Floating point manipulation : ulp and coerce IEEE-754 Double# into Word64#

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

Trac metadata

Trac field	Value
Version	6.12.2
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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