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Diffstat (limited to 'src/math/powl.c')
| -rw-r--r-- | src/math/powl.c | 562 |
1 files changed, 562 insertions, 0 deletions
diff --git a/src/math/powl.c b/src/math/powl.c new file mode 100644 index 00000000..690f2942 --- /dev/null +++ b/src/math/powl.c @@ -0,0 +1,562 @@ +/* origin: OpenBSD /usr/src/lib/libm/src/ld80/e_powl.c */ +/* + * Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net> + * + * Permission to use, copy, modify, and distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + */ +/* powl.c + * + * Power function, long double precision + * + * + * SYNOPSIS: + * + * long double x, y, z, powl(); + * + * z = powl( x, y ); + * + * + * DESCRIPTION: + * + * Computes x raised to the yth power. Analytically, + * + * x**y = exp( y log(x) ). + * + * Following Cody and Waite, this program uses a lookup table + * of 2**-i/32 and pseudo extended precision arithmetic to + * obtain several extra bits of accuracy in both the logarithm + * and the exponential. + * + * + * ACCURACY: + * + * The relative error of pow(x,y) can be estimated + * by y dl ln(2), where dl is the absolute error of + * the internally computed base 2 logarithm. At the ends + * of the approximation interval the logarithm equal 1/32 + * and its relative error is about 1 lsb = 1.1e-19. Hence + * the predicted relative error in the result is 2.3e-21 y . + * + * Relative error: + * arithmetic domain # trials peak rms + * + * IEEE +-1000 40000 2.8e-18 3.7e-19 + * .001 < x < 1000, with log(x) uniformly distributed. + * -1000 < y < 1000, y uniformly distributed. + * + * IEEE 0,8700 60000 6.5e-18 1.0e-18 + * 0.99 < x < 1.01, 0 < y < 8700, uniformly distributed. + * + * + * ERROR MESSAGES: + * + * message condition value returned + * pow overflow x**y > MAXNUM INFINITY + * pow underflow x**y < 1/MAXNUM 0.0 + * pow domain x<0 and y noninteger 0.0 + * + */ + +#include "libm.h" + +#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024 +long double powl(long double x, long double y) +{ + return pow(x, y); +} +#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384 + +/* Table size */ +#define NXT 32 +/* log2(Table size) */ +#define LNXT 5 + +/* log(1+x) = x - .5x^2 + x^3 * P(z)/Q(z) + * on the domain 2^(-1/32) - 1 <= x <= 2^(1/32) - 1 + */ +static long double P[] = { + 8.3319510773868690346226E-4L, + 4.9000050881978028599627E-1L, + 1.7500123722550302671919E0L, + 1.4000100839971580279335E0L, +}; +static long double Q[] = { +/* 1.0000000000000000000000E0L,*/ + 5.2500282295834889175431E0L, + 8.4000598057587009834666E0L, + 4.2000302519914740834728E0L, +}; +/* A[i] = 2^(-i/32), rounded to IEEE long double precision. + * If i is even, A[i] + B[i/2] gives additional accuracy. + */ +static long double A[33] = { + 1.0000000000000000000000E0L, + 9.7857206208770013448287E-1L, + 9.5760328069857364691013E-1L, + 9.3708381705514995065011E-1L, + 9.1700404320467123175367E-1L, + 8.9735453750155359320742E-1L, + 8.7812608018664974155474E-1L, + 8.5930964906123895780165E-1L, + 8.4089641525371454301892E-1L, + 8.2287773907698242225554E-1L, + 8.0524516597462715409607E-1L, + 