glibc/sysdeps/powerpc/fpu/s_logbl.c

/* Get exponent of a floating-point value.  PowerPC version.
   Copyright (C) 2012-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/* ISA 2.07 provides fast GPR to FP instruction (mfvsr{d,wz}) which make
   generic implementation faster.  */
#if defined(_ARCH_PWR8) || !defined(_ARCH_PWR7)
# include <./sysdeps/ieee754/ldbl-128ibm/s_logbl.c>
#else
# include <math.h>
# include <math_private.h>
# include <math_ldbl_opt.h>

/* This implementation avoids FP to INT conversions by using VSX
   bitwise instructions over FP values.  */
long double
__logbl (long double x)
{
  double xh, xl;
  double ret;
  int64_t hx;

  if (__glibc_unlikely (x == 0.0))
    /* Raise FE_DIVBYZERO and return -HUGE_VAL[LF].  */
    return -1.0L / __builtin_fabsl (x);

  ldbl_unpack (x, &xh, &xl);
  EXTRACT_WORDS64 (hx, xh);

  /* Mask to extract the exponent.  */
  asm ("xxland %x0,%x1,%x2\n"
       "fcfid  %0,%0"
       : "=d" (ret)
       : "d" (xh), "d" (0x7ff0000000000000ULL));
  ret = (ret * 0x1p-52) - 1023.0;
  if (ret > 1023.0)
    /* Multiplication is used to set logb (+-INF) = INF.  */
    return (xh * xh);
  else if (ret == -1023.0)
    {
      /* POSIX specifies that denormal number is treated as
         though it were normalized.  */
      return (long double) (- (__builtin_clzll (hx & 0x7fffffffffffffffLL) \
			       - 12) - 1023);
    }
  else if ((hx & 0x000fffffffffffffLL) == 0)
    {
      /* If the high part is a power of 2, and the low part is nonzero
	 with the opposite sign, the low part affects the
	 exponent.  */
      int64_t lx, rhx;
      EXTRACT_WORDS64 (lx, xl);
      rhx = (hx & 0x7ff0000000000000LL) >> 52;
      if ((hx ^ lx) < 0 && (lx & 0x7fffffffffffffffLL) != 0)
	rhx--;
      return (long double) (rhx - 1023);
    }
  /* Test to avoid logb_downward (0.0) == -0.0.  */
  return ret == -0.0 ? 0.0 : ret;
}
# ifndef __logbl
long_double_symbol (libm, __logbl, logbl);
# endif
#endif