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

25 Sep 2016

      #12622: Unboxed static pointers lead to missing SPT entries
-------------------------------------+-------------------------------------
        Reporter:  mboes             |                Owner:
                                     |  facundominguez
            Type:  bug               |               Status:  new
        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):
       Wiki Page:                    |
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Comment (by facundo.dominguez):

 Here's a smaller test case:
 {{{
 -- A.hs
 {-# LANGUAGE BangPatterns #-}
 {-# LANGUAGE StaticPointers #-}
 module A where

 import Data.Typeable
 import GHC.StaticPtr

 g :: a -> Bool
 g _ = True

 data T a = T {-# UNPACK #-} !(StaticPtr a)

 sg :: Typeable a => T (a -> Bool)
 sg = T (static g)
 }}}
 {{{
 -- Main.hs
 {-# LANGUAGE StaticPointers #-}
 {-# LANGUAGE LambdaCase #-}

 import GHC.StaticPtr
 import A

 g = True

 main :: IO ()
 main = do
   let T s = sg :: T (Bool -> Bool)
   lookupKey s >>= \f -> print (f True)

 lookupKey :: StaticPtr a -> IO a
 lookupKey p = unsafeLookupStaticPtr (staticKey p) >>= \case
   Just p -> return $ deRefStaticPtr p
   Nothing -> error $ "couldn't find " ++ show (staticPtrInfo p)
 }}}
 Build with
 {{{
 $ ghc -O Main.hs
 [1 of 2] Compiling A                ( A.hs, A.o )
 [2 of 2] Compiling Main             ( Main.hs, Main.o )
 Linking Main ...
 $ ./Main
 Main: couldn't find StaticPtrInfo {spInfoUnitId = "main", spInfoModuleName
 = "A", spInfoSrcLoc = (14,16)}
 CallStack (from HasCallStack):
   error, called at Main.hs:17:14 in main:Main
 }}}

 Using `-dverbose-core2core` one can see that the FloatOut pass does the
 right thing (i.e. moving the static form to the top-level in Main.hs),
 {{{
 lvl_sG5 :: forall a_aEy. StaticPtr (a_aEy -> Bool)
 lvl_sG5 =
   \ (@ a_aEy) ->
     GHC.StaticPtr.StaticPtr
       @ (a_aEy -> Bool)
       13520098690657238824##
       6110703080284699228##
       lvl_sG4
       (g @ a_aEy)
 }}}
 but the simplifier later rewrites the top-level binding to use the T
 constructor instead:
 {{{
 lvl_sG7 :: forall a_aEy. T (a_aEy -> Bool)
 lvl_sG7 =
   \ (@ a_aEy) ->
     A.T
       @ (a_aEy -> Bool)
       13520098690657238824##
       6110703080284699228##
       lvl_sGn
       (g @ a_aEy)
 }}}
 Thus, when the SPT is built, the StaticPtr constructor is not found and
 the entry is never inserted.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12622#comment:2
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