[GHC] #8508: Inlining Unsaturated Function Applications

[GHC]

#8508: Inlining Unsaturated Function Applications
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       Reporter:  crockeea    |             Owner:
           Type:  bug         |            Status:  new
       Priority:  normal      |         Milestone:
      Component:  Compiler    |           Version:  7.6.2
       Keywords:              |  Operating System:  Linux
   Architecture:  x86_64      |   Type of failure:  Runtime performance bug
  (amd64)                     |         Test Case:
     Difficulty:  Unknown     |          Blocking:
     Blocked By:              |
Related Tickets:              |
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 After trying a simple test, I noticed some strange performance results
 from stylistic changes to the code.

 For example,
 {{{#!haskell
 import qualified Data.Vector.Unboxed as U

 {-# INLINE f #-}
 f :: U.Vector Int -> U.Vector Int -> U.Vector Int
 f = U.zipWith (+) -- version 1
 --f x = U.zipWith (+) x -- version 2
 --f x = (U.zipWith (+) x) . id -- version 3
 --f x y = U.zipWith (+) x y -- version 4

 main = do
   let iters = 100
       dim = 221184
       y = U.replicate dim 0 :: U.Vector Int
   let ans = iterate (f y) y !! iters
   putStr $ (show $ U.foldl1' (+) ans)
 }}}

 Versions 1 and 2 of `f` run in 1.6 seconds, while versions 3 and 4 run in
 0.09 seconds (with vector-0.10.9.1 and GHC 7.6.2, compiling with -O2).

 According to an answer on the Vector trac (link below), this problem is
 because GHC only inlines on saturated function applications. Is there any
 way to expand the cases when GHC inlines to avoid having coding style
 affect performance?

 * [https://github.com/haskell/vector/issues/4 Vector Trac]

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8508>
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