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Justine Tunney 2020-06-15 07:18:57 -07:00
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libc/math/log.c Normal file
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/*
* Double-precision log(x) function.
*
* Copyright (c) 2018, Arm Limited.
* SPDX-License-Identifier: MIT
*/
#include "libc/math/math.h"
#include "libc/math/libm.h"
#include "libc/math/log_data.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 log(double x)
{
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);
#define LO asuint64(1.0 - 0x1p-4)
#define HI asuint64(1.0 + 0x1.09p-4)
if (predict_false(ix - LO < HI - LO)) {
/* Handle close to 1.0 inputs separately. */
/* Fix sign of zero with downward rounding when x==1. */
if (WANT_ROUNDING && predict_false(ix == asuint64(1.0)))
return 0;
r = x - 1.0;
r2 = r * r;
r3 = r * r2;
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])));
/* 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;
return eval_as_double(y);
}
if (predict_false(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 __FP_FAST_FMA
/* rounding error: 0x1p-55/N. */
r = __builtin_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). */
y = lo + r2 * A[0] +
r * r2 * (A[1] + r * A[2] + r2 * (A[3] + r * A[4])) + hi;
return eval_as_double(y);
}