diff options
Diffstat (limited to 'sysdeps/ieee754/ldbl-96/e_gammal_r.c')
| -rw-r--r-- | sysdeps/ieee754/ldbl-96/e_gammal_r.c | 143 |
1 files changed, 132 insertions, 11 deletions
diff --git a/sysdeps/ieee754/ldbl-96/e_gammal_r.c b/sysdeps/ieee754/ldbl-96/e_gammal_r.c index 0974351a10..7cb3e8563a 100644 --- a/sysdeps/ieee754/ldbl-96/e_gammal_r.c +++ b/sysdeps/ieee754/ldbl-96/e_gammal_r.c @@ -19,14 +19,102 @@ #include <math.h> #include <math_private.h> +#include <float.h> +/* Coefficients B_2k / 2k(2k-1) of x^-(2k-1) inside exp in Stirling's + approximation to gamma function. */ + +static const long double gamma_coeff[] = + { + 0x1.5555555555555556p-4L, + -0xb.60b60b60b60b60bp-12L, + 0x3.4034034034034034p-12L, + -0x2.7027027027027028p-12L, + 0x3.72a3c5631fe46aep-12L, + -0x7.daac36664f1f208p-12L, + 0x1.a41a41a41a41a41ap-8L, + -0x7.90a1b2c3d4e5f708p-8L, + }; + +#define NCOEFF (sizeof (gamma_coeff) / sizeof (gamma_coeff[0])) + +/* Return gamma (X), for positive X less than 1766, in the form R * + 2^(*EXP2_ADJ), where R is the return value and *EXP2_ADJ is set to + avoid overflow or underflow in intermediate calculations. */ + +static long double +gammal_positive (long double x, int *exp2_adj) +{ + int local_signgam; + if (x < 0.5L) + { + *exp2_adj = 0; + return __ieee754_expl (__ieee754_lgammal_r (x + 1, &local_signgam)) / x; + } + else if (x <= 1.5L) + { + *exp2_adj = 0; + return __ieee754_expl (__ieee754_lgammal_r (x, &local_signgam)); + } + else if (x < 7.5L) + { + /* Adjust into the range for using exp (lgamma). */ + *exp2_adj = 0; + long double n = __ceill (x - 1.5L); + long double x_adj = x - n; + long double eps; + long double prod = __gamma_productl (x_adj, 0, n, &eps); + return (__ieee754_expl (__ieee754_lgammal_r (x_adj, &local_signgam)) + * prod * (1.0L + eps)); + } + else + { + long double eps = 0; + long double x_eps = 0; + long double x_adj = x; + long double prod = 1; + if (x < 13.0L) + { + /* Adjust into the range for applying Stirling's + approximation. */ + long double n = __ceill (13.0L - x); + x_adj = x + n; + x_eps = (x - (x_adj - n)); + prod = __gamma_productl (x_adj - n, x_eps, n, &eps); + } + /* The result is now gamma (X_ADJ + X_EPS) / (PROD * (1 + EPS)). + Compute gamma (X_ADJ + X_EPS) using Stirling's approximation, + starting by computing pow (X_ADJ, X_ADJ) with a power of 2 + factored out. */ + long double exp_adj = -eps; + long double x_adj_int = __roundl (x_adj); + long double x_adj_frac = x_adj - x_adj_int; + int x_adj_log2; + long double x_adj_mant = __frexpl (x_adj, &x_adj_log2); + if (x_adj_mant < M_SQRT1_2l) + { + x_adj_log2--; + x_adj_mant *= 2.0L; + } + *exp2_adj = x_adj_log2 * (int) x_adj_int; + long double ret = (__ieee754_powl (x_adj_mant, x_adj) + * __ieee754_exp2l (x_adj_log2 * x_adj_frac) + * __ieee754_expl (-x_adj) + * __ieee754_sqrtl (2 * M_PIl / x_adj) + / prod); + exp_adj += x_eps * __ieee754_logl (x); + long double bsum = gamma_coeff[NCOEFF - 1]; + long double x_adj2 = x_adj * x_adj; + for (size_t i = 1; i <= NCOEFF - 1; i++) + bsum = bsum / x_adj2 + gamma_coeff[NCOEFF - 1 - i]; + exp_adj += bsum / x_adj; + return ret + ret * __expm1l (exp_adj); + } +} long double __ieee754_gammal_r (long double x, int *signgamp) { - /* We don't have a real gamma implementation now. We'll use lgamma - and the exp function. But due to the required boundary - conditions we must check some values separately. */ u_int32_t es, hx, lx; GET_LDOUBLE_WORDS (es, hx, lx, x); @@ -43,22 +131,55 @@ __ieee754_gammal_r (long double x, int *signgamp) *signgamp = 0; return x - x; } - if (__builtin_expect ((es & 0x7fff) == 0x7fff, 0) - && ((hx & 0x7fffffff) | lx) != 0) + if (__builtin_expect ((es & 0x7fff) == 0x7fff, 0)) { - /* NaN, return it. */ + /* Positive infinity (return positive infinity) or NaN (return + NaN). */ *signgamp = 0; - return x; + return x + x; } - if (__builtin_expect ((es & 0x8000) != 0, 0) - && x < 0xffffffff && __rintl (x) == x) + if (__builtin_expect ((es & 0x8000) != 0, 0) && __rintl (x) == x) { /* Return value for integer x < 0 is NaN with invalid exception. */ *signgamp = 0; return (x - x) / (x - x); } - /* XXX FIXME. */ - return __ieee754_expl (__ieee754_lgammal_r (x, signgamp)); + if (x >= 1756.0L) + { + /* Overflow. */ + *signgamp = 0; + return LDBL_MAX * LDBL_MAX; + } + else if (x > 0.0L) + { + *signgamp = 0; + int exp2_adj; + long double ret = gammal_positive (x, &exp2_adj); + return __scalbnl (ret, exp2_adj); + } + else if (x >= -LDBL_EPSILON / 4.0L) + { + *signgamp = 0; + return 1.0f / x; + } + else + { + long double tx = __truncl (x); + *signgamp = (tx == 2.0L * __truncl (tx / 2.0L)) ? -1 : 1; + if (x <= -1766.0L) + /* Underflow. */ + return LDBL_MIN * LDBL_MIN; + long double frac = tx - x; + if (frac > 0.5L) + frac = 1.0L - frac; + long double sinpix = (frac <= 0.25L + ? __sinl (M_PIl * frac) + : __cosl (M_PIl * (0.5L - frac))); + int exp2_adj; + long double ret = M_PIl / (-x * sinpix + * gammal_positive (-x, &exp2_adj)); + return __scalbnl (ret, -exp2_adj); + } } strong_alias (__ieee754_gammal_r, __gammal_r_finite) |