[GHC] #4092: Floating point manipulation : ulp and coerce IEEE-754 Double# into Word64#

[GHC]

#4092: Floating point manipulation : ulp and coerce IEEE-754 Double# into Word64#
---------------------------------+------------------------------------------
    Reporter:  malosh            |       Owner:              
        Type:  feature request   |      Status:  new         
    Priority:  normal            |   Component:  Compiler    
     Version:  6.12.2            |    Keywords:              
          Os:  Unknown/Multiple  |    Testcase:              
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown
---------------------------------+------------------------------------------
 There are currently two ways to compute the ulp of double numbers with GHC
 :
  * Calling a C function, which requires to allocate a pointer. This is way
 too expensive when using interval arithmetic (for instance), that compute
 two ULPs at each arithmetic operations.
  * Programming it by hand in haskell with GHC primitive operations, which
 requires using unsafeCoerce# : this does not work in GHC 6.12.2.

 unsafeCoerce# should work, and there should be a primitive ulp# function
 in GHC, operating on Doubles at least.

 By the way, here is my haskell code using C for computing it :

 {{{
 foreign import ccall unsafe "math.h frexp" c_frexp::CDouble->(Ptr
 CInt)->IO ()
 foreign import ccall unsafe "math.h ldexp" c_ldexp::CDouble->CInt->IO
 CDouble
 ulp::Double->Double
 ulp x=unsafePerformIO $ do
   expon<-alloca (\e->do
                     c_frexp (realToFrac x) e
                     peek e)
   (c_ldexp 0.5 $ expon-52) >>= return.realToFrac
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4092>
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