[GHC] #15176: Superclass `Monad m =>` makes program run 100 times slower

[GHC]

#15176: Superclass `Monad m =>` makes program run 100 times slower
-------------------------------------+-------------------------------------
        Reporter:  danilo2           |                Owner:  osa1
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by osa1):

 I attached four files for ticky and prof outputs of the program in
 comment:10 with and wihtout `Monad m =>` patch. I can't make sense of the
 ticky output -- it's really hard to see what's wrong in a hundred lines
 long Core function but perhaps someone else can figure it out.

 One other thing I tried was to test the patch with `-O0`, and the numbers
 are almost identical:

 {{{

 === ORIGINAL
 ===================================================================

 luna git:(master) $ time (cabal-run bench-test +RTS -s)
   77,264,754,848 bytes allocated in the heap
      114,241,080 bytes copied during GC
          240,688 bytes maximum residency (2 sample(s))
           33,152 bytes maximum slop
                2 MB total memory in use (0 MB lost due to fragmentation)

                                      Tot time (elapsed)  Avg pause  Max
 pause
   Gen  0     74218 colls,     0 par    0.256s   0.246s     0.0000s
 0.0002s
   Gen  1         2 colls,     0 par    0.001s   0.001s     0.0003s
 0.0006s

   INIT    time    0.000s  (  0.000s elapsed)
   MUT     time   25.042s  ( 25.168s elapsed)
   GC      time    0.257s  (  0.247s elapsed)
   EXIT    time    0.000s  (  0.000s elapsed)
   Total   time   25.298s  ( 25.415s elapsed)

   %GC     time       1.0%  (1.0% elapsed)

   Alloc rate    3,085,464,250 bytes per MUT second

   Productivity  99.0% of total user, 99.0% of total elapsed

 ( cabal-run bench-test +RTS -s; )  25,30s user 0,12s system 99% cpu 25,423
 total


 === PATCHED
 ===================================================================

 luna git:(master) $ time (cabal-run bench-test +RTS -s)
   77,200,755,440 bytes allocated in the heap
      114,115,976 bytes copied during GC
          241,064 bytes maximum residency (2 sample(s))
           33,152 bytes maximum slop
                2 MB total memory in use (0 MB lost due to fragmentation)

                                      Tot time (elapsed)  Avg pause  Max
 pause
   Gen  0     74218 colls,     0 par    0.263s   0.254s     0.0000s
 0.0002s
   Gen  1         2 colls,     0 par    0.000s   0.001s     0.0003s
 0.0006s

   INIT    time    0.000s  (  0.000s elapsed)
   MUT     time   25.487s  ( 25.573s elapsed)
   GC      time    0.263s  (  0.254s elapsed)
   EXIT    time    0.000s  (  0.000s elapsed)
   Total   time   25.750s  ( 25.827s elapsed)

   %GC     time       1.0%  (1.0% elapsed)

   Alloc rate    3,029,012,929 bytes per MUT second

   Productivity  99.0% of total user, 99.0% of total elapsed

 ( cabal-run bench-test +RTS -s; )  25,75s user 0,08s system 100% cpu
 25,831 total
 }}}

 So it seems to me that with the different dictionary representation we're
 losing some optimization opportunities. I guess we could try to enable all
 optimizations again (with -O2) and selectively disable single optimization
 passes to see which one makes these two versions more similar. That may
 give an idea about which optimization is not applicable with the `Monad m
 =>` patch.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15176#comment:11>
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