Understanding core2core optimisation pipeline

[Nicolas Frisby]

I implemented -flate-dmd-anal last year

Here's some outdated notes about my initial implementation. I share it in
order to indicate what thoughts were in my mind at the time (eg re
motivation).

https://ghc.haskell.org/trac/ghc/wiki/Frisby2013Q1#LateStrictnessWW

Aha! More up-to-date info here, including links to some of the older,
motivating tickets.

https://ghc.haskell.org/trac/ghc/ticket/7782
https://ghc.haskell.org/trac/ghc/wiki/LateDmd

Also, I now suspect this pass is risky: I think it may enter unused
arguments. I realize I didn't understand that stuff well enough at the
time. This definitely deserves some attention if people are using the flag.

To answer your question directly: I do not recall explicitly considering
single-entry thunks when implementing -flate-dmd-anal.

HTH.


On Thu, Oct 30, 2014 at 7:15 AM, Jan Stolarek <jan.stolarek at p.lodz.pl>
wrote:

> I'm resurrecting this 3-month old thread as I have some more questions
> about cardinality analysis.
>
> 1. I'm still a bit confused about terminology. Demand analysis, strictness
> analysis, cardinality
> analysis - do these three terms mean exactly the same thing? If no, then
> what are the
> differences?
>
> 2. First pass of full laziness is followed by floating in. At that stage
> we have not yet run the
> demand analysis and yet the code that does the floating-in checks whether
> a binder is one-shot
> (FloatIn.okToFloatInside called by FloatIn.fiExpr AnnLam case). This
> suggests that cardinality
> analysis is done earlier (but when?) and that demand analysis is not the
> same thing as
> cardinality analysis.
>
> 3. Does demand analyser perform any transformations? Or does it only
> annotate Core with demand
> information that can be used by subsequent passes?
>
> 4. BasicTypes module defines:
>
> data OneShotInfo = NoOneShotInfo -- ^ No information
>                  | ProbOneShot   -- ^ The lambda is probably applied at
> most once
>                  | OneShotLam    -- ^ The lambda is applied at most once.
>
> Do I understand correctly that `NoOneShotInfo` really means no information,
> ie. a binding annotated with this might in fact be one shot? If so, then
> do we
> have means of saying that a binding is certainly not a one-shot binding?
>
> 5. What is the purpose of SetLevels.lvlMFE function?
>
> Janek
>
> > The wiki page just went live:
> >
> >
> https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/Core2CorePipeline
> >
> > It's not yet perfect but it should be a good start.
> >
> > > Roughtly, a complete run of the simplifier means "run the simplifier
> > > repeatedly until nothing further happens".  The iterations are the
> > > successive iterations of this loop. Currently there's a (rather
> arbitrary)
> > > limit of four such iterations before we give up and declare victory.
> >
> > A limit or a default value for that limit?
> >
> > To Ilya:
> > > If you grep for the "late_dmd_anal" option variable in the
> compiler/simplCore/SimplCore.lhs
> > > module, you'll see that it triggers a phase close to the endo of
> getCoreToDo's tasks, which
> > > contains, in particular, the "CoreDoStrictness" pass. This is the
> "late" phase.
> >
> > The paper said that the late pass is run to detect single-entry thunks
> and the reason why it is
> > run late in the pipeline is that if it were run earlier this information
> could be invalidated
> > by the transformations. But in the source code I see that this late pass
> is followed by the
> > simplifier, which can invalidate the information. Also, the
> documentation for -flate-dmd-anal
> > says: "We found some opportunities for discovering strictness that were
> not visible earlier;
> > and optimisations like -fspec-constr can create functions with unused
> arguments which are
> > eliminated by late demand analysis". This says nothing about
> single-netry thunks. So, is the
> > single-entry thunk optimisation performed by GHC?
> >
> > Janek
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