Consider the following C program:
#include <stdlib.h>
#include <stdio.h>
#include <math.h>
int main()
{
double x = 0.5;
printf("%f, %f\n", -expm1(x), 1-exp(x));
exit(0);
}
If I compile this under mingw-w64-1.0-bin_i686-mingw_20110401.zip I
get the correct:
-0.648721, -0.648721
However, if I compile it under mingw-w64-bin_i686-mingw_20110401.zip
(or TDM's 4.5.2 build) I get
-0.271537, -0.648721
It seems that this is using a function from libmingwex.a, and that is
faulty. The code used would appear to be from
mingw/mingw-w64-crt/math/expm1.def.h, and
if (__FLT_ABI (fabs) (x) < __FLT_LOGE2)
{
x *= __FLT_LOGE2;
__asm__ __volatile__ ("f2xm1" : "=t" (x) : "0" (x));
return x;
}
is simply wrong: you need
if (__FLT_ABI (fabs) (x) < __FLT_LOGE2)
{
x /= __FLT_LOGE2;
__asm__ __volatile__ ("f2xm1" : "=t" (x) : "0" (x));
return x;
}
since exp(x) - 1 is 2^(x/log(2)) - 1.
Brian Ripley
--
Brian D. Ripley, [email protected]
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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