#5441: unsafeCoerce'ing function to GHC.Prim.Any causes segfault
-----------------------------------------+----------------------------------
Reporter: ulfn | Owner: simonmar
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler | Version: 7.2.1
Resolution: fixed | Keywords:
Testcase: simplCore/should_run/T5441 | Blockedby:
Difficulty: | Os: Unknown/Multiple
Blocking: | Architecture: x86
Failure: Runtime crash |
-----------------------------------------+----------------------------------
Changes (by simonpj):
* status: new => closed
* testcase: => simplCore/should_run/T5441
* resolution: => fixed
Old description:
> The following module is the smallest one I've been able to find
> exhibiting the bug. Any simplification (that I've tried) results in the
> problem going away.
>
> module Any where
>
> import Unsafe.Coerce (unsafeCoerce)
> import GHC.Prim (Any)
>
> listmap :: (a -> b) -> [a] -> [b]
> listmap f [] = []
> listmap f (x : xs) = f x : listmap f xs
>
> data Nat = Z | S Nat
>
> {-# NOINLINE inject #-}
> inject :: Nat -> Nat -> Nat
> inject m i = i
>
> {-# NOINLINE look #-}
> look :: Nat -> String -> Char
> look Z _ = '0'
>
> showDigit :: Nat -> () -> Nat -> Char
> showDigit base prf d = look (inject base d) ""
>
> toDigits :: Nat -> Nat -> [Nat]
> toDigits Z Z = [Z]
>
> coe1 :: (Nat -> String) -> Any
> coe1 = unsafeCoerce
>
> coe2 :: Any -> (Nat -> String)
> coe2 = unsafeCoerce
>
> showInBase :: Nat -> Any
> showInBase base
> = coe1 (\n -> listmap
> (showDigit base ())
> (toDigits base n))
>
> showNat :: Nat -> String
> showNat = coe2 (showInBase Z)
>
> Now given this main module (it needs to be a different module)
>
> import Any
> main = putStrLn (showNat Z)
>
> Compiling with optimisations:
>
> $ ghc -O --make Main.hs
> ...
> $ ./Main
> Bus error: 10
>
> Without optimizations it prints 0 as expected.
>
> This has been reproduced on 7.0.3 (Mac OS X) and 7.2.1 (some Linux).
New description:
The following module is the smallest one I've been able to find exhibiting
the bug. Any simplification (that I've tried) results in the problem going
away.
{{{
module Any where
import Unsafe.Coerce (unsafeCoerce)
import GHC.Prim (Any)
listmap :: (a -> b) -> [a] -> [b]
listmap f [] = []
listmap f (x : xs) = f x : listmap f xs
data Nat = Z | S Nat
{-# NOINLINE inject #-}
inject :: Nat -> Nat -> Nat
inject m i = i
{-# NOINLINE look #-}
look :: Nat -> String -> Char
look Z _ = '0'
showDigit :: Nat -> () -> Nat -> Char
showDigit base prf d = look (inject base d) ""
toDigits :: Nat -> Nat -> [Nat]
toDigits Z Z = [Z]
coe1 :: (Nat -> String) -> Any
coe1 = unsafeCoerce
coe2 :: Any -> (Nat -> String)
coe2 = unsafeCoerce
showInBase :: Nat -> Any
showInBase base
= coe1 (\n -> listmap
(showDigit base ())
(toDigits base n))
showNat :: Nat -> String
showNat = coe2 (showInBase Z)
}}}
Now given this main module (it needs to be a different module)
{{{
import Any
main = putStrLn (showNat Z)
}}}
Compiling with optimisations:
{{{
$ ghc -O --make Main.hs
...
$ ./Main
Bus error: 10
}}}
Without optimizations it prints 0 as expected.
This has been reproduced on 7.0.3 (Mac OS X) and 7.2.1 (some Linux).
--
Comment:
Ulf, thank you for identifying a nice small test case. Very helpful.
Agda stresses `unsafeCoerce` like nothing else.
Try now.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5441#comment:5>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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