Re: [GHC] #3831: SpecConstr should exploit cases where there is exactly one call pattern

22 Feb 2018

      #3831: SpecConstr should exploit cases where there is exactly one call pattern
-------------------------------------+-------------------------------------
        Reporter:  igloo             |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  6.13
      Resolution:                    |             Keywords:  SpecConstr
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Compile-time      |            Test Case:
  performance bug                    |  simplCore/should_compile/T3831
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by sgraf):

 I'm trying to reproduce what the missed opportunity was here. I ran T3831
 and realized that the test is ineffective for detecting a regression of
 this ticket: The compiler invocation misses a `-O2`.

 Re: missed tantalising opportunity: I think you are referring to
 `setAttributes`, specifically to this nesting of join points:

 {{{
 join {
   $j1
   $j1 x1 m1
     = case m1 of {
         Nothing -> (# x1, Nothing #);
         Just x1 -> case ... x1 of {
           (# y1, n1 #) ->
             join {
               $j2
               $j2 x2 m2
                 = ... -- many more
                   join {
                     $j12 x12 m12
                       = case m12
                         of {
                           Nothing ->
                             (# x12, Nothing #);
                           Just f ->
                             (# x12, Just (\a -> ...) #)
                         } } in
                   ...
                   case m2
                   of {
                     Nothing -> jump $j12 x2 Nothing;
                     Just f -> case ... x2 of {
                       (# x12', m12' #) ->
                       case m12'
                       of {
                         Nothing -> jump $j12 x12' (Just (x6 lvl7));
                         Just _ -> jump $j12 x12' (Just ...)
                       }
                       }
                   } } in
             case n1
             of {
               Nothing -> jump $j2 y1 Nothing;
               Just _ -> jump $j2 y1 (Just ...)
             } } in
 }}}

 I probably have made some errors while simplifying, but the idea is that
 these join points aren't specialised at all.

 1. These are all non-recursive. I was under the impression that we have
 the inliner for non-recursive bindings to decide if inlining is
 worthwhile.
 2. I don't see how there's only one call-pattern for each join point.
 There are 3: one Nothing, two Justs. Specialising (branching factor 2) or
 even inlining (3) these would mean exponential blow-up in code size.

 So either I'm referring to the wrong fragment of the code (I found no
 other join points, let alone recursive ones) or things have changed since
 then.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/3831#comment:17
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