[GHC] #13629: sqrt should use machine instruction on x86_64

[GHC]

#13629: sqrt should use machine instruction on x86_64
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        Reporter:  bgamari           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (NCG)    |              Version:  8.0.1
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime           |            Test Case:
  performance bug                    |  numeric/num009
      Blocked By:                    |             Blocking:
 Related Tickets:  #13570            |  Differential Rev(s):  Phab:D3508
       Wiki Page:                    |
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Comment (by kavon):

 Replying to [comment:4 bgamari]:
 > It's made in the native code generator, `genCCall` in
 `compiler/nativeGen/X86/CodeGen.hs`. While we use the `fsin` instruction
 on i386, we don't on x86_64 (and i386 with `-msse2`).

 If there is already x87 FPU instruction support in the NCG for x86-32, it
 might be profitable to reuse that support for x86-64 to speed up trig
 functions, etc.

 The simplest way I see it is to expand the foreign call into an
 instruction sequence that moves the float from XMM registers to the x87
 stack, computes the value, and moves it back to XMM registers. This way we
 no longer have a C call in a potentially bad place.

 It's worth comparing x87 on modern processors against the assembly routine
 backing the C function first. It seems platforms like Skylake the x87
 `fsin` takes 50-120 cycles [1], but I'm not sure about the library
 versions. If they're roughly equivalent, there's likely a benefit to
 eliding the C call.

 [1] http://www.agner.org/optimize/instruction_tables.pdf  (Page 223 for
 Skylake x87)

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13629#comment:9>
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