Re: port-sparc64/59310: t_fe_round:fe_nearbyint_rint tests are failing

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

All the test cases that are failing are in long double functions with
non-default rounding modes.

On sparc64, long double is implemented via softfloat ABI, from what I
understand -- `objdump --disassemble=rintl libm.so' shows various
_Qp_* function calls:

00000000000776e0 <rintl>:
...
   77760:       40 04 54 c8     call  18ca80 <_Qp_stoq@plt>
...
   77790:       40 04 57 b4     call  18d660 <_Qp_add@plt>
...
   777b8:       40 04 55 32     call  18cc80 <_Qp_sub@plt>
...
   777f4:       40 04 55 a3     call  18ce80 <_Qp_feq@plt>
...
   77824:       40 04 54 97     call  18ca80 <_Qp_stoq@plt>
...
   77850:       40 04 57 84     call  18d660 <_Qp_add@plt>

I bet the reason these are failing is that the C99 fesetround() in
libm doesn't update _softfloat_float_rounding_mode, so the _Qp_*
functions are all working in round-to-nearest:

    227 int
    228 fesetround(int round)
    229 {
    230 	fenv_t r;
    231 
    232 	_DIAGASSERT((round & ~_ROUND_MASK) == 0);
    233 	if (round & ~_ROUND_MASK)
    234 		return -1;
    235 
    236 	__stxfsr(&r);
    237 	r &= ~(_ROUND_MASK << _ROUND_SHIFT);
    238 	r |= round << _ROUND_SHIFT;
    239 	__ldxfsr(r);
    240 
    241 	/* Success */
    242 	return 0;
    243 }

https://nxr.netbsd.org/xref/src/lib/libm/arch/sparc64/fenv.c?r=1.4#227

In contrast, the legacy BSD fpsetround() in libc does set
_softfloat_float_rounding_mode:

     26 fp_rnd
     27 fpsetround(fp_rnd rnd_dir)
     28 {
     29 	fp_rnd old;
     30 	fp_rnd new;
     31 
     32 	__asm("st %%fsr,%0" : "=m" (*&old));
     33 
     34 #ifdef SOFTFLOATSPARC64_FOR_GCC
=>   35 	_softfloat_float_rounding_mode = rnd_dir;
     36 #endif
     37 
     38 	new = old;
     39 	new &= ~(0x03U << 30);
     40 	new |= ((rnd_dir & 0x03U) << 30);
     41 
     42 	__asm("ld %0,%%fsr" : : "m" (*&new));
     43 
     44 	return ((uint32_t)old >> 30) & 0x03;
     45 }

https://nxr.netbsd.org/xref/src/lib/libc/arch/sparc64/gen/fpsetround.c?r=1.9#35