diff options
Diffstat (limited to 'src/math/sqrt.c')
| -rw-r--r-- | src/math/sqrt.c | 320 |
1 files changed, 147 insertions, 173 deletions
diff --git a/src/math/sqrt.c b/src/math/sqrt.c index f1f6d76c..5ba26559 100644 --- a/src/math/sqrt.c +++ b/src/math/sqrt.c @@ -1,184 +1,158 @@ -/* origin: FreeBSD /usr/src/lib/msun/src/e_sqrt.c */ -/* - * ==================================================== - * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. - * - * Developed at SunSoft, a Sun Microsystems, Inc. business. - * Permission to use, copy, modify, and distribute this - * software is freely granted, provided that this notice - * is preserved. - * ==================================================== - */ -/* sqrt(x) - * Return correctly rounded sqrt. - * ------------------------------------------ - * | Use the hardware sqrt if you have one | - * ------------------------------------------ - * Method: - * Bit by bit method using integer arithmetic. (Slow, but portable) - * 1. Normalization - * Scale x to y in [1,4) with even powers of 2: - * find an integer k such that 1 <= (y=x*2^(2k)) < 4, then - * sqrt(x) = 2^k * sqrt(y) - * 2. Bit by bit computation - * Let q = sqrt(y) truncated to i bit after binary point (q = 1), - * i 0 - * i+1 2 - * s = 2*q , and y = 2 * ( y - q ). (1) - * i i i i - * - * To compute q from q , one checks whether - * i+1 i - * - * -(i+1) 2 - * (q + 2 ) <= y. (2) - * i - * -(i+1) - * If (2) is false, then q = q ; otherwise q = q + 2 . - * i+1 i i+1 i - * - * With some algebric manipulation, it is not difficult to see - * that (2) is equivalent to - * -(i+1) - * s + 2 <= y (3) - * i i - * - * The advantage of (3) is that s and y can be computed by - * i i - * the following recurrence formula: - * if (3) is false - * - * s = s , y = y ; (4) - * i+1 i i+1 i - * - * otherwise, - * -i -(i+1) - * s = s + 2 , y = y - s - 2 (5) - * i+1 i i+1 i i - * - * One may easily use induction to prove (4) and (5). - * Note. Since the left hand side of (3) contain only i+2 bits, - * it does not necessary to do a full (53-bit) comparison - * in (3). - * 3. Final rounding - * After generating the 53 bits result, we compute one more bit. - * Together with the remainder, we can decide whether the - * result is exact, bigger than 1/2ulp, or less than 1/2ulp - * (it will never equal to 1/2ulp). - * The rounding mode can be detected by checking whether - * huge + tiny is equal to huge, and whether huge - tiny is - * equal to huge for some floating point number "huge" and "tiny". - * - * Special cases: - * sqrt(+-0) = +-0 ... exact - * sqrt(inf) = inf - * sqrt(-ve) = NaN ... with invalid signal - * sqrt(NaN) = NaN ... with invalid signal for signaling NaN - */ - +#include <stdint.h> +#include <math.h> #include "libm.h" +#include "sqrt_data.h" -static const double tiny = 1.0e-300; +#define FENV_SUPPORT 1 -double sqrt(double x) +/* returns a*b*2^-32 - e, with error 0 <= e < 1. */ +static inline uint32_t mul32(uint32_t a, uint32_t b) { - double z; - int32_t sign = (int)0x80000000; - int32_t ix0,s0,q,m,t,i; - uint32_t