[GHC] #13966: Skip-less stream fusion: a missed opportunity

[GHC]

#13966: Skip-less stream fusion: a missed opportunity
-------------------------------------+-------------------------------------
        Reporter:  jmspiewak         |                Owner:  mpickering
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1-rc3
      Resolution:                    |             Keywords:  JoinPoints,
                                     |  StaticArgumentTransformation
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14067 #14068     |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by mpickering):

 Implementing this leads to:

 {{{
 benchmarking Skip-less
 time                 632.2 ms   (532.7 ms .. 835.8 ms)
                      0.988 R²   (0.976 R² .. 1.000 R²)
 mean                 1.187 s    (1.044 s .. 1.308 s)
 std dev              191.0 ms   (0.0 s .. 207.9 ms)
 variance introduced by outliers: 46% (moderately inflated)

 benchmarking Skip
 time                 904.3 ms   (904.1 ms .. 904.8 ms)
                      1.000 R²   (1.000 R² .. 1.000 R²)
 mean                 1.230 s    (1.130 s .. 1.306 s)
 std dev              114.9 ms   (0.0 s .. 130.3 ms)
 variance introduced by outliers: 22% (moderately inflated)
 }}}

 And the core looks quite similar:

 {{{
 chain1
   = \ (w_s9yo :: Int) ->
       case w_s9yo of { GHC.Types.I# ww1_s9yr ->
       joinrec {
         $wsat_worker2_s9yn [InlPrag=NOUSERINLINE[0], Occ=LoopBreaker]
           :: Int# -> Int# -> Int
         [LclId[JoinId(2)], Arity=2, Str=<S,U><S,U>m, Unf=OtherCon []]
         $wsat_worker2_s9yn (ww2_s9yh :: Int#) (ww3_s9yl :: Int#)
           = join {
               lvl_s9BD [Dmd=<L,U(U)>] :: Int
               [LclId[JoinId(0)], Str=m, Unf=OtherCon []]
               lvl_s9BD = GHC.Types.I# ww2_s9yh } in
             joinrec {
               $wsat_worker3_s9yc [InlPrag=NOUSERINLINE[0],
 Occ=LoopBreaker]
                 :: Int# -> Int
               [LclId[JoinId(1)], Arity=1, Str=<S,U>m, Unf=OtherCon []]
               $wsat_worker3_s9yc (ww4_s9ya :: Int#)
                 = case ># ww4_s9ya ww1_s9yr of {
                     __DEFAULT ->
                       case remInt# ww4_s9ya 2# of {
                         __DEFAULT -> jump $wsat_worker3_s9yc (+# ww4_s9ya
 1#);
                         0# ->
                           jump $wsat_worker2_s9yn (+# ww2_s9yh ww4_s9ya)
 (+# ww4_s9ya 1#)
                       };
                     1# -> jump lvl_s9BD
                   }; } in
             jump $wsat_worker3_s9yc ww3_s9yl; } in
       jump $wsat_worker2_s9yn 0# 1#
       }
 }}}

 {{{
 chain2
   = \ (w_s9xY :: Int) ->
       case w_s9xY of { GHC.Types.I# ww1_s9y1 ->
       joinrec {
         $wsat_worker2_s9xX [InlPrag=NOUSERINLINE[0], Occ=LoopBreaker]
           :: Int# -> Int# -> Int
         [LclId[JoinId(2)], Arity=2, Str=<S,U><S,U>m, Unf=OtherCon []]
         $wsat_worker2_s9xX (ww2_s9xR :: Int#) (ww3_s9xV :: Int#)
           = case ># ww3_s9xV ww1_s9y1 of {
               __DEFAULT ->
                 case remInt# ww3_s9xV 2# of {
                   __DEFAULT -> jump $wsat_worker2_s9xX ww2_s9xR (+#
 ww3_s9xV 1#);
                   0# ->
                     jump $wsat_worker2_s9xX (+# ww2_s9xR ww3_s9xV) (+#
 ww3_s9xV 1#)
                 };
               1# -> GHC.Types.I# ww2_s9xR
             }; } in
       jump $wsat_worker2_s9xX 0# 1#
       }
 }}}

 Still need to investigate the general impact more.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13966#comment:15>
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