New subject: [GHC] #15445: SPECIALIZE one of two identical functions does not fire well (was: SPCIALIZE one of two identical functions does not fire well)

27 Jul 2018

      #15445: SPCIALIZE one of two identical functions does not fire well
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           Reporter:  nobrakal        |             Owner:  (none)
               Type:  bug             |            Status:  new
           Priority:  normal          |         Milestone:  8.6.1
          Component:  Compiler        |           Version:  8.4.3
           Keywords:                  |  Operating System:  Linux
       Architecture:  x86_64 (amd64)  |   Type of failure:  None/Unknown
          Test Case:                  |        Blocked By:
           Blocking:                  |   Related Tickets:
Differential Rev(s):                  |         Wiki Page:
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 Hi,

 I am playing with `SPECIALIZE` pragma:

 {{{#!hs
 module Todo where

 {-# SPECIALIZE plusTwoRec :: [Int] -> [Int] #-}
 plusTwoRec :: Num a => [a] -> [a]
 plusTwoRec [] = []
 plusTwoRec (x:xs) = x+2:plusTwoRec xs

 plusTwoRec' :: Num a => [a] -> [a]
 plusTwoRec' [] = []
 plusTwoRec' (x:xs) = x+2:plusTwoRec' xs
 }}}

 And wanted to benchmark it with (in `Main.hs`):

 {{{#!hs
 import Todo
 import Criterion.Main

 aListOfInt :: [Int]
 aListOfInt = [1..10000]

 main :: IO ()
 main = defaultMain
   [ bench "plusTwoRec" $ nf plusTwoRec aListOfInt
   , bench "plusTwoRec'" $  nf plusTwoRec' aListOfInt
   ]
 }}}

 Sadly, the rule of specialization of `plusTwoRec` does not fire in Main.hs
 (I compiled with:
 `ghc Main.hs -O -dynamic -ddump-rule-firings` (the `-dynamic` part is due
 to my ArchLinux installaltion)).

 The result is:
 {{{
 [1 of 2] Compiling Todo             ( Todo.hs, Todo.o )
 Rule fired: Class op + (BUILTIN)
 Rule fired: Class op fromInteger (BUILTIN)
 Rule fired: integerToInt (BUILTIN)
 Rule fired: SPEC plusTwoRec (Todo)
 [2 of 2] Compiling Main             ( Main.hs, Main.o )
 Rule fired: Class op enumFromTo (BUILTIN)
 Rule fired: unpack (GHC.Base)
 Rule fired: unpack (GHC.Base)
 Rule fired: eftIntList (GHC.Enum)
 Rule fired: unpack-list (GHC.Base)
 Rule fired: unpack-list (GHC.Base)
 Linking Main ...
 }}}

 I have inspected a bit the code produced after the simplifications passes
 (with `-ddump-simpl`) and here is the suspicious part:

 {{{
 plusTwoRec :: forall a. Num a => [a] -> [a]
 [GblId,
  Arity=2,
  Caf=NoCafRefs,
  Str=,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=0,unsat_ok=True,boring_ok=True)}]
 plusTwoRec = plusTwoRec'
 }}}

 I believe that `plusTwoRec` is inlined before the specialization has a
 chance to fire, but I am not sure at all !

 Separating the two functions definitions in two different files works.

 So I don't know if this is a GHC bug, myself that does not read the right
 part of the GHC manual, if it is only a lack of documentation, or anything
 else.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15445
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler

[GHC] #15445: SPCIALIZE one of two identical functions does not fire well

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