[GHC] #14137: Do more inlining into non-recursive join points

[GHC]

#14137: Do more inlining into non-recursive join points
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        Reporter:  simonpj           |                Owner:  nomeata
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by nomeata):

 That example is what #10918 is about. The Cardinality Analyser does not
 see that `ys` is evaluated at most once, but Call Arity does. We can make
 it inline, but the tricky bit is to prevent it from being floated out
 again. That’s what I feel that a `call` to a join point outside of
 `letrec` would be more explicit and more stable – although I see the
 clumsiness of that. There you write:

 >  But it does seem hard. The float-out pass assumes, crudely, that it's
 good to float out a redex of any called-many lambda. But, as we see here,
 that's wrong for case branches that are only evaluated on one of those
 calls (the final one in this case). Not only is that info hard to record
 in the syntax tree, but it's also potentially quite fragile to program
 transformation, like other sorts of cardinality information.

 And a `call` to a join-point outside the `letrec` is so far the only way I
 can think of to “record it in the syntax tree” – but maybe there are
 better ways?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14137#comment:16>
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