[GHC] #9246: GHC generates poor code for repeated uses of min/max

[GHC]

#9246: GHC generates poor code for repeated uses of min/max
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        Reporter:  arotenberg               |            Owner:
            Type:  feature request          |           Status:  new
        Priority:  normal                   |        Milestone:  7.10.1
       Component:  Compiler                 |          Version:  7.8.2
      Resolution:                           |         Keywords:
Operating System:  Windows                  |     Architecture:  x86_64
 Type of failure:  Runtime performance bug  |  (amd64)
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:  #6135
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Changes (by carter):

 * type:  bug => feature request
 * milestone:   => 7.10.1


Comment:

 So if  you take a look at the haskell level primops that GHC provides
 http://www.haskell.org/ghc/docs/latest/html/libraries/ghc-prim-0.3.1.0
 /GHC-Prim.html
 you'll notice that  theres no primops for min or max on the underlying
 unlifted types Double# or Float# (or Int#).

 So if you need a branchless inner loop today, you'll either need to write
 a teeny bit of C you'll call out to, encode the logic as some bit fiddling
 on the floats, OR do something like describe the algorithm
 programmatically in a Haskell DSL, and at runtime use LLVM-General to
 generate the code you want  http://hackage.haskell.org/package/llvm-
 general  (i'm told some video codecs actually use LLVM for runtime code
 gen!)

 Adding these to ghc is a pretty reasonable feature request, though such
 wouldn't land till 7.10. If you're interested in doing some of the leg
 work , i'm happy to try to guide you through the process!  (If not, thats
 fine too, its a really concrete task i'll give to someone who wants to do
 their first ghc patch, )


 zooming out,
  1. do you have any benchmarks (say, using criterion) for this code?
  2. how does eg using -fllvm as your backend fair vs -fasm for this code
 in such a benchmark

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9246#comment:1>
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