[GHC] #8404: Default to turning on architecture specific optimizations in the codegen

[GHC]

#8404: Default to turning on architecture specific optimizations in the codegen
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       Reporter:  tibbe             |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 LLVM defaults to turning on whatever architecture specific optimizations
 are supported on the user's machine (e.g. `-msse4.2`). This makes programs
 run faster during testing and generally gives users a better perception of
 the compiler. If the user wants to compile for some more portable subset
 (e.g. when cross-compiling) he/she adds `-march`, `-mcpu`, etc flags on
 the command line.

 To implement this we'd have to detect what CPU type is used etc and
 initialize our `DynFlags` based on that.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8404>
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