libgcc/config/libbid/bid128_logb.c

/* Copyright (C) 2007-2021 Free Software Foundation, Inc.

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.

GCC 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 General Public License
for more details.

Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.

You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<http://www.gnu.org/licenses/>.  */

#define BID_128RES
#include "bid_internal.h"

BID128_FUNCTION_ARG1_NORND_CUSTOMRESTYPE(int, bid128_logb, x)

  UINT128 CX;
  UINT64 sign_x;
  SINT64 D;
  int_float f64, fx;
  int exponent_x, bin_expon_cx, digits;

  if (!unpack_BID128_value (&sign_x, &exponent_x, &CX, x)) {
#ifdef SET_STATUS_FLAGS
      __set_status_flags (pfpsf, INVALID_EXCEPTION);
#endif
    BID_RETURN_VAL (0x80000000);
  }
  // find number of digits in coefficient
  // 2^64
  f64.i = 0x5f800000;
  // fx ~ CX
  fx.d = (float) CX.w[1] * f64.d + (float) CX.w[0];
  bin_expon_cx = ((fx.i >> 23) & 0xff) - 0x7f;
  digits = estimate_decimal_digits[bin_expon_cx];
  // scale = 38-estimate_decimal_digits[bin_expon_cx];
  D = CX.w[1] - power10_index_binexp_128[bin_expon_cx].w[1];
  if (D > 0 || (!D && CX.w[0] >= power10_index_binexp_128[bin_expon_cx].w[0])) {
    digits++;
  }

  exponent_x = exponent_x - DECIMAL_EXPONENT_BIAS_128 - 1 + digits;

  BID_RETURN_VAL (exponent_x);

}