > "recomputing" it actually mutates the IR like a transformation.
> > 
> 
> > > >
> > 
> 
> > > > I'd like to preface the below with the following:
> > 
> 
> > > > No matter how we ultimately address this requirement, my
> > > > preference
> > > > is that we do so in a way that applies to the old PM. This is a
> > > > case where the old PM supports a richer set of functionality
> > > > than
> > > > the new PM. By incrementally refactoring the old PM away from
> > > > its
> > > > use of this extra capability and towards whatever "new" way
> > > > there
> > > > is to do it, we will understand better what it is that we
> > > > actually
> > > > need.
> > 
> 
> > > >
> > 
> 
> > > > (and sorry for the brain dump in the rest of this post)
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > I have not seen any mention of a solution to this problem
> > > > besides
> > > > "we shouldn't do that", which is sort of a cop-out. Here is a
> > > > strawman proposal:
> > 
> 
> > > >
> > 
> 
> > > > If it isn't too expensive, one simple alternative is to have
> > > > passes
> > > > just make a call to a utility function to put things in LCSSA
> > > > if
> > > > they need it (this utility would also have to invalidate
> > > > analyses).
> > 
> 
> > > > If that ends up being expensive, we can have a dummy
> > > > "indicator"
> > > > analysis IRIsInLCSSAForm which, if cached, means "don't bother
> > > > to
> > > > call the utility function". We could maybe just use the LCSSA
> > > > pass
> > > > directly to do the transformation. LCSSA could have
> > > > IRIsInLCSSAForm as an member typedef `IndicatorT` so it can be
> > > > accessed generically. We could then support an API like:
> > 
> 

> > > I think this idea makes sense. My understanding is: There is
> > > nothing
> > > that prevents an analysis results from exposing a utility that
> > > transforms IR, and the result can certainly cache whether or not
> > > this transformation has been performed.
> > 
> 
> > Somewhat agreed, but I don't actually think this problem is as bad
> > as
> > it seems in practice.
> 

> > We only have two places that do this (loop simplify and lcssa) and
> > they both *can* be modeled as "check if it is form X, and if not,
> > put it in form X" or as "check if it is form X, and if not, give up
> > on transform". This has been discussed several times, and the
> > direction things have been leaning for a long time has been:
> 
> > - Make LCSSA increasingly fundamental to the IR and always present,
> > *or* don't require LCSSA at all for transforms. Either of these
> > solve the problem.
> 

> > - Check for loop-simplified form if necessary, and skip the
> > transformation if not present. Because simplified form is simple to
> > check this seems to work well.
> 
> Personally, I would find it very disturbing if a transformation ever
> just silently does nothing. Especially if it depends on whether some
> set of previous transformations makes particular changes.
In some sense, this is always true, and it is not particularly disturbing. Optimizations enable other optimizations, and so on. I think you're referring here to a more-specific situation: transformations have hard dependencies on other transformations, and those dependencies are not declared, or automatically satisfied, making them hard to use and debug. i.e. composing an opt command line to replicate a problem will be somewhat-unfortunately subtle. 

> When I talked to you in person at the social (last one or the one
> before IIRC), you also mentioned that you think that silently doing
> nothing is the solution to when an analysis on a larger IRUnit is
> not cached.

