llvm-project/compiler-rt/lib/builtins/floatdidf.c

/*===-- floatdidf.c - Implement __floatdidf -------------------------------===
 *
 * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 * See https://llvm.org/LICENSE.txt for license information.
 * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 *
 *===----------------------------------------------------------------------===
 *
 * This file implements __floatdidf for the compiler_rt library.
 *
 *===----------------------------------------------------------------------===
 */

#include "int_lib.h"

/* Returns: convert a to a double, rounding toward even. */

/* Assumption: double is a IEEE 64 bit floating point type
 *             di_int is a 64 bit integral type
 */

/* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm
 * mmmm */

#ifndef __SOFT_FP__
/* Support for systems that have hardware floating-point; we'll set the inexact
 * flag as a side-effect of this computation.
 */

COMPILER_RT_ABI double __floatdidf(di_int a) {
  static const double twop52 = 4503599627370496.0; // 0x1.0p52
  static const double twop32 = 4294967296.0;       // 0x1.0p32

  union {
    int64_t x;
    double d;
  } low = {.d = twop52};

  const double high = (int32_t)(a >> 32) * twop32;
  low.x |= a & INT64_C(0x00000000ffffffff);

  const double result = (high - twop52) + low.d;
  return result;
}

#else
/* Support for systems that don't have hardware floating-point; there are no
 * flags to set, and we don't want to code-gen to an unknown soft-float
 * implementation.
 */

COMPILER_RT_ABI double __floatdidf(di_int a) {
  if (a == 0)
    return 0.0;
  const unsigned N = sizeof(di_int) * CHAR_BIT;
  const di_int s = a >> (N - 1);
  a = (a ^ s) - s;
  int sd = N - __builtin_clzll(a); /* number of significant digits */
  int e = sd - 1;                  /* exponent */
  if (sd > DBL_MANT_DIG) {
    /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
     *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
     *                                                12345678901234567890123456
     *  1 = msb 1 bit
     *  P = bit DBL_MANT_DIG-1 bits to the right of 1
     * Q = bit DBL_MANT_DIG bits to the right of 1
     *  R = "or" of all bits to the right of Q
     */
    switch (sd) {
    case DBL_MANT_DIG + 1:
      a <<= 1;
      break;
    case DBL_MANT_DIG + 2:
      break;
    default:
      a = ((du_int)a >> (sd - (DBL_MANT_DIG + 2))) |
          ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG + 2) - sd))) != 0);
    };
    /* finish: */
    a |= (a & 4) != 0; /* Or P into R */
    ++a;               /* round - this step may add a significant bit */
    a >>= 2;           /* dump Q and R */
    /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
    if (a & ((du_int)1 << DBL_MANT_DIG)) {
      a >>= 1;
      ++e;
    }
    /* a is now rounded to DBL_MANT_DIG bits */
  } else {
    a <<= (DBL_MANT_DIG - sd);
    /* a is now rounded to DBL_MANT_DIG bits */
  }
  double_bits fb;
  fb.u.s.high = ((su_int)s & 0x80000000) |        /* sign */
                ((e + 1023) << 20) |              /* exponent */
                ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
  fb.u.s.low = (su_int)a;                         /* mantissa-low */
  return fb.f;
}
#endif

#if defined(__ARM_EABI__)
#if defined(COMPILER_RT_ARMHF_TARGET)
AEABI_RTABI double __aeabi_l2d(di_int a) { return __floatdidf(a); }
#else
AEABI_RTABI double __aeabi_l2d(di_int a) COMPILER_RT_ALIAS(__floatdidf);
#endif
#endif