[GHC] #14672: Make likelyhood of branches/conditions available throughout the compiler.

[GHC]

#14672: Make likelyhood of branches/conditions available throughout the compiler.
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        Reporter:  AndreasK          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D4316
       Wiki Page:                    |  Phab:D4324 Phab:D4327
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Comment (by svenpanne):

 Just a general remark: [https://phabricator.haskell.org/D4327 D4327] talks
 about annotations as a source of likelihood values, which is fine, but
 having actual data from previous runs, i.e. using profile-guided
 optimization, would probably have much more potential. Humans are
 notoriously bad at guessing what actually eats performace, so manual
 annotations can only get you so far.

 As an example: The performance of the CPython interpreter itself increases
 by roughly 10% if it is compiled with PGO and LTO (link-time
 optimization), see e.g.

  * https://bugs.python.org/issue24915
  * https://www.activestate.com/blog/2014/06/python-performance-boost-
 using-profile-guided-optimization

 Of course the effect of PGO heavily depends on the actual program and the
 programming language, but I think it is something which should be
 considered in the long run. Perhaps our LLVM backend already has some
 (relatively) easy way to make profile data available to the LLVM pipeline,
 but I might be wrong here.

 https://msdn.microsoft.com/en-us/library/e7k32f4k.aspx lists a few PGOs,
 and some are definitely worthwhile for GHC, e.g. using profile data in the
 register allocator to decide which registers to spill etc.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14672#comment:8>
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