libm/math/
sqrtf.rs

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1/* origin: FreeBSD /usr/src/lib/msun/src/e_sqrtf.c */
2/*
3 * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
4 */
5/*
6 * ====================================================
7 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
8 *
9 * Developed at SunPro, a Sun Microsystems, Inc. business.
10 * Permission to use, copy, modify, and distribute this
11 * software is freely granted, provided that this notice
12 * is preserved.
13 * ====================================================
14 */
15
16#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
17pub fn sqrtf(x: f32) -> f32 {
18    // On wasm32 we know that LLVM's intrinsic will compile to an optimized
19    // `f32.sqrt` native instruction, so we can leverage this for both code size
20    // and speed.
21    llvm_intrinsically_optimized! {
22        #[cfg(target_arch = "wasm32")] {
23            return if x < 0.0 {
24                ::core::f32::NAN
25            } else {
26                unsafe { ::core::intrinsics::sqrtf32(x) }
27            }
28        }
29    }
30    #[cfg(target_feature = "sse")]
31    {
32        // Note: This path is unlikely since LLVM will usually have already
33        // optimized sqrt calls into hardware instructions if sse is available,
34        // but if someone does end up here they'll apprected the speed increase.
35        #[cfg(target_arch = "x86")]
36        use core::arch::x86::*;
37        #[cfg(target_arch = "x86_64")]
38        use core::arch::x86_64::*;
39        unsafe {
40            let m = _mm_set_ss(x);
41            let m_sqrt = _mm_sqrt_ss(m);
42            _mm_cvtss_f32(m_sqrt)
43        }
44    }
45    #[cfg(not(target_feature = "sse"))]
46    {
47        const TINY: f32 = 1.0e-30;
48
49        let mut z: f32;
50        let sign: i32 = 0x80000000u32 as i32;
51        let mut ix: i32;
52        let mut s: i32;
53        let mut q: i32;
54        let mut m: i32;
55        let mut t: i32;
56        let mut i: i32;
57        let mut r: u32;
58
59        ix = x.to_bits() as i32;
60
61        /* take care of Inf and NaN */
62        if (ix as u32 & 0x7f800000) == 0x7f800000 {
63            return x * x + x; /* sqrt(NaN)=NaN, sqrt(+inf)=+inf, sqrt(-inf)=sNaN */
64        }
65
66        /* take care of zero */
67        if ix <= 0 {
68            if (ix & !sign) == 0 {
69                return x; /* sqrt(+-0) = +-0 */
70            }
71            if ix < 0 {
72                return (x - x) / (x - x); /* sqrt(-ve) = sNaN */
73            }
74        }
75
76        /* normalize x */
77        m = ix >> 23;
78        if m == 0 {
79            /* subnormal x */
80            i = 0;
81            while ix & 0x00800000 == 0 {
82                ix <<= 1;
83                i = i + 1;
84            }
85            m -= i - 1;
86        }
87        m -= 127; /* unbias exponent */
88        ix = (ix & 0x007fffff) | 0x00800000;
89        if m & 1 == 1 {
90            /* odd m, double x to make it even */
91            ix += ix;
92        }
93        m >>= 1; /* m = [m/2] */
94
95        /* generate sqrt(x) bit by bit */
96        ix += ix;
97        q = 0;
98        s = 0;
99        r = 0x01000000; /* r = moving bit from right to left */
100
101        while r != 0 {
102            t = s + r as i32;
103            if t <= ix {
104                s = t + r as i32;
105                ix -= t;
106                q += r as i32;
107            }
108            ix += ix;
109            r >>= 1;
110        }
111
112        /* use floating add to find out rounding direction */
113        if ix != 0 {
114            z = 1.0 - TINY; /* raise inexact flag */
115            if z >= 1.0 {
116                z = 1.0 + TINY;
117                if z > 1.0 {
118                    q += 2;
119                } else {
120                    q += q & 1;
121                }
122            }
123        }
124
125        ix = (q >> 1) + 0x3f000000;
126        ix += m << 23;
127        f32::from_bits(ix as u32)
128    }
129}
130
131// PowerPC tests are failing on LLVM 13: https://github.com/rust-lang/rust/issues/88520
132#[cfg(not(target_arch = "powerpc64"))]
133#[cfg(test)]
134mod tests {
135    use super::*;
136    use core::f32::*;
137
138    #[test]
139    fn sanity_check() {
140        assert_eq!(sqrtf(100.0), 10.0);
141        assert_eq!(sqrtf(4.0), 2.0);
142    }
143
144    /// The spec: https://en.cppreference.com/w/cpp/numeric/math/sqrt
145    #[test]
146    fn spec_tests() {
147        // Not Asserted: FE_INVALID exception is raised if argument is negative.
148        assert!(sqrtf(-1.0).is_nan());
149        assert!(sqrtf(NAN).is_nan());
150        for f in [0.0, -0.0, INFINITY].iter().copied() {
151            assert_eq!(sqrtf(f), f);
152        }
153    }
154
155    #[test]
156    fn conformance_tests() {
157        let values = [
158            3.14159265359f32,
159            10000.0f32,
160            f32::from_bits(0x0000000f),
161            INFINITY,
162        ];
163        let results = [1071833029u32, 1120403456u32, 456082799u32, 2139095040u32];
164
165        for i in 0..values.len() {
166            let bits = f32::to_bits(sqrtf(values[i]));
167            assert_eq!(results[i], bits);
168        }
169    }
170}
libm/math/sqrtf.rs

libm/math/
sqrtf.rs
