[GHC] #12585: GHC duplicates string literals in rodata section and breaks 'Ptr Addr#' equality

9 Sep 2016

      #12585: GHC duplicates string literals in rodata section and breaks 'Ptr Addr#'
equality
-------------------------------------+-------------------------------------
           Reporter:  slyfox         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 -- test-sha256.hs:
 {-# LANGUAGE MagicHash #-}
 module Main (main) where

 import GHC.Prim (Addr#)
 import GHC.Ptr (Ptr(..), minusPtr)

 bug :: Addr# -> IO ()
 bug a = do
     print ("cmp:", Ptr a == Ptr a)
     print ("delta:", Ptr a `minusPtr` Ptr a)
     print ("values:", Ptr a, Ptr a)

 main :: IO ()
 main = bug "Assumptions are subtle!"#
 }}}

 {{{
 $ inplace/bin/ghc-stage2 -fforce-recomp -O1 --make test-sha256.hs &&
 ./test-sha256
 [1 of 1] Compiling Main             ( test-sha256.hs, test-sha256.o )
 Linking test-sha256 ...
 ("cmp:",False)
 ("delta:",-24)
 ("values:",0x000000000072fdc0,0x000000000072fda8)
 }}}

 Stg shows that literal gets inlined:

 {{{
 $ inplace/bin/ghc-stage2 -fforce-recomp -O1 --make test-sha256 -ddump-stg
 -dsuppress-all -dsuppress-uniques 2>&1 | grep Assumptions

                                 eqAddr# ["Assumptions are subtle!"#
 "Assumptions are subtle!"#]
                                 minusAddr# ["Assumptions are subtle!"#
 "Assumptions are subtle!"#]
                                                     $w$cshowsPrec
 "Assumptions are subtle!"# w2
                                                     $w$cshowsPrec
 "Assumptions are subtle!"# w2
                                 eqAddr# ["Assumptions are subtle!"#
 "Assumptions are subtle!"#]
                                 minusAddr# ["Assumptions are subtle!"#
 "Assumptions are subtle!"#]
                                                     $w$cshowsPrec
 "Assumptions are subtle!"# w2
                                                     $w$cshowsPrec
 "Assumptions are subtle!"# w2
 }}}

 I've found this bug as a SIGSEGV on testsuite cryptohash-sha256-0.11.100.1
 from hackage.

 Bytestring assumes that address does not change and implements
 loops over Ptrs
 https://github.com/haskell/bytestring/blob/master/Data/ByteString.hs#L1171
 :

 {{{#!hs
 filter :: (Word8 -> Bool) -> ByteString -> ByteString
 filter k ps@(PS x s l)
     | null ps   = ps
     | otherwise = unsafePerformIO $ createAndTrim l $ \p -> withForeignPtr
 x $ \f -> do
         t <- go (f `plusPtr` s) p (f `plusPtr` (s + l))
         return $! t `minusPtr` p -- actual length
     where
         go !f !t !end | f == end  = return t
                       | otherwise = do
                           w <- peek f
                           if k w
                             then poke t w >> go (f `plusPtr` 1) (t
 `plusPtr` 1) end
                             else             go (f `plusPtr` 1) t
 end
 {-# INLINE filter #-}
 }}}

 In case of cryptohash-sha256-0.11.100.1 '''t <- go (f `plusPtr` s) p (f
 `plusPtr` (s + l))''' for literal inlined righ at 'f' call which caused
 testsuite failure.

 It seems sensible not to emit the literal more than once into
 '''.rodata''' section.

 It won't guard against problems where literal is exported as a part of .hi
 file but might be good enough for common cases like this.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12585
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