[GHC] #14461: Reuse free variable lists through nested closures

[GHC]

#14461: Reuse free variable lists through nested closures
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        Reporter:  tdammers          |                Owner:  alexbiehl
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  7258              |  Differential Rev(s):
       Wiki Page:  NestedClosures    |
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Comment (by alexbiehl):

 I hacked up a POC of this transformation (it excludes updatable thunks
 from consideration). I had a quick (non-scientific) glance over the amount
 of CMM code generated for W2.hs (example from #7258):

 {{{
 vanilla ghc ~380000 LOC
 hacked  ghc ~270000 LOC
 }}}

 I noticed a lots of small, nested closures which only needed a fraction of
 the free variables of their outer closures and hence didn't benefit from
 the transformation.

 > Some data along these lines would be extremely helpful in guiding what
 we do.
 I will instrument my GHC to collect some data to guide us from here.



 > Another variant: currently we insist that `[gv1,..gvm]` is a
 '''subset''' of `[fv1,..fvn]`, to avoid space leaks.  But with some RTS
 support we could do better: the closure for `x` could point to the closure
 for `y`, but ''also'' have a bitmap (in `x`'s info table) to say which of
 `y`'s free vars are actually used by `x`.  That would open up the
 possibility for much more flexible sharing, at the cost of more bitmaps
 etc.  E.g.
 > {{{
 > x = [fv1,...fv100] \ [a] ->
 >     let y = [fv1,...fv99, a] \ [] -> ...
 >     in ...
 > }}}

 Sound reasonable. Let's see what the data shows..

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14461#comment:11>
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