[GHC] #7684: cgrun071 segfaults

[GHC]

#7684: cgrun071 segfaults
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        Reporter:  tibbe          |            Owner:  leroux
            Type:  bug            |           Status:  new
        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 leroux):

 The problem has been pinpointed to the fact that apple-gcc42 does not
 support SSE4 instructions.

 After carter suggested that it may be a problem with apple-gcc42, I
 rebuilt ghc with gcc48 and cgrun071 passed both the normal and llvm ways.

 It seems that carter's theory was right in that gcc 4.2 does not have
 support for SSE4 instructions. In fact, support for SSE4 was added in with
 the release of gcc43. http://gcc.gnu.org/gcc-4.3/changes.html

 It may be that SSE42 support isn't being detected properly by nativeGen.
 https://github.com/ghc/ghc/blob/master/compiler/nativeGen/X86/CodeGen.hs#L1686-L1689

 When reproducing this, #8148 may get in the way. carter's helpful fix is
 to simply symlink to `gcc-4.8` as `gcc` and, add it to your PATH, then
 build ghc as normal.
 {{{
 $ mkdir ~/bin
 $ ln -s `which gcc-4.8` ~/bin/gcc
 $ export PATH=~/bin:$PATH
 $ # reload your shell and `gcc -v` should show 4.8 or whatever.
 }}}

 Should we switch to calling the "slower" popCount for this certain case?
 https://github.com/ghc/testsuite/blob/master/tests/codeGen/should_run/cgrun071.hs#L41-L46
 {{{
 slowPopcnt :: Word -> Word
 slowPopcnt x = count' (bitSize x) x 0
   where
     count' 0 _ !acc = acc
     count' n x acc  = count' (n-1) (x `shiftR` 1)
                       (acc + if x .&. 1 == 1 then 1 else 0)
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7684#comment:19>
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