sysdeps/powerpc/power7/fpu/s_logb.c

   1 /* logb(). PowerPC/POWER7 version.
   2    Copyright (C) 2012-2014 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    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <math_ldbl_opt.h>
  20
  21 /* This implementation avoids FP to INT conversions by using VSX
  22    bitwise instructions over FP values.  */
  23
  24 static const double two1div52 = 2.220446049250313e-16;  /* 1/2**52  */
  25 static const double two10m1   = -1023.0;                /* 2**10 -1  */
  26
  27 /* FP mask to extract the exponent.  */
  28 static const union {
  29   unsigned long long mask;
  30   double d;
  31 } mask = { 0x7ff0000000000000ULL };
  32
  33 double
  34 __logb (double x)
  35 {
  36   double ret;
  37
  38   if (__builtin_expect (x == 0.0, 0))
  39     /* Raise FE_DIVBYZERO and return -HUGE_VAL[LF].  */
  40     return -1.0 / __builtin_fabs (x);
  41
  42   /* ret = x & 0x7ff0000000000000;  */
  43   asm (
  44     "xxland %x0,%x1,%x2\n"
  45     "fcfid  %0,%0"
  46     : "=f" (ret)
  47     : "f" (x), "f" (mask.d));
  48   /* ret = (ret >> 52) - 1023.0;  */
  49   ret = (ret * two1div52) + two10m1;
  50   if (__builtin_expect (ret > -two10m1, 0))
  51     /* Multiplication is used to set logb (+-INF) = INF.  */
  52     return (x * x);
  53   else if (__builtin_expect (ret == two10m1, 0))
  54     {
  55       /* POSIX specifies that denormal numbers are treated as
  56          though they were normalized.  */
  57       int32_t lx, ix;
  58       int ma;
  59
  60       EXTRACT_WORDS (ix, lx, x);
  61       if (ix == 0)
  62         ma = __builtin_clz (lx) + 32;
  63       else
  64         ma = __builtin_clz (ix);
  65       return (double) (-1023 - (ma - 12));
  66     }
  67   /* Test to avoid logb_downward (0.0) == -0.0.  */
  68   return ret == -0.0 ? 0.0 : ret;
  69 }
  70 weak_alias (__logb, logb)
  71 #ifdef NO_LONG_DOUBLE
  72 strong_alias (__logb, __logbl)
  73 weak_alias (__logb, logbl)
  74 #endif
  75
  76 #if LONG_DOUBLE_COMPAT (libm, GLIBC_2_0)
  77 compat_symbol (libm, logb, logbl, GLIBC_2_0);
  78 #endif