[GHC] #7858: Fix definitions of abs/signum for Floats/Doubles

[GHC]

#7858: Fix definitions of abs/signum for Floats/Doubles
-------------------------------------+-------------------------------------
              Reporter:  lerkok      |            Owner:
                  Type:  bug         |           Status:  patch
              Priority:  normal      |        Milestone:  7.10.1
             Component:              |          Version:  7.6.3
  libraries/base                     |         Keywords:  floating point
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #9238
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------
Changes (by rwbarton):

 * milestone:   => 7.10.1


Comment:

 We can replace the expensive `isNegativeZero x` test in `abs` with `x ==
 0`, since `abs` of positive zero is also zero.
 {{{
     abs x
       | x == 0    = 0
       | x >= 0    = x
       | otherwise = negate x
 }}}
 Of course this merits a comment on the first guard!

 As for `signum`, I don't see off-hand a simple way to work around the bug
 #9238, so I suggest we apply the `abs` patch, then mark this ticket as
 blocked by #9238. If someone gets around to implementing efficient `abs`
 and `signum` with fancy bit tricks before 7.10 then it will become moot
 anyway.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7858#comment:6>
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