[GHC] #8949: switch -msse2 to be on by default

[GHC]

#8949: switch -msse2 to be on by default
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       Reporter:  errge                       |             Owner:
           Type:  bug                         |            Status:  new
       Priority:  normal                      |         Milestone:
      Component:  Compiler (CodeGen)          |           Version:  7.9
       Keywords:                              |  Operating System:
   Architecture:  x86                         |  Unknown/Multiple
     Difficulty:  Moderate (less than a day)  |   Type of failure:  Other
     Blocked By:                              |         Test Case:
Related Tickets:                              |          Blocking:
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 I propose msse2 to be on by default.  I guess the default was chosen way
 back, when Pentium III support was still relevant.

 Nowadays we don't really win on the CPU support, because e.g.
 https://github.com/tibbe/hashable/blob/master/hashable.cabal is built by
 default with sse2 on the injected C code level.  And hashable has a lot of
 reverse depends, therefore on an end user system (RedHat or Debian) the
 user is most probably unlucky with a Pentium III CPU anyways.

 Flipping this default would also fix the excess precision problem for most
 users of GHC on the i686 platform.

 GHC should provide a -mno-sse2 flag for the cases when this needs to be
 disabled.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8949>
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