[GHC] #7574: Register allocator chokes on certain branches with literals

[GHC]

#7574: Register allocator chokes on certain branches with literals
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    Reporter:  thoughtpolice     |       Owner:                    
        Type:  bug               |      Status:  new               
    Priority:  normal            |   Milestone:                    
   Component:  Compiler (NCG)    |     Version:  7.7               
    Keywords:  ncg, codegen      |          Os:  Unknown/Multiple  
Architecture:  Unknown/Multiple  |     Failure:  Compile-time crash
  Difficulty:  Unknown           |    Testcase:                    
   Blockedby:  7573              |    Blocking:                    
     Related:  #7571             |  
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Changes (by simonmar):

  * difficulty:  => Unknown


Comment:

 The problem here is that the else-block becomes unreachable after
 `cmmStmtConFold` optimises away the conditional, and the register
 allocator doesn't like unreachable code.

 The right fix is to discard unreachable code before register allocation.
 It already does a strongly-connected-component analysis so this shouldn't
 be too hard, but IIRC when I tried to do this before it wasn't
 straightforward because `Digraph` doesn't expose the right bits.  We need
 an SCC pass that starts from a particular node, not the entire set of
 nodes.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7574#comment:5>
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