[GHC] #12603: INLINE and manually inlining produce different code

[GHC]

#12603: INLINE and manually inlining produce different code
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        Reporter:  bgamari           |                Owner:  bgamari
            Type:  task              |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       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:  #12747 #12781     |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by MikolajKonarski):

 * priority:  normal => high


@@ -10,1 +10,1 @@
- Edit: here is a minimal example:
+ Edit, by Mikolaj: here is a minimal example:

New description:

 Mikolaj reported that he was seeing significantly different code generated
 in the case of an `INLINE` pragma versus manually inlining. I haven't
 looked into what the cause it and this isn't necessarily problematic; this
 is just a reminder to look into what is happening.

 See
 https://github.com/LambdaHack/LambdaHack/blob/97724fe8c73e80b329ddf326a8eb001020870b2d/Game/LambdaHack/Common/Color.hs#L99.


 Edit, by Mikolaj: here is a minimal example:

 {{{
 -- ghc --make Main.hs -O1; ./Main +RTS -s -RTS

 seqFrame2 :: [Int] -> IO ()
 {-# NOINLINE seqFrame2 #-}
 seqFrame2 l = do
   let crux = attrFromInt
   --   Total   time    2.052s  (  2.072s elapsed)
   -- but the following version is many times slower:
   -- let crux = attrFromIntINLINE
   --   Total   time    7.896s  (  7.929s elapsed)
   mapM_ (\a -> crux a `seq` return ()) l

 main :: IO ()
 main = seqFrame2 $ replicate 100000000 0


 data Attr = Attr !Int  --- the bang is essential

 attrFromInt :: Int -> Attr
 {-# NOINLINE attrFromInt #-}
 attrFromInt w = Attr (w + (2 ^ (8 :: Int)))

 fgFromInt :: Int -> Int
 {-# INLINE fgFromInt #-}  -- removing this INLINE makes it many times
 faster
                           -- just like the manually inlined version
                           -- and NOINLINE lands in between
 fgFromInt w = w + (2 ^ (8 :: Int))

 attrFromIntINLINE :: Int -> Attr
 {-# NOINLINE attrFromIntINLINE #-}
 attrFromIntINLINE w = Attr (fgFromInt w)
 }}}

--

Comment:

 IMHO, inlining forced with INLINE is just broken in GHC 8.0.1 --- it omits
 some optimizations that ghc normally does when performing the inlining
 without pragmas (may be related to the fact that .hi files function code
 with INLINE is not optimized, as opposed INLINABLE or without pragmas). So
 I'm bumping the priority.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12603#comment:21>
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