[GHC] #2269: Word type to Double or Float conversions are slower than Int conversions

[GHC]

#2269: Word type to Double or Float conversions are slower than Int conversions
-------------------------------------+-------------------------------------
        Reporter:  dons              |                Owner:  dons@…
            Type:  feature request   |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  6.8.2
      Resolution:                    |             Keywords:  rules,
                                     |  performance, double
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 Replying to [comment:16 thomie]:
 > But `testWord` is still 3 times slower than `testInt`.
 > {{{
 > $ time ./testWord
 > 5.00000005e15
 >
 > real  0m0.579s
 > user  0m0.575s
 > sys   0m0.003s
 >
 > $ time ./testInt
 > 5.00000005e15
 >
 > real  0m0.196s
 > user  0m0.191s
 > sys   0m0.004s
 > }}}
 >
 > As I can not easily explain this difference, I'll leave this ticket open
 for now.

 It's because the x86 NCG implements the new `MO_UF_Conv` as a call to a C
 function, rather than generating code inline like `MO_SF_Conv`
 (`cvtsi2sdq`).

 Unfortunately there's no corresponding instruction for converting an
 unsigned 64-bit integer to float or double, but for converting to double
 the code generated by `clang -O` is pretty small and simple and probably
 worth inlining. It will still be somewhat slower than `Int` though.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/2269#comment:21>
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