7.8799042255394324325455E-1L, + 7.7110541270397041179298E-1L, + 7.5458221379671136985669E-1L, + 7.3841307296974965571198E-1L, + 7.2259040348852331001267E-1L, + 7.0710678118654752438189E-1L, + 6.9195494098191597746178E-1L, + 6.7712777346844636413344E-1L, + 6.6261832157987064729696E-1L, + 6.4841977732550483296079E-1L, + 6.3452547859586661129850E-1L, + 6.2092890603674202431705E-1L, + 6.0762367999023443907803E-1L, + 5.9460355750136053334378E-1L, + 5.8186242938878875689693E-1L, + 5.6939431737834582684856E-1L, + 5.5719337129794626814472E-1L, + 5.4525386633262882960438E-1L, + 5.3357020033841180906486E-1L, + 5.2213689121370692017331E-1L, + 5.1094857432705833910408E-1L, + 5.0000000000000000000000E-1L, +}; +static long double B[17] = { + 0.0000000000000000000000E0L, + 2.6176170809902549338711E-20L, +-1.0126791927256478897086E-20L, + 1.3438228172316276937655E-21L, + 1.2207982955417546912101E-20L, +-6.3084814358060867200133E-21L, + 1.3164426894366316434230E-20L, +-1.8527916071632873716786E-20L, + 1.8950325588932570796551E-20L, + 1.5564775779538780478155E-20L, + 6.0859793637556860974380E-21L, +-2.0208749253662532228949E-20L, + 1.4966292219224761844552E-20L, + 3.3540909728056476875639E-21L, +-8.6987564101742849540743E-22L, +-1.2327176863327626135542E-20L, + 0.0000000000000000000000E0L, +}; + +/* 2^x = 1 + x P(x), + * on the interval -1/32 <= x <= 0 + */ +static long double R[] = { + 1.5089970579127659901157E-5L, + 1.5402715328927013076125E-4L, + 1.3333556028915671091390E-3L, + 9.6181291046036762031786E-3L, + 5.5504108664798463044015E-2L, + 2.4022650695910062854352E-1L, + 6.9314718055994530931447E-1L, +}; + +#define douba(k) A[k] +#define doubb(k) B[k] +#define MEXP (NXT*16384.0L) +/* The following if denormal numbers are supported, else -MEXP: */ +#define MNEXP (-NXT*(16384.0L+64.0L)) +/* log2(e) - 1 */ +#define LOG2EA 0.44269504088896340735992L + +#define F W +#define Fa Wa +#define Fb Wb +#define G W +#define Ga Wa +#define Gb u +#define H W +#define Ha Wb +#define Hb Wb + +static const long double MAXLOGL = 1.1356523406294143949492E4L; +static const long double MINLOGL = -1.13994985314888605586758E4L; +static const long double LOGE2L = 6.9314718055994530941723E-1L; +static volatile long double z; +static long double w, W, Wa, Wb, ya, yb, u; +static const long double huge = 0x1p10000L; +/* XXX Prevent gcc from erroneously constant folding this. */ +static volatile long double twom10000 = 0x1p-10000L; + +static long double reducl(long double); +static long double powil(long double, int); + +long double powl(long double x, long double y) +{ + /* double F, Fa, Fb, G, Ga, Gb, H, Ha, Hb */ + int i, nflg, iyflg, yoddint; + long e; + + if (y == 0.0L) + return 1.0L; + if (isnan(x)) + return x; + if (isnan(y)) + return y; + if (y == 1.0L) + return x; + + // FIXME: this is wrong, see pow special cases in c99 F.9.4.4 + if (!isfinite(y) && (x == -1.0L || x == 1.0L) ) + return y - y; /* +-1**inf is NaN */ + if (x == 1.0L) + return 1.0L; + if (y >= LDBL_MAX) { + if (x > 1.0L) + return INFINITY; + if (x > 0.0L && x < 1.0L) + return 0.0L; + if (x < -1.0L) + return INFINITY; + if (x > -1.0L && x < 0.0L) + return 