[GHC] #15304: Huge increase of compile time and memory use from 8.0.2 to 8.2.2 or 8.4.2

[GHC]

#15304: Huge increase of compile time and memory use from 8.0.2 to 8.2.2 or 8.4.2
-------------------------------------+-------------------------------------
        Reporter:  NathanWaivio      |                Owner:  tdammers
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Compile-time      |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by tdammers):

 This is very useful. The reduced `Cl3.hs` with all `Floating` methods
 except `exp` and `log` removed gives a much cleaner result. Looking at
 `-ddump-rule-firings` output, here's counters of how often each rule fired
 on 8.0.2 and 8.4.3:

 {{{
 rule                      |  8.0  8.4
 --------------------------+------------
 *##                       |  127   44
 +##                       |   26  372
 ^2/Integer                |   55   55
 Class op -                |  419  417
 Class op /                |   15    8
 Class op *                | 1698 1693
 Class op **               |    3    2
 Class op +                |  737  734
 Class op abs              |    5    5
 Class op atan2            |    1    1
 Class op cos              |    7    5
 Class op cosh             |    3    2
 Class op exp              |   14   10
 Class op fromInteger      |   13    8
 Class op fromRational     |    4    2
 Class op log              |   20   14
 Class op log1p            |    6    4
 Class op negate           |  109  106
 Class op $p1Floating      |   25   16
 Class op $p1Fractional    |   20   12
 Class op pi               |    1    1
 Class op recip            |    4    3
 Class op sin              |    7    5
 Class op sinh             |    3    2
 Class op sqrt             |   14   14
 doubleFromInteger         |    9    7
 SC:$clog0                 |    6    0
 SC:$w$catan20             |    1    0
 }}}

 So it looks like a change to the way `SpecConstr`s are handled is
 preventing either specializations, or `RULES` that follow from them.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15304#comment:13>
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