[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):

 Replying to [comment:10 AndreasK]:
 > I was talking about annotations primarily because they seem like the low
 hanging fruit.

 I assumed that, and it's definitely the right way to start. And
 annotations can sometimes be really useful, e.g. GCC has
 ''_builtin_expect'' for a reason. :-) In general, it's just a bit hard to
 find the places for such annotations: You have to profile your code,
 identify the hot spots, and figure out if a branch hint helps. PGO
 automates that process.

 > [...] Once we have a way to pass/use the information throughout the
 passes adding PGO is the next logical step and should be possible without
 touching anything past core.

 PGO is not about branch hints alone, it is e.g. tremendously helpful to
 have an estimate how often a looping construct actually loops, know the
 usual target of an indirect branch etc. (Well, in a sense these are all
 some kind of branch hint, but in a much broader sense.)

 So it might be a good idea to plan for something more general than a
 relative branch frequency. But I guess even threading that simple
 information through the compiler and use it wisely is not a simple task,
 so let's not overengineer in the beginning...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14672#comment:11>
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