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

[GHC]

#12603: INLINE and manually inlining produce different code
-------------------------------------+-------------------------------------
        Reporter:  bgamari           |                Owner:  bgamari
            Type:  task              |               Status:  new
        Priority:  normal            |            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:                    |
-------------------------------------+-------------------------------------
Changes (by MikolajKonarski):

 * related:   => 12747, 12781


@@ -8,0 +8,38 @@
+
+
+ Edit: 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)
+ }}}

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: 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:

 I've created a separate ticket for the case 2 (much more allocation with
 INLINE than NOINLINE): https://ghc.haskell.org/trac/ghc/ticket/12781

 So, now cases 3 and 2 have separate tickets and so I move (a new version
 of) minimal example for case 1 to the main ticket description.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12603#comment:19>
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