r,t1,s1,ix1,q1; + return (uint64_t)a*b >> 32; +} - EXTRACT_WORDS(ix0, ix1, x); +/* returns a*b*2^-64 - e, with error 0 <= e < 3. */ +static inline uint64_t mul64(uint64_t a, uint64_t b) +{ + uint64_t ahi = a>>32; + uint64_t alo = a&0xffffffff; + uint64_t bhi = b>>32; + uint64_t blo = b&0xffffffff; + return ahi*bhi + (ahi*blo >> 32) + (alo*bhi >> 32); +} - /* take care of Inf and NaN */ - if ((ix0&0x7ff00000) == 0x7ff00000) { - return x*x + x; /* sqrt(NaN)=NaN, sqrt(+inf)=+inf, sqrt(-inf)=sNaN */ - } - /* take care of zero */ - if (ix0 <= 0) { - if (((ix0&~sign)|ix1) == 0) - return x; /* sqrt(+-0) = +-0 */ - if (ix0 < 0) - return (x-x)/(x-x); /* sqrt(-ve) = sNaN */ - } - /* normalize x */ - m = ix0>>20; - if (m == 0) { /* subnormal x */ - while (ix0 == 0) { - m -= 21; - ix0 |= (ix1>>11); - ix1 <<= 21; - } - for (i=0; (ix0&0x00100000) == 0; i++) - ix0<<=1; - m -= i - 1; - ix0 |= ix1>>(32-i); - ix1 <<= i; - } - m -= 1023; /* unbias exponent */ - ix0 = (ix0&0x000fffff)|0x00100000; - if (m & 1) { /* odd m, double x to make it even */ - ix0 += ix0 + ((ix1&sign)>>31); - ix1 += ix1; - } - m >>= 1; /* m = [m/2] */ - - /* generate sqrt(x) bit by bit */ - ix0 += ix0 + ((ix1&sign)>>31); - ix1 += ix1; - q = q1 = s0 = s1 = 0; /* [q,q1] = sqrt(x) */ - r = 0x00200000; /* r = moving bit from right to left */ - - while (r != 0) { - t = s0 + r; - if (t <= ix0) { - s0 = t + r; - ix0 -= t; - q += r; - } - ix0 += ix0 + ((ix1&sign)>>31); - ix1 += ix1; - r >>= 1; - } +double sqrt(double x) +{ + uint64_t ix, top, m; - r = sign; - while (r != 0) { - t1 = s1 + r; - t = s0; - if (t < ix0 || (t == ix0 && t1 <= ix1)) { - s1 = t1 + r; - if ((t1&sign) == sign && (s1&sign) == 0) - s0++; - ix0 -= t; - if (ix1 < t1) - ix0--; - ix1 -= t1; - q1 += r; - } - ix0 += ix0 + ((ix1&sign)>>31); - ix1 += ix1; - r >>= 1; + /* special case handling. */ + ix = asuint64(x); + top = ix >> 52; + if (predict_false(top - 0x001 >= 0x7ff - 0x001)) { + /* x < 0x1p-1022 or inf or nan. */ + if (ix * 2 == 0) + return x; + if (ix == 0x7ff0000000000000) + return x; + if (ix > 0x7ff0000000000000) + return __math_invalid(x); + /* x is subnormal, normalize it. */ + ix = asuint64(x * 0x1p52); + top = ix >> 52; + top -= 52; } - /* use floating add to find out rounding direction */ - if ((ix0|ix1) != 0) { - z = 1.0 - tiny; /* raise inexact flag */ - if (z >= 1.0) { - z = 1.0 + tiny; - if (q1 == (uint32_t)0xffffffff) { - q1 = 0; - q++; - } else if (z > 1.0) { - if (q1 == (uint32_t)0xfffffffe) - q++; - q1 += 2; - } else - q1 += q1 & 1; - } + /* argument reduction: + x = 4^e m; with integer e, and m in [1, 4) + m: fixed point representation [2.62] + 2^e is the exponent part of the result. */ + int even = top & 1; + m = (ix << 11) | 0x8000000000000000; + if (even) m >>= 1; + top = (top + 0x3ff) >> 1; + + /* approximate r ~ 1/sqrt(m) and s ~ sqrt(m) when m in [1,4) + + initial estimate: + 7bit table lookup (1bit exponent and 6bit significand). + + iterative approximation: + using 2 goldschmidt iterations with 32bit int arithmetics + and a final iteration with 64bit int arithmetics. + + details: + + the