[Haskell-cafe] floating point operations and representation

Jacob Schwartz quark at bluespec.com
Wed Mar 12 20:35:46 EDT 2008

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.



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