#2289: Needless reboxing of values when returning from a tight loop -------------------------------------+------------------------------------- Reporter: dons | Owner: Type: bug | Status: new Priority: lowest | Milestone: Component: Compiler | Version: 6.8.2 Resolution: | Keywords: boxing, | loops, performance Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #2387,#1600 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by twhitehead): I was just going to open a ticket to report that I had discovered that the compiler can't unbox multiple return values. That is, anytime you create a non-inlined function (such as a loop) that returns multiple values, you must pay the boxing and unboxing penalty. My reading of these tickets (#1600, #2289, and #2387) though is that this is already known. I'll just add a simple example here {{{ #!haskell main :: IO () main = case loop2 100 (10,10) of (au,ad) -> print (au - ad) loop2 :: Int -> (Int,Int) -> (Int,Int) loop2 n (au,ad) | n > 0 = loop2 (n-1) (au+1,ad-1) | otherwise = (au,ad) }}} yielding (with ''-ddump-simpl -dsuppress-all -dno-suppress-type- signatures'') {{{ #!haskell Rec { main_$s$wloop2 [Occ=LoopBreaker] :: Int# -> Int# -> Int# -> (# Int, Int #) [GblId, Arity=3, Caf=NoCafRefs, Str=DmdType <L,U><L,U><L,U>] main_$s$wloop2 = \ (sc_s3vn :: Int#) (sc1_s3vo :: Int#) (sc2_s3vp :: Int#) -> case tagToEnum# (># sc_s3vn 0) of _ [Occ=Dead] { False -> (# I# sc1_s3vo, I# sc2_s3vp #); True -> main_$s$wloop2 (-# sc_s3vn 1) (+# sc1_s3vo 1) (-# sc2_s3vp 1) } end Rec } main1 :: State# RealWorld -> (# State# RealWorld, () #) [GblId, Arity=1, Str=DmdType <L,U>, Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True, WorkFree=True, Expandable=True, Guidance=IF_ARGS [0] 171 0}] main1 = \ (eta_B1 [OS=OneShot] :: State# RealWorld) -> case main_$s$wloop2 100 10 10 of _ [Occ=Dead] { (# ww1_s3uF, ww2_s3uG #) -> hPutStr2 stdout (case ww1_s3uF of _ [Occ=Dead] { I# x_a1d4 -> case ww2_s3uG of _ [Occ=Dead] { I# y_a1d8 -> case $wshowSignedInt 0 (-# x_a1d4 y_a1d8) ([]) of _ [Occ=Dead] { (# ww5_a1xQ, ww6_a1xR #) -> : ww5_a1xQ ww6_a1xR } } }) True eta_B1 } }}} Cheers! -Tyson PS: The perfect performance storm comes when you have a very fast inner loop returning multiple values to an outer loop that evaluates the inner loop many many times. * The inner loop can't be inlined because it is a loop. * The return values from the inner loop must be boxed and unboxed because GHC doesn't seem to be able to do otherwise when there are multiple return values. * The inner loop being very fast means the relative cost of boxing and unboxing is high. * The outer loop repeating the inner loop many many times turns the high relative cost into a high absolute cost. In my specific case I was processing around 10G worth of text records. The inner loop found the next field separator. The outer loop built the data records. It was a nasty hit. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2289#comment:44> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler