#12603: INLINE and manually inlining produce different code -------------------------------------+------------------------------------- Reporter: bgamari | Owner: bgamari Type: bug | Status: new Priority: high | Milestone: 8.2.1 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: #12747 #12781 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by MikolajKonarski: @@ -46,0 +46,4 @@ + + My guess is the inlining forced with INLINE is just broken in GHC 8.0.1 + --- it omits some optimizations that ghc would normally do on manually + inlined code, such as floating out common subexpressions. 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/97724fe8c73e80b329ddf326a8eb00.... 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) }}} My guess is the inlining forced with INLINE is just broken in GHC 8.0.1 --- it omits some optimizations that ghc would normally do on manually inlined code, such as floating out common subexpressions. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12603#comment:25 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler