[GHC] #7361: Segmentation fault on 5f37e0c71ff4af8539c5aebc739b006b4f0c6ebf

[GHC]

#7361: Segmentation fault on 5f37e0c71ff4af8539c5aebc739b006b4f0c6ebf
---------------------------------+------------------------------------------
    Reporter:  bgamari           |       Owner:  simonmar        
        Type:  bug               |      Status:  infoneeded      
    Priority:  highest           |   Milestone:  7.8.1           
   Component:  Compiler          |     Version:  7.7             
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  Runtime crash   
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Changes (by simonmar):

  * status:  new => infoneeded


Comment:

 I reproduced it again today, but the bug still eludes me.  The code is
 reading and writing beyond the end of a `ByteArray#`, and I can't tell
 whether this is a codegen bug or a library bug that has gone unnoticed so
 far.

 The crash still happens with `-fno-cmm-sink` and with `-fllvm`, which
 eliminates the two things that I would be most suspicious of (the Cmm
 sinking pass and the native code generator).

 The code that writes over the end of the array is
 `Data.Random.Source.MWC.$wa`.  This function is large and complicated and
 the code is TH-generated, so I'm a bit lost here.  I tried to follow the
 code and couldn't see any codegen bugs though.

 The erroneous access happens on the 3rd call, where the arguments are a
 normal-looking `MVector`:

 {{{
 (gdb) p4 0x2aaaac40bde9
 0x2aaaac40be00: 0x102
 0x2aaaac40bdf8: 0x0
 0x2aaaac40bdf0: 0x2aaaac405628
 0x2aaaac40bde8: 0x4cc3d0
 <vectorzm0zi10zi0zi1_DataziVectorziPrimitiveziMutable_MVector_con_info>
 }}}

 and an `Int#` value 1.

 Maybe someone familiar with `random-source` could add some debugging tests
 and try to narrow down the failure?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7361#comment:21>
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