Alex Crichton
d166a30c11
* Move everything to azure pipelines * Inline docker configuration in this repo (no `cross`) * Delete `no-panic` example, use `#[no_panic]` instead.
74 lines
2.0 KiB
Rust
74 lines
2.0 KiB
Rust
/* origin: FreeBSD /usr/src/lib/msun/src/s_cbrtf.c */
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/*
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* Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
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* Debugged and optimized by Bruce D. Evans.
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*/
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/* cbrtf(x)
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* Return cube root of x
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*/
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use core::f32;
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const B1: u32 = 709958130; /* B1 = (127-127.0/3-0.03306235651)*2**23 */
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const B2: u32 = 642849266; /* B2 = (127-127.0/3-24/3-0.03306235651)*2**23 */
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#[inline]
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#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
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pub fn cbrtf(x: f32) -> f32 {
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let x1p24 = f32::from_bits(0x4b800000); // 0x1p24f === 2 ^ 24
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let mut r: f64;
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let mut t: f64;
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let mut ui: u32 = x.to_bits();
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let mut hx: u32 = ui & 0x7fffffff;
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if hx >= 0x7f800000 {
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/* cbrt(NaN,INF) is itself */
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return x + x;
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}
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/* rough cbrt to 5 bits */
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if hx < 0x00800000 {
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/* zero or subnormal? */
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if hx == 0 {
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return x; /* cbrt(+-0) is itself */
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}
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ui = (x * x1p24).to_bits();
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hx = ui & 0x7fffffff;
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hx = hx / 3 + B2;
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} else {
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hx = hx / 3 + B1;
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}
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ui &= 0x80000000;
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ui |= hx;
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/*
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* First step Newton iteration (solving t*t-x/t == 0) to 16 bits. In
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* double precision so that its terms can be arranged for efficiency
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* without causing overflow or underflow.
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*/
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t = f32::from_bits(ui) as f64;
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r = t * t * t;
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t = t * (x as f64 + x as f64 + r) / (x as f64 + r + r);
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/*
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* Second step Newton iteration to 47 bits. In double precision for
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* efficiency and accuracy.
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*/
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r = t * t * t;
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t = t * (x as f64 + x as f64 + r) / (x as f64 + r + r);
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/* rounding to 24 bits is perfect in round-to-nearest mode */
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t as f32
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}
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