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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