[GHC] #15560: Full laziness destroys opportunities for join points

[GHC]

#15560: Full laziness destroys opportunities for join points
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        Reporter:  AndreasK          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
  (CodeGen)                          |
      Resolution:                    |             Keywords:  JoinPoints
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14287 #13286     |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Ideally we would combine the checks for all branches into a single one
 to begin with.

 Do you mean that in
 {{{
    case x of
      True -> e1
      False -> e2
 }}}
 instead of a heap-check at the start of `e1` and another at the start of
 `e2`, we could have a single one before the case?

 No, we can't do this: evaluating `x` might force a thunk, and hence
 allocate an arbitrary amount of stuff.

 If the `case` is strutinising an unlifted type (which does not require
 evaluating) then yes it's different, and indeed in that case we sometimes
 ''do'' move the heap check up.
 See the long `Note [Compiling case expressions]` in `StgCmmExpr.hs`.

 (Another possibility that looks unattractive, and that I have not
 explored: put the heap check after returning from evaluating `x` but
 before doing the case-analysis to decide which branch to take.  That might
 reduce code size, but would never eliminate a heap check altogether;
 indeed it might put one in the code path that was not there before, for a
 branch that did not allocate at all.)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15560#comment:11>
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