0928368f62
This adds the Arm Optimized Routines (see https://github.com/ARM-software/optimized-routines) source code under the the LLVM license. The version of the code provided in this patch is v20.02 of the Arm Optimized Routines project. This entire contribution is being committed as is even though it does not currently fit the LLVM libc model and does not follow the LLVM coding style. In the near future, implementations from this patch will be moved over to their right place in the LLVM-libc tree. This will be done over many small patches, all of which will go through the normal LLVM code review process. See this libc-dev post for the plan: http://lists.llvm.org/pipermail/libc-dev/2020-March/000044.html Differential revision of the original upload: https://reviews.llvm.org/D75355
82 lines
2.1 KiB
C
82 lines
2.1 KiB
C
/*
|
|
* Single-precision log2 function.
|
|
*
|
|
* Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
|
|
* See https://llvm.org/LICENSE.txt for license information.
|
|
* SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
|
|
*/
|
|
|
|
#include <math.h>
|
|
#include <stdint.h>
|
|
#include "math_config.h"
|
|
|
|
/*
|
|
LOG2F_TABLE_BITS = 4
|
|
LOG2F_POLY_ORDER = 4
|
|
|
|
ULP error: 0.752 (nearest rounding.)
|
|
Relative error: 1.9 * 2^-26 (before rounding.)
|
|
*/
|
|
|
|
#define N (1 << LOG2F_TABLE_BITS)
|
|
#define T __log2f_data.tab
|
|
#define A __log2f_data.poly
|
|
#define OFF 0x3f330000
|
|
|
|
float
|
|
log2f (float x)
|
|
{
|
|
/* double_t for better performance on targets with FLT_EVAL_METHOD==2. */
|
|
double_t z, r, r2, p, y, y0, invc, logc;
|
|
uint32_t ix, iz, top, tmp;
|
|
int k, i;
|
|
|
|
ix = asuint (x);
|
|
#if WANT_ROUNDING
|
|
/* Fix sign of zero with downward rounding when x==1. */
|
|
if (unlikely (ix == 0x3f800000))
|
|
return 0;
|
|
#endif
|
|
if (unlikely (ix - 0x00800000 >= 0x7f800000 - 0x00800000))
|
|
{
|
|
/* x < 0x1p-126 or inf or nan. */
|
|
if (ix * 2 == 0)
|
|
return __math_divzerof (1);
|
|
if (ix == 0x7f800000) /* log2(inf) == inf. */
|
|
return x;
|
|
if ((ix & 0x80000000) || ix * 2 >= 0xff000000)
|
|
return __math_invalidf (x);
|
|
/* x is subnormal, normalize it. */
|
|
ix = asuint (x * 0x1p23f);
|
|
ix -= 23 << 23;
|
|
}
|
|
|
|
/* 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 >> (23 - LOG2F_TABLE_BITS)) % N;
|
|
top = tmp & 0xff800000;
|
|
iz = ix - top;
|
|
k = (int32_t) tmp >> 23; /* arithmetic shift */
|
|
invc = T[i].invc;
|
|
logc = T[i].logc;
|
|
z = (double_t) asfloat (iz);
|
|
|
|
/* log2(x) = log1p(z/c-1)/ln2 + log2(c) + k */
|
|
r = z * invc - 1;
|
|
y0 = logc + (double_t) k;
|
|
|
|
/* Pipelined polynomial evaluation to approximate log1p(r)/ln2. */
|
|
r2 = r * r;
|
|
y = A[1] * r + A[2];
|
|
y = A[0] * r2 + y;
|
|
p = A[3] * r + y0;
|
|
y = y * r2 + p;
|
|
return eval_as_float (y);
|
|
}
|
|
#if USE_GLIBC_ABI
|
|
strong_alias (log2f, __log2f_finite)
|
|
hidden_alias (log2f, __ieee754_log2f)
|
|
#endif
|