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

/*===-- floatundisf.c - Implement __floatundisf ---------------------------===
 *
 * 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 __floatundisf for the compiler_rt library.
 *
 *===----------------------------------------------------------------------===
 */

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

/* Assumption: float is a IEEE 32 bit floating point type
 *            du_int is a 64 bit integral type
 */

/* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */

#include "int_lib.h"

COMPILER_RT_ABI float __floatundisf(du_int a) {
  if (a == 0)
    return 0.0F;
  const unsigned N = sizeof(du_int) * CHAR_BIT;
  int sd = N - __builtin_clzll(a); /* number of significant digits */
  int e = sd - 1;                  /* 8 exponent */
  if (sd > FLT_MANT_DIG) {
    /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
     *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
     *                                                12345678901234567890123456
     *  1 = msb 1 bit
     *  P = bit FLT_MANT_DIG-1 bits to the right of 1
     *  Q = bit FLT_MANT_DIG bits to the right of 1
     *  R = "or" of all bits to the right of Q
     */
    switch (sd) {
    case FLT_MANT_DIG + 1:
      a <<= 1;
      break;
    case FLT_MANT_DIG + 2:
      break;
    default:
      a = (a >> (sd - (FLT_MANT_DIG + 2))) |
          ((a & ((du_int)(-1) >> ((N + FLT_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 FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
    if (a & ((du_int)1 << FLT_MANT_DIG)) {
      a >>= 1;
      ++e;
    }
    /* a is now rounded to FLT_MANT_DIG bits */
  } else {
    a <<= (FLT_MANT_DIG - sd);
    /* a is now rounded to FLT_MANT_DIG bits */
  }
  float_bits fb;
  fb.u = ((e + 127) << 23) |       /* exponent */
         ((su_int)a & 0x007FFFFF); /* mantissa */
  return fb.f;
}

#if defined(__ARM_EABI__)
#if defined(COMPILER_RT_ARMHF_TARGET)
AEABI_RTABI float __aeabi_ul2f(du_int a) { return __floatundisf(a); }
#else
AEABI_RTABI float __aeabi_ul2f(du_int a) COMPILER_RT_ALIAS(__floatundisf);
#endif
#endif