2 * Single-precision log function.
4 * Copyright (c) 2017-2019, Arm Limited.
5 * SPDX-License-Identifier: MIT
10 #include "math_config.h"
16 ULP error: 0.818 (nearest rounding.)
17 Relative error: 1.957 * 2^-26 (before rounding.)
20 #define T __logf_data.tab
21 #define A __logf_data.poly
22 #define Ln2 __logf_data.ln2
23 #define N (1 << LOGF_TABLE_BITS)
24 #define OFF 0x3f330000
29 /* double_t for better performance on targets with FLT_EVAL_METHOD==2. */
30 double_t z, r, r2, y, y0, invc, logc;
36 /* Fix sign of zero with downward rounding when x==1. */
37 if (unlikely (ix == 0x3f800000))
40 if (unlikely (ix - 0x00800000 >= 0x7f800000 - 0x00800000))
42 /* x < 0x1p-126 or inf or nan. */
44 return __math_divzerof (1);
45 if (ix == 0x7f800000) /* log(inf) == inf. */
47 if ((ix & 0x80000000) || ix * 2 >= 0xff000000)
48 return __math_invalidf (x);
49 /* x is subnormal, normalize it. */
50 ix = asuint (x * 0x1p23f);
54 /* x = 2^k z; where z is in range [OFF,2*OFF] and exact.
55 The range is split into N subintervals.
56 The ith subinterval contains z and c is near its center. */
58 i = (tmp >> (23 - LOGF_TABLE_BITS)) % N;
59 k = (int32_t) tmp >> 23; /* arithmetic shift */
60 iz = ix - (tmp & 0x1ff << 23);
63 z = (double_t) asfloat (iz);
65 /* log(x) = log1p(z/c-1) + log(c) + k*Ln2 */
67 y0 = logc + (double_t) k * Ln2;
69 /* Pipelined polynomial evaluation to approximate log1p(r). */
73 y = y * r2 + (y0 + r);
74 return eval_as_float (y);
77 strong_alias (logf, __logf_finite)
78 hidden_alias (logf, __ieee754_logf)