diff options
Diffstat (limited to 'sysdeps/ieee754/flt-32/e_expf.c')
| -rw-r--r-- | sysdeps/ieee754/flt-32/e_expf.c | 185 |
1 files changed, 77 insertions, 108 deletions
diff --git a/sysdeps/ieee754/flt-32/e_expf.c b/sysdeps/ieee754/flt-32/e_expf.c index 782072f213..12239e1862 100644 --- a/sysdeps/ieee754/flt-32/e_expf.c +++ b/sysdeps/ieee754/flt-32/e_expf.c @@ -1,7 +1,6 @@ -/* Single-precision floating point e^x. - Copyright (C) 1997-2017 Free Software Foundation, Inc. +/* Single-precision e^x function. + Copyright (C) 2017 Free Software Foundation, Inc. This file is part of the GNU C Library. - Contributed by Geoffrey Keating <geoffk@ozemail.com.au> The GNU C Library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public @@ -17,117 +16,87 @@ License along with the GNU C Library; if not, see <http://www.gnu.org/licenses/>. */ -/* How this works: - - The input value, x, is written as - - x = n * ln(2) + t/512 + delta[t] + x; - - where: - - n is an integer, 127 >= n >= -150; - - t is an integer, 177 >= t >= -177 - - delta is based on a table entry, delta[t] < 2^-28 - - x is whatever is left, |x| < 2^-10 - - Then e^x is approximated as - - e^x = 2^n ( e^(t/512 + delta[t]) - + ( e^(t/512 + delta[t]) - * ( p(x + delta[t] + n * ln(2)) - delta ) ) ) - - where - - p(x) is a polynomial approximating e(x)-1; - - e^(t/512 + delta[t]) is obtained from a table. - - The table used is the same one as for the double precision version; - since we have the table, we might as well use it. - - It turns out to be faster to do calculations in double precision than - to perform an 'accurate table method' expf, because of the range reduction - overhead (compare exp2f). - */ -#include <float.h> -#include <ieee754.h> #include <math.h> -#include <fenv.h> -#include <inttypes.h> -#include <math_private.h> - -extern const float __exp_deltatable[178]; -extern const double __exp_atable[355] /* __attribute__((mode(DF))) */; - -static const float TWOM100 = 7.88860905e-31; -static const float TWO127 = 1.7014118346e+38; +#include <stdint.h> +#include "math_config.h" + +/* +EXP2F_TABLE_BITS = 5 +EXP2F_POLY_ORDER = 3 + +ULP error: 0.502 (nearest rounding.) +Relative error: 1.69 * 2^-34 in [-ln2/64, ln2/64] (before rounding.) +Wrong count: 170635 (all nearest rounding wrong results with fma.) +Non-nearest ULP error: 1 (rounded ULP error) +*/ + +#define N (1 << EXP2F_TABLE_BITS) +#define InvLn2N __exp2f_data.invln2_scaled +#define T __exp2f_data.tab +#define C __exp2f_data.poly_scaled + +static inline uint32_t +top12 (float x) +{ + return asuint (x) >> 20; +} float __ieee754_expf (float x) { - static const float himark = 88.72283935546875; - static const float lomark = -103.972084045410; - /* Check for usual case. */ - if (isless (x, himark) && isgreater (x, lomark)) + uint32_t abstop; + uint64_t ki, t; + /* double_t for better performance on targets with FLT_EVAL_METHOD==2. */ + double_t kd, xd, z, r, r2, y, s; + + xd = (double_t) x; + abstop = top12 (x) & 0x7ff; + if (__glibc_unlikely (abstop >= top12 (88.0f))) { - static const float THREEp42 = 13194139533312.0; - static const float THREEp22 = 12582912.0; - /* 1/ln(2). */ -#undef M_1_LN2 - static const float M_1_LN2 = 1.44269502163f; - /* ln(2) */ -#undef M_LN2 - static const double M_LN2 = .6931471805599452862; - - int tval; - double x22, t, result, dx; - float n, delta; - union ieee754_double ex2_u; - - { - SET_RESTORE_ROUND_NOEXF (FE_TONEAREST); - - /* Calculate n. */ - n = x * M_1_LN2 + THREEp22; - n -= THREEp22; - dx = x - n*M_LN2; - - /* Calculate t/512. */ - t = dx + THREEp42; - t -= THREEp42; - dx -= t; - - /* Compute tval = t. */ - tval = (int) (t * 512.0); - - if (t >= 0) - delta = - __exp_deltatable[tval]; - else - delta = __exp_deltatable[-tval]; - - /* Compute ex2 = 2^n e^(t/512+delta[t]). */ - ex2_u.d = __exp_atable[tval+177]; - ex2_u.ieee.exponent += (int) n; - - /* Approximate e^(dx+delta) - 1, using a second-degree polynomial, - with maximum error in [-2^-10-2^-28,2^-10+2^-28] - less than 5e-11. */ - x22 = (0.5000000496709180453 * dx + 1.0000001192102037084) * dx + delta; - } - - /* Return result. */ - result = x22 * ex2_u.d + ex2_u.d; - return (float) result; + /* |x| >= 88 or x is nan. */ + if (asuint (x) == asuint (-INFINITY)) + return 0.0f; + if (abstop >= top12 (INFINITY)) + return x + x; + if (x > 0x1.62e42ep6f) /* x > log(0x1p128) ~= 88.72 */ + return __math_oflowf (0); + if (x < -0x1.9fe368p6f) /* x < log(0x1p-150) ~= -103.97 */ + return __math_uflowf (0); +#if WANT_ERRNO_UFLOW + if (x < -0x1.9d1d9ep6f) /* x < log(0x1p-149) ~= -103.28 */ + return __math_may_uflowf (0); +#endif } - /* Exceptional cases: */ - else if (isless (x, himark)) - { - if (isinf (x)) - /* e^-inf == 0, with no error. */ - return 0; - else - /* Underflow */ - return TWOM100 * TWOM100; - } - else - /* Return x, if x is a NaN or Inf; or overflow, otherwise. */ - return TWO127*x; + + /* x*N/Ln2 = k + r with r in [-1/2, 1/2] and int k. */ + z = InvLn2N * xd; + + /* Round and convert z to int, the result is in [-150*N, 128*N] and + ideally ties-to-even rule is used, otherwise the magnitude of r + can be bigger which gives larger approximation error. */ +#if TOINT_INTRINSICS + kd = roundtoint (z); + ki = converttoint (z); +#elif TOINT_RINT + kd = rint (z); + ki = (long) kd; +#elif TOINT_SHIFT +# define SHIFT __exp2f_data.shift + kd = math_narrow_eval ((double) (z + SHIFT)); /* Needs to be double. */ + ki = asuint64 (kd); + kd -= SHIFT; +#endif + r = z - kd; + + /* exp(x) = 2^(k/N) * 2^(r/N) ~= s * (C0*r^3 + C1*r^2 + C2*r + 1) */ + t = T[ki % N]; + t += ki << (52 - EXP2F_TABLE_BITS); + s = asdouble (t); + z = C[0] * r + C[1]; + r2 = r * r; + y = C[2] * r + 1; + y = z * r2 + y; + y = y * s; + return (float) y; } strong_alias (__ieee754_expf, __expf_finite) |