libm/math/
copysign.rs

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/// Sign of Y, magnitude of X (f64)
///
/// Constructs a number with the magnitude (absolute value) of its
/// first argument, `x`, and the sign of its second argument, `y`.
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn copysign(x: f64, y: f64) -> f64 {
    let mut ux = x.to_bits();
    let uy = y.to_bits();
    ux &= (!0) >> 1;
    ux |= uy & (1 << 63);
    f64::from_bits(ux)
}