[Haskell-cafe] floating point operations and representation
Alfonso Acosta
alfonso.acosta at gmail.com
Thu Mar 13 10:27:01 EDT 2008
On Thu, Mar 13, 2008 at 1:35 AM, Jacob Schwartz <quark at bluespec.com> wrote:
> I have two questions about using the Double data type and the
> operations in the Floating typeclass on a computer that uses IEEE
> floating point numbers.
>
> I notice that the Floating class only provides "log" (presumably log
> base 'e') and "logBase" (which, in the latest source that I see for
> GHC is defined as "log y / log x"). However, in C, the "math.h"
> library provides specific "log2" and "log10" functions, for extra
> precision. A test on IEEE computers (x86 and x86-64), shows that for
> a range of 64-bit "double" values, the answers in C do differ (in the
> last bit) if you use "log2(x)" and "log10(x)" versus "log (x) /
> log(2)" and "log(x) / log(10)".
>
> I am under the restriction that I need to write Haskell programs using
> Double which mimic existing C/C++ programs or generated data sets, and
> get the same answers. (It's silly, but take it as a given
> requirement.) If the C programs are using "log2", then I need "log2"
> in the Haskell, or else I run the risk of not producing the same
> answers. My first thought is to import "log2" and "log10" through the
> FFI. I was wondering if anyone on Haskell-Cafe has already done this
> and/or has a better suggestion about how to get specialized "log2" and
> "log10" (among the many specialized functions that the "math.h"
> library provides, for better precision -- for now, I'm just concerned
> with "log2" and "log10").
The RULES pragma + FFI solution (as suggested by Henning) seems to be
the most sensible one so far.
> My second question is how to get at the IEEE bit representation for a
> Double. I am already checking "isIEEE n" in my source code (and
> "floatRadix n == 2"). So I know that I am operating on hardware that
> implements floating point numbers by the IEEE standard. I would like
> to get at the 64 bits of a Double. Again, I can convert to a CDouble
> and use the FFI to wrap a C function which casts the "double" to a
> 64-bit number and returns it. But I'm wondering if there's not a
> better way to do this natively in Haskell/GHC (perhaps some crazy use
> of the Storable typeclass?).
Posted in a a previous haskell-cafe message [1]
{-# LANGUAGE MagicHash #-}
import Data.Int
import GHC.Exts
foo :: Double -> Int64 --ghc only
foo (D# x) = I64# (unsafeCoerce# x)
[1] http://www.haskell.org/pipermail/haskell-cafe/2008-February/040000.html
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