0.0L; + } + if (y <= -LDBL_MAX) { + if (x > 1.0L) + return 0.0L; + if (x > 0.0L && x < 1.0L) + return INFINITY; + if (x < -1.0L) + return 0.0L; + if (x > -1.0L && x < 0.0L) + return INFINITY; + } + if (x >= LDBL_MAX) { + if (y > 0.0L) + return INFINITY; + return 0.0L; + } + + w = floorl(y); + /* Set iyflg to 1 if y is an integer. */ + iyflg = 0; + if (w == y) + iyflg = 1; + + /* Test for odd integer y. */ + yoddint = 0; + if (iyflg) { + ya = fabsl(y); + ya = floorl(0.5L * ya); + yb = 0.5L * fabsl(w); + if( ya != yb ) + yoddint = 1; + } + + if (x <= -LDBL_MAX) { + if (y > 0.0L) { + if (yoddint) + return -INFINITY; + return INFINITY; + } + if (y < 0.0L) { + if (yoddint) + return -0.0L; + return 0.0; + } + } + + + nflg = 0; /* flag = 1 if x<0 raised to integer power */ + if (x <= 0.0L) { + if (x == 0.0L) { + if (y < 0.0) { + if (signbit(x) && yoddint) + return -INFINITY; + return INFINITY; + } + if (y > 0.0) { + if (signbit(x) && yoddint) + return -0.0L; + return 0.0; + } + if (y == 0.0L) + return 1.0L; /* 0**0 */ + return 0.0L; /* 0**y */ + } + if (iyflg == 0) + return (x - x) / (x - x); /* (x<0)**(non-int) is NaN */ + nflg = 1; + } + + /* Integer power of an integer. */ + if (iyflg) { + i = w; + w = floorl(x); + if (w == x && fabsl(y) < 32768.0) { + w = powil(x, (int)y); + return w; + } + } + + if (nflg) + x = fabsl(x); + + /* separate significand from exponent */ + x = frexpl(x, &i); + e = i; + + /* find significand in antilog table A[] */ + i = 1; + if (x <= douba(17)) + i = 17; + if (x <= douba(i+8)) + i += 8; + if (x <= douba(i+4)) + i += 4; + if (x <= douba(i+2)) + i += 2; + if (x >= douba(1)) + i = -1; + i += 1; + + /* Find (x - A[i])/A[i] + * in order to compute log(x/A[i]): + * + * log(x) = log( a x/a ) = log(a) + log(x/a) + * + * log(x/a) = log(1+v), v = x/a - 1 = (x-a)/a + */ + x -= douba(i); + x -= doubb(i/2); + x /= douba(i); + + /* rational approximation for log(1+v): + * + * log(1+v) = v - v**2/2 + v**3 P(v) / Q(v) + */ + z = x*x; + w = x * (z * __polevll(x, P, 3) / __p1evll(x, Q, 3)); + w = w - ldexpl(z, -1); /* w - 0.5 * z */ + + /* Convert to base 2 logarithm: + * multiply by log2(e) = 1 + LOG2EA + */ + z = LOG2EA * w; + z += w; + z += LOG2EA * x; + z += x; + + /* Compute exponent term of the base 2 logarithm. */ + w = -i; + w = ldexpl(w, -LNXT); /* divide by NXT */ + w += e; + /* Now base 2 log of x is w + z. */ + + /* Multiply base 2 log by y, in extended precision. */ + + /* separate y into large part ya + * and small part yb less than 1/NXT + */ + ya = reducl(y); + yb = y - ya; + + /* (w+z)(ya+yb) + * = w*ya + w*yb + z*y + */ + F = z * y + w * yb; + Fa = reducl(F); + Fb = F - Fa; + + G = Fa + w * ya; + Ga = reducl(G); + Gb = G - Ga; + + H = Fb + Gb; + Ha = reducl(H); + w = ldexpl( Ga+Ha, LNXT ); + + /* Test the power of 2 for overflow */ + if (w > MEXP) + return huge * huge; /* overflow */ + if (w < MNEXP) + return twom10000 * twom10000; /* underflow */ + + e = w; + Hb = H - Ha; + + if (Hb > 0.0L) { + e += 1; + Hb -= 1.0L/NXT; /*0.0625L;*/ + } + + /* Now the product y * log2(x) = Hb + e/NXT. + * + * Compute base 2 exponential of Hb, + * where -0.0625 <= Hb <= 0. + */ + z = Hb * __polevll(Hb, R, 6); /* z = 2**Hb - 1 */ + + /* Express e/NXT as an integer plus a negative number of (1/NXT)ths. + * Find lookup table entry for the fractional power of 2. + */ + if (e < 0) + i = 0; + else + i = 1; + i = e/NXT + i; + e = NXT*i - e; + w = douba(e); + z = w * z; /* 2**-e * ( 1 + (2**Hb-1) ) */ + z = z + w; + z = ldexpl(z, i); /* multiply by integer power of 2 */ + + if (nflg) { + /* For negative x, + * find out if the integer exponent + * is odd or even. + */ + w = ldexpl(y, -1); + w = floorl(w); + w = ldexpl(w, 1); + if (w != y) + z = -z; /* odd exponent */ + } + + return z; +} + + +/* Find a multiple of 1/NXT that is within 1/NXT of x. */ +static long double reducl(long double x) +{ + long double t; + + t = ldexpl(x, LNXT); + t = floorl(t); + t = ldexpl(t, -LNXT); + return t; +} + +/* powil.c + * + * Real raised to integer power, long double precision + * + * + * SYNOPSIS: + * + * long double x, y, powil(); + * int n; + * + * y = powil( x, n ); + * + * + * DESCRIPTION: + * + * Returns argument x raised to the nth power. + * The routine efficiently decomposes n as a sum of powers of + * two. The desired power is a product of two-to-the-kth + * powers of x. Thus to compute the 32767 power of x requires + * 28 multiplications instead of 32767 multiplications. + * + * + * ACCURACY: + * + * Relative error: + * arithmetic x domain n domain # trials peak rms + * IEEE .001,1000 -1022,1023 50000 4.3e-17 7.8e-18 + * IEEE 1,2 -1022,1023 20000 3.9e-17 7.6e-18 + * IEEE .99,1.01 0,8700 10000 3.6e-16 7.2e-17 + * + * Returns MAXNUM on overflow, zero on underflow. + */ + +static long double powil(long double x, int nn) +{ + long double ww, y; + long double s; + int n, e, sign, asign, lx; + + if (x == 0.0L) { + if (nn == 0) + return 1.0L; + else if (nn < 0) + return LDBL_MAX; + return 0.0L; + } + + if (nn == 0) + return 1.0L; + + if (x < 0.0L) { + asign = -1; + x = -x; + } else + asign = 0; + + if (nn < 0) { + sign = -1; + n = -nn; + } else { + sign = 1; + n = nn; + } + + /* Overflow detection */ + + /* Calculate approximate logarithm of answer */ + s = x; + s = frexpl( s, &lx); + e = (lx - 1)*n; + if ((e == 0) || (e > 64) || (e < -64)) { + s = (s - 7.0710678118654752e-1L) / (s + 7.0710678118654752e-1L); + s = (2.9142135623730950L * s - 0.5L + lx) * nn * LOGE2L; + } else { + s = LOGE2L * e; + } + + if (s > MAXLOGL) + return huge * huge; /* overflow */ + + if (s < MINLOGL) + return twom10000 * twom10000; /* underflow */ + /* Handle tiny denormal answer, but with less accuracy + * since roundoff error in 1.0/x will be amplified. + * The precise demarcation should be the gradual underflow threshold. + */ + if (s < -MAXLOGL+2.0L) { + x = 1.0L/x; + sign = -sign; + } + + /* First bit of the power */ + if (n & 1) + y = x; + else { + y = 1.0L; + asign = 0; + } + + ww = x; + n >>= 1; + while (n) { + ww = ww * ww; /* arg to the 2-to-the-kth power */ + if (n & 1) /* if that bit is set, then include in product */ + y *= ww; + n >>= 1; + } + + if (asign) + y = -y; /* odd power of negative number */ + if (sign < 0) + y = 1.0L/y; + return y; +} + +#endif |