The following short program compares the value of pow(x, 0.5) to that
returned by sqrt(x) for a range of values of x, and prints out any first
case where the two do not agree. With the mingw family of compilers I had
expected the Microsoft C library to be in use.
What I appear to see is
linux (ubuntu 11.04 64-bit tested) no output
Mac (snow leopard) no output
gcc (cygwin) no output
gcc-3 -mno-cygwin )
i686-w64-mingw32-gcc ) all different from each other
x86_64-w64-mingw32-gcc ) but all find discrepancies.
It is a totally fair cop if the Microsoft C library gives different
results in the last bit for floating point elementary functions to gnu
libraries, but is there an obvious reason why the three mingw varients
all give different answers. This showed up in regression testing something
much larger across platforms.
Arthur
====================
#include <stdio.h>
#include <math.h>
int main(int argc, char *argv[])
{
double a, b, c;
int i;
for (i=0, a=1.0; i<10000000; i++)
{ a *= 1.00000001;
b = sqrt(a); c = pow(a, 0.5);
if (b == c) continue;
printf("%.17g %.17g %.17g\n", a, b, c);
return 0;
}
return 0;
}
====================
acn1@panamint temp
$ gcc expt.c -o expt
acn1@panamint temp
$ ./expt
acn1@panamint temp
$ gcc-3 -mno-cygwin expt.c -o expt
acn1@panamint temp
$ ./expt
1.0241210164238141 1.011988644414459 1.0119886444144588
acn1@panamint temp
$ i686-w64-mingw32-gcc expt.c -o expt
acn1@panamint temp
$ ./expt
1.0004603659307831 1.000230156479389 1.0002301564793892
acn1@panamint temp
$ x86_64-w64-mingw32-gcc expt.c -o expt
acn1@panamint temp
$ ./expt
1.0000179101601867 1.0000089550399969 1.0000089550399971
acn1@panamint temp
==============================================================
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