[Haskell-cafe] floating point operations and representation

Lennart Augustsson lennart at augustsson.net
Thu Mar 13 08:20:28 EDT 2008

Wow, you have a tough mission if you want to replicate the bit level answers
for double (btw, hi Jacob).
Libraries differ for transcendental function, and even worse, CPUs differ.
You may get different answers on an Intel and and AMD.
That said, I think your best bet is to import log2 and log10 from C and use

Haskell sadly lacks an efficient way to go from a Double to its bit pattern.

  -- Lennart

On Thu, Mar 13, 2008 at 12:35 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").
> 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?).  Or if someone has already tackled this
> problem with FFI, that would be interesting to know.
> Thanks.
> Jacob
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