sqrtf.rs - source
Description: Source of the Rust file `/Users/erlendbasso/.cargo/registry/src/github.com-1ecc6299db9ec823/libm-0.2.6/src/math/sqrtf.rs`.
Keywords: rust, rustlang, rust-lang

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    /* origin: FreeBSD /usr/src/lib/msun/src/e_sqrtf.c */
    /*
     * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
     */
    /*
     * ====================================================
     * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
     *
     * Developed at SunPro, a Sun Microsystems, Inc. business.
     * Permission to use, copy, modify, and distribute this
     * software is freely granted, provided that this notice
     * is preserved.
     * ====================================================
     */

    #[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
    pub fn sqrtf(x: f32) -> f32 {
        // On wasm32 we know that LLVM's intrinsic will compile to an optimized
        // `f32.sqrt` native instruction, so we can leverage this for both code size
        // and speed.
        llvm_intrinsically_optimized! {
            #[cfg(target_arch = "wasm32")] {
                return if x < 0.0 {
                    ::core::f32::NAN
                } else {
                    unsafe { ::core::intrinsics::sqrtf32(x) }
                }
            }
        }
        #[cfg(target_feature = "sse")]
        {
            // Note: This path is unlikely since LLVM will usually have already
            // optimized sqrt calls into hardware instructions if sse is available,
            // but if someone does end up here they'll apprected the speed increase.
            #[cfg(target_arch = "x86")]
            use core::arch::x86::*;
            #[cfg(target_arch = "x86_64")]
            use core::arch::x86_64::*;
            unsafe {
                let m = _mm_set_ss(x);
                let m_sqrt = _mm_sqrt_ss(m);
                _mm_cvtss_f32(m_sqrt)
            }
        }
        #[cfg(not(target_feature = "sse"))]
        {
            const TINY: f32 = 1.0e-30;

            let mut z: f32;
            let sign: i32 = 0x80000000u32 as i32;
            let mut ix: i32;
            let mut s: i32;
            let mut q: i32;
            let mut m: i32;
            let mut t: i32;
            let mut i: i32;
            let mut r: u32;

            ix = x.to_bits() as i32;

            /* take care of Inf and NaN */
            if (ix as u32 & 0x7f800000) == 0x7f800000 {
                return x * x + x; /* sqrt(NaN)=NaN, sqrt(+inf)=+inf, sqrt(-inf)=sNaN */
            }

            /* take care of zero */
            if ix <= 0 {
                if (ix & !sign) == 0 {
                    return x; /* sqrt(+-0) = +-0 */
                }
                if ix < 0 {
                    return (x - x) / (x - x); /* sqrt(-ve) = sNaN */
                }
            }

            /* normalize x */
            m = ix >> 23;
            if m == 0 {
                /* subnormal x */
                i = 0;
                while ix & 0x00800000 == 0 {
                    ix <<= 1;
                    i = i + 1;
                }
                m -= i - 1;
            }
            m -= 127; /* unbias exponent */
            ix = (ix & 0x007fffff) | 0x00800000;
            if m & 1 == 1 {
                /* odd m, double x to make it even */
                ix += ix;
            }
            m >>= 1; /* m = [m/2] */

            /* generate sqrt(x) bit by bit */
            ix += ix;
            q = 0;
            s = 0;
            r = 0x01000000; /* r = moving bit from right to left */

            while r != 0 {
                t = s + r as i32;
                if t <= ix {
                    s = t + r as i32;
                    ix -= t;
                    q += r as i32;
                }
                ix += ix;
                r >>= 1;
            }

            /* use floating add to find out rounding direction */
            if ix != 0 {
                z = 1.0 - TINY; /* raise inexact flag */
                if z >= 1.0 {
                    z = 1.0 + TINY;
                    if z > 1.0 {
                        q += 2;
                    } else {
                        q += q & 1;
                    }
                }
            }

            ix = (q >> 1) + 0x3f000000;
            ix += m << 23;
            f32::from_bits(ix as u32)
        }
    }

    // PowerPC tests are failing on LLVM 13: https://github.com/rust-lang/rust/issues/88520
    #[cfg(not(target_arch = "powerpc64"))]
    #[cfg(test)]
    mod tests {
        use super::*;
        use core::f32::*;

        #[test]
        fn sanity_check() {
            assert_eq!(sqrtf(100.0), 10.0);
            assert_eq!(sqrtf(4.0), 2.0);
        }

        /// The spec: https://en.cppreference.com/w/cpp/numeric/math/sqrt
        #[test]
        fn spec_tests() {
            // Not Asserted: FE_INVALID exception is raised if argument is negative.
            assert!(sqrtf(-1.0).is_nan());
            assert!(sqrtf(NAN).is_nan());
            for f in [0.0, -0.0, INFINITY].iter().copied() {
                assert_eq!(sqrtf(f), f);
            }
        }
    }