relative error (e = r0 sqrt(m)-1) of a linear estimate + (r0 = a m + b) is |e| < 0.085955 ~ 0x1.6p-4 at best, + a table lookup is faster and needs one less iteration + 6 bit lookup table (128b) gives |e| < 0x1.f9p-8 + 7 bit lookup table (256b) gives |e| < 0x1.fdp-9 + for single and double prec 6bit is enough but for quad + prec 7bit is needed (or modified iterations). to avoid + one more iteration >=13bit table would be needed (16k). + + a newton-raphson iteration for r is + w = r*r + u = 3 - m*w + r = r*u/2 + can use a goldschmidt iteration for s at the end or + s = m*r + + first goldschmidt iteration is + s = m*r + u = 3 - s*r + r = r*u/2 + s = s*u/2 + next goldschmidt iteration is + u = 3 - s*r + r = r*u/2 + s = s*u/2 + and at the end r is not computed only s. + + they use the same amount of operations and converge at the + same quadratic rate, i.e. if + r1 sqrt(m) - 1 = e, then + r2 sqrt(m) - 1 = -3/2 e^2 - 1/2 e^3 + the advantage of goldschmidt is that the mul for s and r + are independent (computed in parallel), however it is not + "self synchronizing": it only uses the input m in the + first iteration so rounding errors accumulate. at the end + or when switching to larger precision arithmetics rounding + errors dominate so the first iteration should be used. + + the fixed point representations are + m: 2.30 r: 0.32, s: 2.30, d: 2.30, u: 2.30, three: 2.30 + and after switching to 64 bit + m: 2.62 r: 0.64, s: 2.62, d: 2.62, u: 2.62, three: 2.62 */ + + static const uint64_t three = 0xc0000000; + uint64_t r, s, d, u, i; + + i = (ix >> 46) % 128; + r = (uint32_t)__rsqrt_tab[i] << 16; + /* |r sqrt(m) - 1| < 0x1.fdp-9 */ + s = mul32(m>>32, r); + /* |s/sqrt(m) - 1| < 0x1.fdp-9 */ + d = mul32(s, r); + u = three - d; + r = mul32(r, u) << 1; + /* |r sqrt(m) - 1| < 0x1.7bp-16 */ + s = mul32(s, u) << 1; + /* |s/sqrt(m) - 1| < 0x1.7bp-16 */ + d = mul32(s, r); + u = three - d; + r = mul32(r, u) << 1; + /* |r sqrt(m) - 1| < 0x1.3704p-29 (measured worst-case) */ + r = r << 32; + s = mul64(m, r); + d = mul64(s, r); + u = (three<<32) - d; + s = mul64(s, u); /* repr: 3.61 */ + /* -0x1p-57 < s - sqrt(m) < 0x1.8001p-61 */ + s = (s - 2) >> 9; /* repr: 12.52 */ + /* -0x1.09p-52 < s - sqrt(m) < -0x1.fffcp-63 */ + + /* s < sqrt(m) < s + 0x1.09p-52, + compute nearest rounded result: + the nearest result to 52 bits is either s or s+0x1p-52, + we can decide by comparing (2^52 s + 0.5)^2 to 2^104 m. */ + uint64_t d0, d1, d2; + double y, t; + d0 = (m << 42) - s*s; + d1 = s - d0; + d2 = d1 + s + 1; + s += d1 >> 63; + s &= 0x000fffffffffffff; + s |= top << 52; + y = asdouble(s); + if (FENV_SUPPORT) { + /* handle rounding modes and inexact exception: + only (s+1)^2 == 2^42 m case is exact otherwise + add a tiny value to cause the fenv effects. */ + uint64_t tiny = predict_false(d2==0) ? 0 : 0x0010000000000000; + tiny |= (d1^d2) & 0x8000000000000000; + t = asdouble(tiny); + y = eval_as_double(y + t); } - ix0 = (q>>1) + 0x3fe00000; - ix1 = q1>>1; - if (q&1) - ix1 |= sign; - INSERT_WORDS(z, ix0 + ((uint32_t)m << 20), ix1); - return z; + return y; } |