[Bug libgcc/116017] libgcc/riscv/softfp: Fix loss of sign when truncating NaN values

[keithp at keithp dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116017

--- Comment #5 from keithp at keithp dot com <keithp at keithp dot com> ---
You're quite correct; conversion from double to float also loses the sign bit.
It never occurred to me that RISC-V would be different from every other GCC
target in this regard. I'll go stick a whole pile of risc-v specific code where
I needs to preserve the sign; I'm glad I don't also have to preserve the
significand. Yay for standards!


Soft float, double, long double:
    test/test-nansign_rv64imac_lp64
    ld: -nan (sign 1) d: nan (sign 0) f: nan (sign 0)

Hard float, soft double, long double:
    test/test-nansign_rv64imafc_lp64f
    ld: -nan (sign 1) d: nan (sign 0) f: nan (sign 0)

Hard float, hard double, soft long double:
    test/test-nansign_rv64ifd_lp64d
    ld: -nan (sign 1) d: nan (sign 0) f: nan (sign 0)

Here's the code which generated the above:

#define __STDC_WANT_IEC_60559_BFP_EXT__
#include <stdio.h>
#include <math.h>
#include <stdlib.h>

int main(void)
{
        volatile long double    ld;
        volatile double d;
        volatile float f;
        int     ldbit, dbit, fbit;
        char    ldstr[64];
        char    dstr[64];
        char    fstr[64];

        ld = (long double) NAN;
        ld = -ld;
        ldbit = signbit(ld);
        strfroml(ldstr, sizeof(ldstr), "%a", ld);

        d = (double) ld;
        dbit = signbit(d);
        strfromd(dstr, sizeof(dstr), "%a", d);

        d = (double) NAN;
        d = -d;
        f = (float) d;
        fbit = signbit(f);
        strfromf(fstr, sizeof(fstr), "%a", f);

        printf("ld: %s (sign %d) d: %s (sign %d) f: %s (sign %d)\n",
               ldstr, ldbit, dstr, dbit, fstr, fbit);
        if (ldbit == 0 || dbit == 0 || fbit == 0)
            return 1;
        return 0;
}