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

30 May 2018


      #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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