Re: [issue18062] m68k FPU precision issue

[Thorsten Glaser <tg@mirbsd.de>]

mirabilos dixit:

>mirabilos added the comment:

>Yes, that’s precisely what’s not working on the most widespread
>emulator, at the very least. (I have working code for that, in
>three(!) variants, but it just ignores those requests to change
>precision.)

Here’s the Python testcase made into a standalone test:

-----BEGIN cutting here may damage your screen surface-----
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#ifndef __MirBSD__
#include <fpu_control.h>
#endif

void runtests(void);

void
runtests(void)
{
	volatile double x, y, z;

	/* 1./(1-2**-53) -> 1+2**-52 (correct), 1.0 (double rounding) */
	x = 0.99999999999999989; /* 1-2**-53 */
	y = 1./x;
	printf("test#1 %s: %.17E\n", (y == 1.) ? "fail" : "good", y);
	/* 1e16+2.99999 -> 1e16+2. (correct), 1e16+4. (double rounding) */
	x = 1e16;
	y = 2.99999;
	z = x + y;
	printf("test#2 %s: %.17E\n", z == 1e16+4. ? "fail" : "good", z);
}

int
main(void) {
#ifdef _FPU_SETCW
	fpu_control_t cwold, cwnew, cwgot;
#endif

	runtests();
#if (defined(__i386__) || defined(__m68k__)) && defined(_FPU_SETCW)
	_FPU_GETCW(cwold);
#ifdef __i386__
	cwnew = 0x127f;
#else
	cwnew = _FPU_RC_NEAREST | _FPU_DOUBLE;
#endif
	_FPU_SETCW(cwnew);
	_FPU_GETCW(cwgot);
	printf("changing FPU control word from %08X to %08X => %08X\n",
	    cwold, cwnew, cwgot);
	runtests();
#endif

	return (0);
}
-----END cutting here may damage your screen surface-----

Here’s the result of running it on the latest ARAnyM, which
did get MPFR-based FPU emulation bugfixes, but apparently
still ignores any FPUCW changes (or, at least the ones relating
to precision):

root@ara3:~ # ./a.out
test#1 fail: 1.00000000000000000E+00
test#2 fail: 1.00000000000000040E+16
changing FPU control word from 00000000 to 00000080 => 00000080
test#1 fail: 1.00000000000000000E+00
test#2 fail: 1.00000000000000040E+16


bye,
//mirabilos
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