[GHC] #14035: Weird performance results.

[GHC]

#14035: Weird performance results.
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        Reporter:  danilo2           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 I have made some progress.

 (1), I discovered that `-XStrict` was generating some stunningly bad
 desugarings for very ordinary function bindings. I have a fix in the
 works.  This seems to be responsible for almost all the performance loss.

 (2), look at your code
 {{{
 FailParser ma >>= f = FailParser $ do
     T b  a  <- ma
     T b' a' <- fromFailParser $ f a
     return $ T (b ||| b') a'
 }}}
 If `b` turns out to be `XFalse`, this amounts to
 {{{
 FailParser ma >>= f = FailParser $ do
     T b  a  <- ma
     T b' a' <- fromFailParser $ f a
     return $ T b' a'
 }}}
 and GHC can re-use the (T b' a') that `fromFailParser` returned.  Moreover
 in the critical inner loop `b` is indeed `XFalse`:
 {{{
 parserLoop = parserStep >> parserLoop
 }}}
 because `return` returns `XFalse`.

 But if you change the `(>==)` to
 {{{
     return $ T b a'
 }}}
 now GHC can't re-use anthing, and so allocate a fresh `T` every time round
 the loop.  So an apparently simpler program is actually more complicated!

 But (2) is not a massive effect.  The big thing is the desugaring.  Stay
 tuned.

 Meanwhile, try without `-XStrict`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14035#comment:6>
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