[GHC] #12817: Degraded performance with constraint synonyms

[GHC]

#12817: Degraded performance with constraint synonyms
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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:                    |
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Description changed by crockeea:

@@ -27,0 +27,4 @@
+
+ For some context, in my real code from which this example was extracted,
+ the 'fast' (no constraint synonym) version takes 78 microseconds, while
+ the 'slow' version with the synonym takes 360 microseconds.

New description:

 The attached bug demonstrates how using constraint synonyms can negatively
 impact performance.

 I have two functions which are identical, except that one uses a
 constraint synonym:

 {{{
 type CElt r = (Num r, Eq r)

 fooSlow :: (CElt r) => Bar r -> Bar r
 {-# INLINABLE fooSlow #-}
 fooSlow (C1 u) = either (fromEitherBar . eitherFoo) (fromEitherBar .
 eitherFoo) u
 fooSlow (C2 c) = C2 $ fooSlow c

 fooFast :: (Num r, Eq r) => Bar r -> Bar r
 {-# INLINABLE fooFast #-}
 fooFast (C1 u) = either (fromEitherBar . eitherFoo) (fromEitherBar .
 eitherFoo) u
 fooFast (C2 c) = C2 $ fooFast c
 }}}

 Using criterion, `fooSlow` is about 3x slower than `fooFast`. Main.hs
 needs `criterion` and `deepseq`, but the problem should be evident just
 from inspecting the core of Bar.hs, which only relies on `base`. Code was
 compiled with GHC-8.0.1 with `-O2`.

 For some context, in my real code from which this example was extracted,
 the 'fast' (no constraint synonym) version takes 78 microseconds, while
 the 'slow' version with the synonym takes 360 microseconds.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12817#comment:2>
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