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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; +} |