**diff options**

author | Rich Felker <dalias@aerifal.cx> | 2012-03-13 01:17:53 -0400 |
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committer | Rich Felker <dalias@aerifal.cx> | 2012-03-13 01:17:53 -0400 |

commit | b69f695acedd4ce2798ef9ea28d834ceccc789bd (patch) | |

tree | eafd98b9b75160210f3295ac074d699f863d958e /src/math/cbrt.c | |

parent | d46cf2e14cc4df7cc75e77d7009fcb6df1f48a33 (diff) | |

download | musl-b69f695acedd4ce2798ef9ea28d834ceccc789bd.tar.gz |

first commit of the new libm!

thanks to the hard work of Szabolcs Nagy (nsz), identifying the best
(from correctness and license standpoint) implementations from freebsd
and openbsd and cleaning them up! musl should now fully support c99
float and long double math functions, and has near-complete complex
math support. tgmath should also work (fully on gcc-compatible
compilers, and mostly on any c99 compiler).
based largely on commit 0376d44a890fea261506f1fc63833e7a686dca19 from
nsz's libm git repo, with some additions (dummy versions of a few
missing long double complex functions, etc.) by me.
various cleanups still need to be made, including re-adding (if
they're correct) some asm functions that were dropped.

Diffstat (limited to 'src/math/cbrt.c')

-rw-r--r-- | src/math/cbrt.c | 105 |

1 files changed, 105 insertions, 0 deletions

diff --git a/src/math/cbrt.c b/src/math/cbrt.c new file mode 100644 index 00000000..f4253428 --- /dev/null +++ b/src/math/cbrt.c @@ -0,0 +1,105 @@ +/* origin: FreeBSD /usr/src/lib/msun/src/s_cbrt.c */ +/* + * ==================================================== + * 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. + * ==================================================== + * + * Optimized by Bruce D. Evans. + */ +/* cbrt(x) + * Return cube root of x + */ + +#include "libm.h" + +static const uint32_t +B1 = 715094163, /* B1 = (1023-1023/3-0.03306235651)*2**20 */ +B2 = 696219795; /* B2 = (1023-1023/3-54/3-0.03306235651)*2**20 */ + +/* |1/cbrt(x) - p(x)| < 2**-23.5 (~[-7.93e-8, 7.929e-8]). */ +static const double +P0 = 1.87595182427177009643, /* 0x3ffe03e6, 0x0f61e692 */ +P1 = -1.88497979543377169875, /* 0xbffe28e0, 0x92f02420 */ +P2 = 1.621429720105354466140, /* 0x3ff9f160, 0x4a49d6c2 */ +P3 = -0.758397934778766047437, /* 0xbfe844cb, 0xbee751d9 */ +P4 = 0.145996192886612446982; /* 0x3fc2b000, 0xd4e4edd7 */ + +double cbrt(double x) +{ + int32_t hx; + union dshape u; + double r,s,t=0.0,w; + uint32_t sign; + uint32_t high,low; + + EXTRACT_WORDS(hx, low, x); + sign = hx & 0x80000000; + hx ^= sign; + if (hx >= 0x7ff00000) /* cbrt(NaN,INF) is itself */ + return x+x; + + /* + * Rough cbrt to 5 bits: + * cbrt(2**e*(1+m) ~= 2**(e/3)*(1+(e%3+m)/3) + * where e is integral and >= 0, m is real and in [0, 1), and "/" and + * "%" are integer division and modulus with rounding towards minus + * infinity. The RHS is always >= the LHS and has a maximum relative + * error of about 1 in 16. Adding a bias of -0.03306235651 to the + * (e%3+m)/3 term reduces the error to about 1 in 32. With the IEEE + * floating point representation, for finite positive normal values, + * ordinary integer divison of the value in bits magically gives + * almost exactly the RHS of the above provided we first subtract the + * exponent bias (1023 for doubles) and later add it back. We do the + * subtraction virtually to keep e >= 0 so that ordinary integer + * division rounds towards minus infinity; this is also efficient. + */ + if (hx < 0x00100000) { /* zero or subnormal? */ + if ((hx|low) == 0) + return x; /* cbrt(0) is itself */ + SET_HIGH_WORD(t, 0x43500000); /* set t = 2**54 */ + t *= x; + GET_HIGH_WORD(high, t); + INSERT_WORDS(t, sign|((high&0x7fffffff)/3+B2), 0); + } else + INSERT_WORDS(t, sign|(hx/3+B1), 0); + + /* + * New cbrt to 23 bits: + * cbrt(x) = t*cbrt(x/t**3) ~= t*P(t**3/x) + * where P(r) is a polynomial of degree 4 that approximates 1/cbrt(r) + * to within 2**-23.5 when |r - 1| < 1/10. The rough approximation + * has produced t such than |t/cbrt(x) - 1| ~< 1/32, and cubing this + * gives us bounds for r = t**3/x. + * + * Try to optimize for parallel evaluation as in k_tanf.c. + */ + r = (t*t)*(t/x); + t = t*((P0+r*(P1+r*P2))+((r*r)*r)*(P3+r*P4)); + + /* + * Round t away from zero to 23 bits (sloppily except for ensuring that + * the result is larger in magnitude than cbrt(x) but not much more than + * 2 23-bit ulps larger). With rounding towards zero, the error bound + * would be ~5/6 instead of ~4/6. With a maximum error of 2 23-bit ulps + * in the rounded t, the infinite-precision error in the Newton + * approximation barely affects third digit in the final error + * 0.667; the error in the rounded t can be up to about 3 23-bit ulps + * before the final error is larger than 0.667 ulps. + */ + u.value = t; + u.bits = (u.bits + 0x80000000) & 0xffffffffc0000000ULL; + t = u.value; + + /* one step Newton iteration to 53 bits with error < 0.667 ulps */ + s = t*t; /* t*t is exact */ + r = x/s; /* error <= 0.5 ulps; |r| < |t| */ + w = t+t; /* t+t is exact */ + r = (r-t)/(w+r); /* r-t is exact; w+r ~= 3*t */ + t = t+t*r; /* error <= 0.5 + 0.5/3 + epsilon */ + return t; +} |