----Original Message----
>From: Brian Dessent
>Sent: 30 June 2005 16:58
> Dave Korn wrote:
>
>> Absolutely, there's a rounding error of some sort. Compare the
>> difference when compiling the testcase with -mno-cygwin (i.e. using
>> mingw maths lib):
>
> Isn't this just a case of the Cygwin math library choosing "round to
> even" and the MSVCRT/mingw library choosing "0.5 always rounds up"?
No, I really don't think that's the case, in fact I reckon they both
default to the same roundingmode, and the discrepancy remains the same in
any rounding mode. Have a go with the attached version of the testcase,
which allows you to choose any rounding mode you like at runtime: just pass
a hex constant from the set (0, 0x400, 0x800, 0xc00) on the command line.
cheers,
DaveK
--
Can't think of a witty .sigline today....
#include <stdio.h>
#include <math.h>
#ifdef __MINGW32__
#include <fenv.h>
#endif
#ifndef __MINGW32__
#define FE_TONEAREST 0x0000
#define FE_DOWNWARD 0x0400
#define FE_UPWARD 0x0800
#define FE_TOWARDZERO 0x0c00
/* 7.6.3.2
The fesetround function establishes the rounding direction
represented by its argument round. If the argument is not equal
to the value of a rounding direction macro, the rounding direction
is not changed. */
int fesetround (int mode)
{
unsigned short _cw;
if ((mode & ~(FE_TONEAREST | FE_DOWNWARD | FE_UPWARD | FE_TOWARDZERO))
!= 0)
return -1;
__asm__ volatile ("fnstcw %0;": "=m" (_cw));
_cw &= ~(FE_TONEAREST | FE_DOWNWARD | FE_UPWARD | FE_TOWARDZERO);
_cw |= mode;
__asm__ volatile ("fldcw %0;" : : "m" (_cw));
return 0;
}
/* 7.6.3.1
The fegetround function returns the value of the rounding direction
macro representing the current rounding direction. */
int
fegetround (void)
{
unsigned short _cw;
__asm__ ("fnstcw %0;" : "=m" (_cw));
return _cw
& (FE_TONEAREST | FE_DOWNWARD | FE_UPWARD | FE_TOWARDZERO);
}
#endif
int main(int argc, const char **argv)
{
if (argc >= 2)
{
int nrm, n;
n = sscanf (argv[1], "%i", &nrm);
if (n == 1) switch (nrm)
{
case 0:
case 0x400:
case 0x800:
case 0xc00:
fesetround (nrm);
}
}
printf ("Rmode $%08x\n", fegetround ());
printf("%0.2f\n", 0.105);
printf("%0.2f\n", 0.115);
printf("%0.2f\n", 0.125);
printf("%0.2f\n", 0.135);
return 0;
}
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