[GHC] #14346: 8.2.1 regression: heap corruption after safe foreign calls

[GHC]

#14346: 8.2.1 regression: heap corruption after safe foreign calls
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        Reporter:  andrewchen        |                Owner:  (none)
            Type:  bug               |               Status:  infoneeded
        Priority:  highest           |            Milestone:
       Component:  Runtime System    |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime crash     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonmar):

 Interesting, can someone boil down the transformation that dropped the
 `touch#`?

 Simon: `touch#` is keeping the `ByteArray#` alive until after the
 `action`, in `allocaBytes` (see comment:18). The action itself doesn't
 keep the array alive, because it is working with the raw pointer, not the
 `ByteArray#`.  This is how we allocate temporary memory for marshalling
 data between Haskell and C, because it's a lot faster to allocate memory
 on the Haskell heap than to use `malloc()` and `free()`.

 I imagine the simplifier has proven that `action` never returns and then
 dropped the `case` with the continuation containing the `touch#`.  That
 seems like a reasonable thing to do.

 I like @bgamari's alternative suggestion of `with#`, although we probably
 want it to be

 {{{
 with# :: a -> (State# s -> (# State# s, b #)) -> State# s -> (# State# s,
 b #)
 }}}

 otherwise the second argument must be a thunk (yuck).

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14346#comment:20>
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