darcs patch: Immediately tag initialization code to prevent untagge...

[Edward Z. Yang]

1 patch for repository http://darcs.haskell.org/ghc:

Wed Mar 23 13:43:30 GMT 2011  Edward Z. Yang <ezyang at mit.edu>
  * Immediately tag initialization code to prevent untagged spills.
  
  When allocating new objects on the heap, we previously returned
  a CmmExpr containing the heap pointer as well as the tag expression,
  which would be added to the code graph upon first usage.  Unfortunately,
  this meant that untagged heap pointers living in registers might
  be spilled to the stack, where they interacted poorly with garbage
  collection (we saw this bug specifically with the compacting garbage
  collector.)
  
  This fix immediately tags the register containing the heap pointer,
  so that unless we have extremely unfriendly spill code, the new pointer
  will never be spilled to the stack untagged.
  
  An alternate solution might have been to modify allocDynClosure to
  tag the pointer upon the initial register allocation, but not all
  invocations of allocDynClosure tag the resulting pointer, and
  threading the consequent CgIdInfo for the cases that did would have
  been annoying.
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