[GHC] #1371: Add -O3

[GHC]

#1371: Add  -O3
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        Reporter:           |            Owner:
  simonmar                  |           Status:  new
            Type:  task     |        Milestone:  _|_
        Priority:  normal   |          Version:  6.6.1
       Component:           |         Keywords:
  Compiler                  |     Architecture:  Unknown/Multiple
      Resolution:           |       Difficulty:  Moderate (less than a day)
Operating System:           |       Blocked By:
  Unknown/Multiple          |  Related Tickets:
 Type of failure:           |
  None/Unknown              |
       Test Case:           |
        Blocking:           |
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Comment (by carter):

 @schyler, who is doing that? -O3 on LLVM's opt has very version specific
 meaning, and (currently, according to discussions on the llvm-devs mailing
 list), has much less clear semantics than does -O3 in gcc.  Do you have
 specific examples where llvm's O3 improves ghc code's perf? What LLVM
 version (please share).

 its worth noting that generally on LLVM, many optimizations are in O3 to
 denote experimental / doesn't always work status, and then get promoted ot
 O1 / O2 when they have more robust code improvement characteristics

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/1371#comment:10>
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