Re: [GHC] #12622: Unboxed static pointers lead to missing SPT entries

2 Jan 2017

      #12622: Unboxed static pointers lead to missing SPT entries
-------------------------------------+-------------------------------------
        Reporter:  mboes             |                Owner:
                                     |  facundo.dominguez
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D2709
       Wiki Page:                    |  Phab:D2720
-------------------------------------+-------------------------------------

Comment (by facundo.dominguez):

 Hello, I have an implementation of this approach using `makeStatic`. It
 works most of the time, but I'm having some strange behavior when building
 with `--fno-full-laziness`.

 This is the test program:
 {{{
 {-# LANGUAGE LambdaCase         #-}
 {-# LANGUAGE StaticPointers     #-}

 -- | A test to use symbols produced by the static form.
 module Main(main) where

 import GHC.StaticPtr

 main :: IO ()
 main = do
   lookupKey (static (id . id)) >>= \f -> print $ f (1 :: Int)

 lookupKey :: StaticPtr a -> IO a
 lookupKey p = unsafeLookupStaticPtr (staticKey p) >>= \case
   Just p -> return $ deRefStaticPtr p
   Nothing -> error $ "couldn't find " ++ show (staticPtrInfo p)
 }}}

 At some intermediate phase, core looks like this:
 {{{
 -- RHS size: {terms: 3, types: 2, coercions: 0}
 lvl_s2fm :: StaticPtr (Int -> Int)
 [LclId, Str=x]
 lvl_s2fm =
   base-4.10.0.0:GHC.StaticPtr.Internal.makeStatic
     @ (Int -> Int) lvl_s2fk lvl_s2fl

 -- RHS size: {terms: 3, types: 3, coercions: 0}
 p_aEP [OS=OneShot] :: StaticPtr (Int -> Int)
 [LclId]
 p_aEP =
   fromStaticPtr
     @ StaticPtr
     GHC.StaticPtr.$fIsStaticStaticPtr
     @ (Int -> Int)
     lvl_s2fm

 -- RHS size: {terms: 2, types: 2, coercions: 0}
 main_s2eP :: StaticKey
 [LclId]
 main_s2eP = staticKey @ (Int -> Int) p_aEP

 -- RHS size: {terms: 2, types: 2, coercions: 0}
 main_s2eO :: IO (Maybe (StaticPtr (Int -> Int)))
 [LclId, Arity=1]
 main_s2eO = unsafeLookupStaticPtr @ (Int -> Int) main_s2eP

 ...
 }}}

 Before the call to `makeStatic` is replaced with an entry in the static
 pointer table, a simplifier pass labeled as
 {{{
 ==================== Simplifier ====================
   Max iterations = 4
   SimplMode {Phase = 2 [main],
              inline,
              rules,
              eta-expand,
              case-of-case}
 Result size of Simplifier = {terms: 42, types: 55, coercions: 9}
 }}}

 transforms it to
 {{{
 -- RHS size: {terms: 17, types: 13, coercions: 0}
 lvl_s2fm :: StaticPtr (Int -> Int)
 [LclId,
  Str=x,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=False, ConLike=False,
          WorkFree=False, Expandable=True, Guidance=NEVER}]
 lvl_s2fm =
   base-4.10.0.0:GHC.StaticPtr.Internal.makeStatic
     @ (Int -> Int)
     (GHC.StaticPtr.StaticPtrInfo
        (GHC.Base.build
           @ Char
           (\ (@ b_a2dI) -> GHC.CString.unpackFoldrCString# @ b "main"#))
        (GHC.Base.build
           @ Char
           (\ (@ b_a2dI) -> GHC.CString.unpackFoldrCString# @ b "Main"#))
        (GHC.Types.I# 13#, GHC.Types.I# 21#))
     (\ (x_a2dj :: Int) -> x_a2dj)

 -- RHS size: {terms: 3, types: 5, coercions: 0}
 main_s32D
   :: GHC.Prim.State# GHC.Prim.RealWorld
      -> (# GHC.Prim.State# GHC.Prim.RealWorld, () #)
 [LclId,
  Arity=1,
  Unf=Unf{Src=<vanilla>, TopLvl=True, Value=True, ConLike=True,
          WorkFree=True, Expandable=True,
          Guidance=ALWAYS_IF(arity=1,unsat_ok=True,boring_ok=True)}]
 main_s32D = \ _ [Occ=Dead] -> case lvl_s2fm of wild_00 { }
 }}}
 which looks wrong, as the program becomes now a case with an empty list of
 alternatives.

 This is the definition of `makeStatic`
 {{{
 module GHC.StaticPtr.Internal (makeStatic) where

 import GHC.StaticPtr

 {-# NOINLINE makeStatic #-}
 makeStatic :: StaticPtrInfo -> a -> StaticPtr a
 makeStatic (StaticPtrInfo pkg m (line, col)) _ =
     error $ "makeStatic: Unresolved static form at " ++ pkg ++ ":" ++ m ++
 ":"
             ++ show line ++ ":" ++ show col
 }}}

 Perhaps the simplifier is somehow using the fact that `makeStatic` calls
 to error despite of the function being tagged with `NOINLINE`?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12622#comment:17
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