> > Anyways, I don't think we have to solve this problem 100% to make
> > progress on the pass manager. AT no point have I felt particularly
> > blocked on this.
> 
> > > >
> > 
> 
> > > > ```
> > 
> 
> > > > FooTransformation.cpp :
> > 
> 
> > > >
> > 
> 
> > > > PreservedAnalyses FooTransformation::run(Function &F,
> > > > AnalysisManager AM) {
> > 
> 
> > > > // Must be called before getting analyses, as it might
> > > > invalidate
> > > > some.
> > 
> 
> > > > canonicalizeIR<LCSSA>(F, AM);
> > 
> 
> > > >
> > 
> 
> > > > ...
> > 
> 
> > > > }
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > include/IR/Canonicalization.h:
> > 
> 
> > > >
> > 
> 
> > > > template <typename CanonicalizationT, typename IRUnitT>
> > 
> 
> > > > void canonicalizeIR(IRUnitT &IR, AnalysisManager &AM) {
> > 
> 
> > > > using IndicatorT = typename
> > > > CanonicalizationT::IndicatorAnalysis;
> > 
> 
> > > > if ( AM.getCachedResult <IndicatorT>(IR))
> > 
> 
> > > > return;
> > 
> 
> > > > CanonicalizationT C;
> > 
> 
> > > > PreservedAnalysis PA = C.run (IR, AM);
> > 
> 
> > > > AM.invalidate (IR, PA);
> > 
> 
> > > > (void) AM.getResult <IndicatorT>(IR);
> > 
> 
> > > > }
> > 
> 
> > > >
> > 
> 
> > > > ```
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > One place that talks about this problem of "requiring a
> > > > transformation" is
> > > > http://llvm.org/devmtg/2014-04/PDFs/Talks/Passes.pdf on slide
> > > > 17.
> > 
> 
> > > >
> > 
> 
> > > > One reason it provides for "we shouldn't do that" is that if
> > > > you
> > > > think about these things as "canonicalize the IR into a
> > > > specific
> > > > form", then when you have N>1 such dependencies (like some
> > > > passes
> > > > do on LoopSimplify and LCSSA), one must have a subset of the
> > > > requirements of the other. I.e. you can't have two
> > > > canonicalizations that "fight" each other. Using an explicit
> > > > mutation API like the strawman above is a bit less bulletproof
> > > > than scheduling based on statically known interferences between
> > > > canonicalizations (e.g. CanonicalizationA may invalidate
> > > > CanonicalizationB, but not the reverse, so it would
> > > > automatically
> > > > know to run CanonicalizationA before CanonicalizationB), but
> > > > given
> > > > that we have relatively few "canonicalizations" (to give them a
> > > > name) that use this feature of the old PM, it may be livable
> > > > (at
> > > > least in the middle-end, it seems like there is just LCSSA,
> > > > LoopSimplify, BreakCriticalEdges, and LowerSwitch in calls to
> > > > addPreservedID/addRequiredID).
> > 
> 
> > > >
> > 
> 
> > > > I don't find the "Causes rampant re-running of invalidated
> > > > analyses" argument in that slide convincing. If a pass needs
> > > > the
> > > > IR in LCSSA then it needs it. There isn't much we can do about
> > > > that.
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > >
> > 
> 
> > > > One invariant I'd like to preserve in the new pass manager is
> > > > that
> > > > whatever pipeline is provided on the opt command line, we end
> > > > up
> > > > running something "valid"; so a cop-out like "if a pass needs
> > > > LCSSA, you need to make sure to add LCSSA at an appropriate
> > > > place
> > > > before it in the pipeline" is not something I think is
> > > > reasonable
> > > > (way too error-prone).
> > 
> 
> > > >
> > 
> 
> > > > Small rant:
> > 
> 
> > > >
> > 
> 
> > > > We already are in this error-prone situation in the new PM with
> > > > the
> > > > need to call `getCachedResult` to access analyses from a larger
> > > > IRUnitT (e.g. the situation I explained in the post-commit
> > > > thread
> > > > of r274712);
> > 
> 

> > > Yea, I don't like this either. I think we both agree that we need
> > > a
> > > better solution to this. I think we should fix this now and then
> > > deal with potential concurrency issues when we actually have a
> > > design for that so we know what that means.
> > 
> 
> > FWIW, I strongly disagree.
> 
> Thankfully at least right now we just flat-out assert/crash alerting
> to the issue. I don't want to live in a world where passes start to
> silently become no-ops (possibly in a way that only manifests if
> e.g. a particular other pass in the pipeline happens to make changes
> and hence invalidate a particular analysis).

> That would mean living in a world where e.g. you do you build of
> test-suite with an experimental pipeline and halfway through get a
> message like "warning: licm is doing nothing" (if you even get that)
> and have to go and figure out which `require<...>` you need to put
> at a higher level, or figuring out which loop pass invalidated
> something that licm needed.
> This is exactly the current situation, but instead of a message you
> just get an assertion failure / crash.
I'm happy to have a mode which crashes, for us, to help with test-case reduction, etc. I don't think we should have this by default, however. We can have warnings and collect statistics instead. 

Thanks again, 
Hal 

> FWIW, I've been running realistic pipelines and actually using the
> new PM "for real" (e.g. bisecting a realistic pass pipeline on the
> opt command line to find where a bug is coming from, testing
> different optimization pipelines, etc.) and this is definitely one
> of the main issues.
> I think once you start testing out the new PM "for real" you will
> change your position.
> (Correct me if I'm wrong, but I have to assume that you haven't yet
> because you would have run into the showstopping bug that started
> this thread (filed as PR28622), or PR28400, or even simply PR28577.)

> -- Sean Silva

> > I think it would be better to iterate on this once we understand
> > how
> > the new pass manager works. I think exposing the fact that these
> > things are cached is really important and useful, and it makes
> > querying across IR unit boundaries significantly more clear at the
> > call site.
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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