sysdeps/powerpc/power7/fpu/s_logbf.c

   1 /* logbf(). PowerPC/POWER7 version.
   2    Copyright (C) 2012-2015 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_private.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 static const double two7m1    = -127.0;                 /* -2**7 + 1  */
  27
  28 /* FP mask to extract the exponent.  */
  29 static const union {
  30   unsigned long long mask;
  31   double d;
  32 } mask = { 0x7ff0000000000000ULL };
  33
  34 float
  35 __logbf (float x)
  36 {
  37   /* VSX operation are all done internally as double.  */
  38   double ret;
  39
  40   if (__builtin_expect (x == 0.0, 0))
  41     /* Raise FE_DIVBYZERO and return -HUGE_VAL[LF].  */
  42     return -1.0 / __builtin_fabsf (x);
  43
  44   /* ret = x & 0x7f800000;  */
  45   asm (
  46     "xxland %x0,%x1,%x2\n"
  47     "fcfid  %0,%0"
  48     : "=f"(ret)
  49     : "f" (x), "f" (mask.d));
  50   /* ret = (ret >> 52) - 1023.0, since ret is double.  */
  51   ret = (ret * two1div52) + two10m1;
  52   if (__builtin_expect (ret > -two7m1, 0))
  53     /* Multiplication is used to set logb (+-INF) = INF.  */
  54     return (x * x);
  55   /* Since operations are done with double we don't need
  56      additional tests for subnormal numbers.
  57      The test is to avoid logb_downward (0.0) == -0.0.  */
  58   return ret == -0.0 ? 0.0 : ret;
  59 }
  60 weak_alias (__logbf, logbf)