[GHC] #14644: Improve cmm/assembly for pattern matches with two constants.

[GHC]

#14644: Improve cmm/assembly for pattern matches with two constants.
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        Reporter:  AndreasK          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.2
  (CodeGen)                          |             Keywords:  Codegen, CMM,
      Resolution:                    |  Patterns, Pattern Matching
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by svenpanne):

 I am not sure if things are that easy: To do these kind of switches in a
 really good way, one would need a probability distribution how often the
 individual cases actually happen.

 If we e.g. assume that all Ints happen equally often in the example above,
 it would be best to check for <1 and >7 first, so we would get roughly 1.5
 comparisons on average. Depending on the number of registers available,
 you can even get away with 1 subtraction and 1 unsigned comparison for
 this range check, a classic C hack (ab)using wrap-around for unsigned
 ints.

 If we have some hints, e.g. raising a pattern matching exception, we could
 do better, e.g. assume 0% probability for this case. If we have more
 detailed (estimated) probabilities, we could do a Huffman-like decision
 tree. This is where profile-guided optimization shines.

 Additional things to consider: Performance in tight loops is often vastly
 different, because branch prediction/caching will most likely kick in
 visibly. Correctly predicted branches will cost you almost nothing, while
 unknown/incorrectly predicted branches will be much more costly. In the
 absence of more information from their branch predictor, quite a few
 processors assume that backward branches are taken and forward branches
 are assumed to be not taken. So code layout has a non-trivial performance
 impact.

 Instruction counts are quite misleading nowadays...

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14644#comment:3>
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