[GHC] #7684: cgrun071 segfaults

[GHC]

#7684: cgrun071 segfaults
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        Reporter:  tibbe          |            Owner:
            Type:  bug            |           Status:  infoneeded
        Priority:  normal         |        Milestone:  7.8.1
       Component:  Compiler       |          Version:  7.7
      Resolution:                 |         Keywords:
Operating System:  MacOS X        |     Architecture:  x86_64 (amd64)
 Type of failure:  Runtime crash  |       Difficulty:  Unknown
       Test Case:  cgrun071       |       Blocked By:
        Blocking:                 |  Related Tickets:
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Comment (by tibbe):

 Replying to [comment:42 simonmar]:
 > Well, one bug is that in `emitPopCntCall` we don't cast the argument to
 the correct width.

 I don't think it should.

 The `MO_PopCnt` `MachOp` that's emitted by `emitPopCntCall` also takes the
 width as an argument. The reason we pass the width all the way down to the
 native and LLVM code generators is to avoid doing any narrowing, as we can
 emit a `popcnt` instruction that looks at just part of the word (e.g.
 `popcnt %ax,%ax`).

 If we end up not using the the dedicated instruction, and we're using GCC
 4.2 which doesn't do the narrowing correctly, then we'd have to do the
 narrowing ourselves before calling the C fallback.

 I think we have two choices in working around the buggy behavior of GCC
 4.2:

  1. Change the function prototype as rwbarton suggested.
  2. Do some narrowing in the native code generator before emitting the
 call to the C function.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7684#comment:46>
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