[GHC] #7684: cgrun071 segfaults

[GHC]

#7684: cgrun071 segfaults
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        Reporter:  tibbe          |            Owner:
            Type:  bug            |           Status:  patch
        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 rwbarton):

 Replying to [comment:40 simonmar]:
 > The patch is very likely to be papering over the problem rather than
 fixing it.  While the primop does take a Word# (because there's no
 `Word8#`), the MachOp `MO_PopCnt` really does take an 8-bit value when it
 is generated by the 8-bit version of the primop.

 How so? Of course we want it to be true, but I don't see any place where
 GHC is aware of this.

 Let's trace through the compilation of a PopCnt8Op primop versus a
 PopCnt16Op primop. The only place where these are treated differently is
 in `emitPrimOp`, which calls `emitPopCntCall` with an argument of `W8` in
 the former and `W16` in the latter case. `emitPopCntCall` only uses that
 argument as a field of the `MO_PopCnt` constructor. It doesn't do any kind
 of cast or assertion on the size of its argument. In the native code
 generator, the field of `MO_PopCnt` is only used to build the name of the
 C function to call (assuming no SSE). So, it's unsurprising to me that GHC
 does not, for example, narrow the argument to `hs_popcnt8` to 8 bits, but
 narrow the argument to `hs_popcnt16` to 16 bits.

 Maybe you think some part of the above is a bug, and prefer to fix the
 problem there. I didn't want to guess where the bug was, so I proposed
 this patch to the C implementation to match the calling convention used by
 code generator.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7684#comment:41>
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