1  /* Compute radix independent exponent.
       2     Copyright (C) 2011-2023 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  #include <math.h>
      20  
      21  #include <math_private.h>
      22  #include <libm-alias-double.h>
      23  #include <fix-int-fp-convert-zero.h>
      24  #include <math-use-builtins.h>
      25  
      26  double
      27  __logb (double x)
      28  {
      29  #if USE_LOGB_BUILTIN
      30    return __builtin_logb (x);
      31  #else
      32    int64_t ix, ex;
      33  
      34    EXTRACT_WORDS64 (ix, x);
      35    ix &= UINT64_C(0x7fffffffffffffff);
      36    if (ix == 0)
      37      return -1.0 / fabs (x);
      38    ex = ix >> 52;
      39    if (ex == 0x7ff)
      40      return x * x;
      41    if (__glibc_unlikely (ex == 0))
      42      {
      43        int m = __builtin_clzll (ix);
      44        ex -= m - 12;
      45      }
      46    if (FIX_INT_FP_CONVERT_ZERO && ex == 1023)
      47      return 0.0;
      48    return (double) (ex - 1023);
      49  #endif /* !USE_LOGB_BUILTIN  */
      50  }
      51  #ifndef __logb
      52  libm_alias_double (__logb, logb)
      53  #endif