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Diffstat (limited to 'lib/mlibc/options/ansi/musl-generic-math/exp2f.c')
| -rw-r--r-- | lib/mlibc/options/ansi/musl-generic-math/exp2f.c | 126 |
1 files changed, 126 insertions, 0 deletions
diff --git a/lib/mlibc/options/ansi/musl-generic-math/exp2f.c b/lib/mlibc/options/ansi/musl-generic-math/exp2f.c new file mode 100644 index 0000000..296b634 --- /dev/null +++ b/lib/mlibc/options/ansi/musl-generic-math/exp2f.c @@ -0,0 +1,126 @@ +/* origin: FreeBSD /usr/src/lib/msun/src/s_exp2f.c */ +/*- + * Copyright (c) 2005 David Schultz <das@FreeBSD.ORG> + * All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions + * are met: + * 1. Redistributions of source code must retain the above copyright + * notice, this list of conditions and the following disclaimer. + * 2. Redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in the + * documentation and/or other materials provided with the distribution. + * + * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND + * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE + * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE + * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE + * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL + * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS + * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) + * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT + * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY + * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF + * SUCH DAMAGE. + */ + +#include "libm.h" + +#define TBLSIZE 16 + +static const float +redux = 0x1.8p23f / TBLSIZE, +P1 = 0x1.62e430p-1f, +P2 = 0x1.ebfbe0p-3f, +P3 = 0x1.c6b348p-5f, +P4 = 0x1.3b2c9cp-7f; + +static const double exp2ft[TBLSIZE] = { + 0x1.6a09e667f3bcdp-1, + 0x1.7a11473eb0187p-1, + 0x1.8ace5422aa0dbp-1, + 0x1.9c49182a3f090p-1, + 0x1.ae89f995ad3adp-1, + 0x1.c199bdd85529cp-1, + 0x1.d5818dcfba487p-1, + 0x1.ea4afa2a490dap-1, + 0x1.0000000000000p+0, + 0x1.0b5586cf9890fp+0, + 0x1.172b83c7d517bp+0, + 0x1.2387a6e756238p+0, + 0x1.306fe0a31b715p+0, + 0x1.3dea64c123422p+0, + 0x1.4bfdad5362a27p+0, + 0x1.5ab07dd485429p+0, +}; + +/* + * exp2f(x): compute the base 2 exponential of x + * + * Accuracy: Peak error < 0.501 ulp; location of peak: -0.030110927. + * + * Method: (equally-spaced tables) + * + * Reduce x: + * x = k + y, for integer k and |y| <= 1/2. + * Thus we have exp2f(x) = 2**k * exp2(y). + * + * Reduce y: + * y = i/TBLSIZE + z for integer i near y * TBLSIZE. + * Thus we have exp2(y) = exp2(i/TBLSIZE) * exp2(z), + * with |z| <= 2**-(TBLSIZE+1). + * + * We compute exp2(i/TBLSIZE) via table lookup and exp2(z) via a + * degree-4 minimax polynomial with maximum error under 1.4 * 2**-33. + * Using double precision for everything except the reduction makes + * roundoff error insignificant and simplifies the scaling step. + * + * This method is due to Tang, but I do not use his suggested parameters: + * + * Tang, P. Table-driven Implementation of the Exponential Function + * in IEEE Floating-Point Arithmetic. TOMS 15(2), 144-157 (1989). + */ +float exp2f(float x) +{ + double_t t, r, z; + union {float f; uint32_t i;} u = {x}; + union {double f; uint64_t i;} uk; + uint32_t ix, i0, k; + + /* Filter out exceptional cases. */ + ix = u.i & 0x7fffffff; + if (ix > 0x42fc0000) { /* |x| > 126 */ + if (ix > 0x7f800000) /* NaN */ + return x; + if (u.i >= 0x43000000 && u.i < 0x80000000) { /* x >= 128 */ + x *= 0x1p127f; + return x; + } + if (u.i >= 0x80000000) { /* x < -126 */ + if (u.i >= 0xc3160000 || (u.i & 0x0000ffff)) + FORCE_EVAL(-0x1p-149f/x); + if (u.i >= 0xc3160000) /* x <= -150 */ + return 0; + } + } else if (ix <= 0x33000000) { /* |x| <= 0x1p-25 */ + return 1.0f + x; + } + + /* Reduce x, computing z, i0, and k. */ + u.f = x + redux; + i0 = u.i; + i0 += TBLSIZE / 2; + k = i0 / TBLSIZE; + uk.i = (uint64_t)(0x3ff + k)<<52; + i0 &= TBLSIZE - 1; + u.f -= redux; + z = x - u.f; + /* Compute r = exp2(y) = exp2ft[i0] * p(z). */ + r = exp2ft[i0]; + t = r * z; + r = r + t * (P1 + z * P2) + t * (z * z) * (P3 + z * P4); + + /* Scale by 2**k */ + return r * uk.f; +} |