#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>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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