[GHC] #9661: Branchless ==# is compiled to branchy code

[GHC]

#9661: Branchless ==# is compiled to branchy code
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        Reporter:  dfeuer            |                   Owner:  bgamari
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.9
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  D854
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Comment (by rwbarton):

 Sorry, my previous comment was poorly worded at best. I'm sure there are
 cases where the best non-branching code has better performance than the
 best branching code. However, I'm not at all convinced that it makes sense
 to try to give the user control over whether to generate branching or non-
 branching code. It's at odds with the notion of an optimizing compiler,
 and as I mentioned earlier even C compilers do not give you that kind of
 control once optimizations are enabled. GHC should just generate whatever
 it thinks is best, and if the user really needs to generate particular
 assembly, they can write that assembly manually.

 I would need to see some ''very'' dramatic numbers to change this point of
 view; and currently we have no numbers at all.

 I do think that expanding GHC's bag of tricks for post-Stg code generation
 (as in #10124 for example) is practical and worthwhile, though it will
 probably need more work in the native code generator as well for best
 results.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9661#comment:23>
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