fusl/src/math/copysign.c

#include "libm.h"

double copysign(double x, double y) {
  union {
    double f;
    uint64_t i;
  } ux = {x}, uy = {y};
  ux.i &= -1ULL / 2;
  ux.i |= uy.i & 1ULL << 63;
  return ux.f;
}