1 /* Get exponent of a floating-point value.  PowerPC version.
2    Copyright (C) 2012-2021 Free Software Foundation, Inc.
3    This file is part of the GNU C Library.
4 
5    The GNU C Library is free software; you can redistribute it and/or
6    modify it under the terms of the GNU Lesser General Public
7    License as published by the Free Software Foundation; either
8    version 2.1 of the License, or (at your option) any later version.
9 
10    The GNU C Library is distributed in the hope that it will be useful,
11    but WITHOUT ANY WARRANTY; without even the implied warranty of
12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13    Lesser General Public License for more details.
14 
15    You should have received a copy of the GNU Lesser General Public
16    License along with the GNU C Library; if not, see
17    <https://www.gnu.org/licenses/>.  */
18 
19 /* ISA 2.07 provides fast GPR to FP instruction (mfvsr{d,wz}) which make
20    generic implementation faster.  */
21 #if defined(_ARCH_PWR8) || !defined(_ARCH_PWR7)
22 # include <sysdeps/ieee754/dbl-64/s_logb.c>
23 #else
24 # include <math.h>
25 # include <math_private.h>
26 # include <math_ldbl_opt.h>
27 # include <libm-alias-double.h>
28 
29 /* This implementation avoids FP to INT conversions by using VSX
30    bitwise instructions over FP values.  */
31 double
__logb(double x)32 __logb (double x)
33 {
34   double ret;
35 
36   if (__glibc_unlikely (x == 0.0))
37     /* Raise FE_DIVBYZERO and return -HUGE_VAL[LF].  */
38     return -1.0 / fabs (x);
39 
40   /* Mask to extract the exponent.  */
41   asm ("xxland %x0,%x1,%x2\n"
42        "fcfid  %0,%0"
43        : "=d" (ret)
44        : "d" (x), "d" (0x7ff0000000000000ULL));
45   ret = (ret * 0x1p-52) - 1023.0;
46   if (ret > 1023.0)
47     /* Multiplication is used to set logb (+-INF) = INF.  */
48     return (x * x);
49   else if (ret == -1023.0)
50     {
51       /* POSIX specifies that denormal numbers are treated as
52          though they were normalized.  */
53       int64_t ix;
54       EXTRACT_WORDS64 (ix, x);
55       ix &= UINT64_C (0x7fffffffffffffff);
56       return (double) (-1023 - (__builtin_clzll (ix) - 12));
57     }
58   /* Test to avoid logb_downward (0.0) == -0.0.  */
59   return ret == -0.0 ? 0.0 : ret;
60 }
61 # ifndef __logb
62 libm_alias_double (__logb, logb)
63 # endif
64 #endif
65