[GHC] #15196: Invert floating point comparisons such that no extra parity check is required.

[GHC]

#15196: Invert floating point comparisons such that no extra parity check is
required.
-------------------------------------+-------------------------------------
        Reporter:  AndreasK          |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (NCG)    |              Version:  8.4.3
      Resolution:                    |             Keywords:  CodeGen
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by AndreasK:

Old description:

> This comment pretty much explains it already:
>
> {{{
>         -- We have to worry about unordered operands (eg. comparisons
>         -- against NaN).  If the operands are unordered, the comparison
>         -- sets the parity flag, carry flag and zero flag.
>         -- All comparisons are supposed to return false for unordered
>         -- operands except for !=, which returns true.
>         --
>         -- Optimisation: we don't have to test the parity flag if we
>         -- know the test has already excluded the unordered case: eg >
>         -- and >= test for a zero carry flag, which can only occur for
>         -- ordered operands.
>         --
>         -- ToDo: by reversing comparisons we could avoid testing the
>         -- parity flag in more cases.
> }}}

New description:

 This comment pretty much explains it already:

 {{{
         -- We have to worry about unordered operands (eg. comparisons
         -- against NaN).  If the operands are unordered, the comparison
         -- sets the parity flag, carry flag and zero flag.
         -- All comparisons are supposed to return false for unordered
         -- operands except for !=, which returns true.
         --
         -- Optimisation: we don't have to test the parity flag if we
         -- know the test has already excluded the unordered case: eg >
         -- and >= test for a zero carry flag, which can only occur for
         -- ordered operands.
         --
         -- ToDo: by reversing comparisons we could avoid testing the
         -- parity flag in more cases.
 }}}

 This would turn a sequence of


 {{{
 jp foo
 jb bar
 jmp foo
 }}}

 into:

 {{{
 jge foo
 jmp bar
 }}}

--

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15196#comment:1>
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