third_party/ulib/musl/src/math/fmod.c

#include <math.h>
#include <stdint.h>

double fmod(double x, double y) {
    union {
        double f;
        uint64_t i;
    } ux = {x}, uy = {y};
    int ex = ux.i >> 52 & 0x7ff;
    int ey = uy.i >> 52 & 0x7ff;
    int sx = ux.i >> 63;
    uint64_t i;

    /* in the followings uxi should be ux.i, but then gcc wrongly adds */
    /* float load/store to inner loops ruining performance and code size */
    uint64_t uxi = ux.i;

    if (uy.i << 1 == 0 || isnan(y) || ex == 0x7ff)
        return (x * y) / (x * y);
    if (uxi << 1 <= uy.i << 1) {
        if (uxi << 1 == uy.i << 1)
            return 0 * x;
        return x;
    }

    /* normalize x and y */
    if (!ex) {
        for (i = uxi << 12; i >> 63 == 0; ex--, i <<= 1)
            ;
        uxi <<= -ex + 1;
    } else {
        uxi &= -1ULL >> 12;
        uxi |= 1ULL << 52;
    }
    if (!ey) {
        for (i = uy.i << 12; i >> 63 == 0; ey--, i <<= 1)
            ;
        uy.i <<= -ey + 1;
    } else {
        uy.i &= -1ULL >> 12;
        uy.i |= 1ULL << 52;
    }

    /* x mod y */
    for (; ex > ey; ex--) {
        i = uxi - uy.i;
        if (i >> 63 == 0) {
            if (i == 0)
                return 0 * x;
            uxi = i;
        }
        uxi <<= 1;
    }
    i = uxi - uy.i;
    if (i >> 63 == 0) {
        if (i == 0)
            return 0 * x;
        uxi = i;
    }
    for (; uxi >> 52 == 0; uxi <<= 1, ex--)
        ;

    /* scale result */
    if (ex > 0) {
        uxi -= 1ULL << 52;
        uxi |= (uint64_t)ex << 52;
    } else {
        uxi >>= -ex + 1;
    }
    uxi |= (uint64_t)sx << 63;
    ux.i = uxi;
    return ux.f;
}