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Diffstat (limited to 'pl/math/log.c')
| -rw-r--r-- | pl/math/log.c | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/pl/math/log.c b/pl/math/log.c new file mode 100644 index 0000000..40b0441 --- /dev/null +++ b/pl/math/log.c @@ -0,0 +1,161 @@ +/* + * Double-precision log(x) function. + * + * Copyright (c) 2018-2023, Arm Limited. + * SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception + */ + +#include <float.h> +#include <math.h> +#include <stdint.h> +#include "math_config.h" + +#define T __log_data.tab +#define T2 __log_data.tab2 +#define B __log_data.poly1 +#define A __log_data.poly +#define Ln2hi __log_data.ln2hi +#define Ln2lo __log_data.ln2lo +#define N (1 << LOG_TABLE_BITS) +#define OFF 0x3fe6000000000000 + +/* Top 16 bits of a double. */ +static inline uint32_t +top16 (double x) +{ + return asuint64 (x) >> 48; +} + +double +optr_aor_log_f64 (double x) +{ + /* double_t for better performance on targets with FLT_EVAL_METHOD==2. */ + double_t w, z, r, r2, r3, y, invc, logc, kd, hi, lo; + uint64_t ix, iz, tmp; + uint32_t top; + int k, i; + + ix = asuint64 (x); + top = top16 (x); + +#if LOG_POLY1_ORDER == 10 || LOG_POLY1_ORDER == 11 +#define LO asuint64 (1.0 - 0x1p-5) +#define HI asuint64 (1.0 + 0x1.1p-5) +#elif LOG_POLY1_ORDER == 12 +#define LO asuint64 (1.0 - 0x1p-4) +#define HI asuint64 (1.0 + 0x1.09p-4) +#endif + if (unlikely (ix - LO < HI - LO)) + { + /* Handle close to 1.0 inputs separately. */ + /* Fix sign of zero with downward rounding when x==1. */ + if (WANT_ROUNDING && unlikely (ix == asuint64 (1.0))) + return 0; + r = x - 1.0; + r2 = r * r; + r3 = r * r2; +#if LOG_POLY1_ORDER == 10 + /* Worst-case error is around 0.516 ULP. */ + y = r3 + * (B[1] + r * B[2] + r2 * B[3] + + r3 * (B[4] + r * B[5] + r2 * B[6] + r3 * (B[7] + r * B[8]))); + w = B[0] * r2; /* B[0] == -0.5. */ + hi = r + w; + y += r - hi + w; + y += hi; +#elif LOG_POLY1_ORDER == 11 + /* Worst-case error is around 0.516 ULP. */ + y = r3 + * (B[1] + r * B[2] + + r2 + * (B[3] + r * B[4] + r2 * B[5] + + r3 * (B[6] + r * B[7] + r2 * B[8] + r3 * B[9]))); + w = B[0] * r2; /* B[0] == -0.5. */ + hi = r + w; + y += r - hi + w; + y += hi; +#elif LOG_POLY1_ORDER == 12 + y = r3 + * (B[1] + r * B[2] + r2 * B[3] + + r3 + * (B[4] + r * B[5] + r2 * B[6] + + r3 * (B[7] + r * B[8] + r2 * B[9] + r3 * B[10]))); +#if N <= 64 + /* Worst-case error is around 0.532 ULP. */ + w = B[0] * r2; /* B[0] == -0.5. */ + hi = r + w; + y += r - hi + w; + y += hi; +#else + /* Worst-case error is around 0.507 ULP. */ + w = r * 0x1p27; + double_t rhi = r + w - w; + double_t rlo = r - rhi; + w = rhi * rhi * B[0]; /* B[0] == -0.5. */ + hi = r + w; + lo = r - hi + w; + lo += B[0] * rlo * (rhi + r); + y += lo; + y += hi; +#endif +#endif + return eval_as_double (y); + } + if (unlikely (top - 0x0010 >= 0x7ff0 - 0x0010)) + { + /* x < 0x1p-1022 or inf or nan. */ + if (ix * 2 == 0) + return __math_divzero (1); + if (ix == asuint64 (INFINITY)) /* log(inf) == inf. */ + return x; + if ((top & 0x8000) || (top & 0x7ff0) == 0x7ff0) + return __math_invalid (x); + /* x is subnormal, normalize it. */ + ix = asuint64 (x * 0x1p52); + ix -= 52ULL << 52; + } + + /* x = 2^k z; where z is in range [OFF,2*OFF) and exact. + The range is split into N subintervals. + The ith subinterval contains z and c is near its center. */ + tmp = ix - OFF; + i = (tmp >> (52 - LOG_TABLE_BITS)) % N; + k = (int64_t) tmp >> 52; /* arithmetic shift */ + iz = ix - (tmp & 0xfffULL << 52); + invc = T[i].invc; + logc = T[i].logc; + z = asdouble (iz); + + /* log(x) = log1p(z/c-1) + log(c) + k*Ln2. */ + /* r ~= z/c - 1, |r| < 1/(2*N). */ +#if HAVE_FAST_FMA + /* rounding error: 0x1p-55/N. */ + r = fma (z, invc, -1.0); +#else + /* rounding error: 0x1p-55/N + 0x1p-66. */ + r = (z - T2[i].chi - T2[i].clo) * invc; +#endif + kd = (double_t) k; + + /* hi + lo = r + log(c) + k*Ln2. */ + w = kd * Ln2hi + logc; + hi = w + r; + lo = w - hi + r + kd * Ln2lo; + + /* log(x) = lo + (log1p(r) - r) + hi. */ + r2 = r * r; /* rounding error: 0x1p-54/N^2. */ + /* Worst case error if |y| > 0x1p-5: + 0.5 + 4.13/N + abs-poly-error*2^57 ULP (+ 0.002 ULP without fma) + Worst case error if |y| > 0x1p-4: + 0.5 + 2.06/N + abs-poly-error*2^56 ULP (+ 0.001 ULP without fma). */ +#if LOG_POLY_ORDER == 6 + y = lo + r2 * A[0] + r * r2 * (A[1] + r * A[2] + r2 * (A[3] + r * A[4])) + hi; +#elif LOG_POLY_ORDER == 7 + y = lo + + r2 + * (A[0] + r * A[1] + r2 * (A[2] + r * A[3]) + + r2 * r2 * (A[4] + r * A[5])) + + hi; +#endif + return eval_as_double (y); +} |