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

//===-- lib/floattitf.c - int128 -> quad-precision conversion -----*- C -*-===//
//
// 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 ti_int to quad-precision conversion for the
// compiler-rt library in the IEEE-754 default round-to-nearest, ties-to-even
// mode.
//
//===----------------------------------------------------------------------===//

#define QUAD_PRECISION
#include "fp_lib.h"
#include "int_lib.h"

// Returns: convert a ti_int to a fp_t, rounding toward even.

// Assumption: fp_t is a IEEE 128 bit floating point type
//             ti_int is a 128 bit integral type

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

#if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
COMPILER_RT_ABI fp_t __floattitf(ti_int a) {
  if (a == 0)
    return 0.0;
  const unsigned N = sizeof(ti_int) * CHAR_BIT;
  const ti_int s = a >> (N - 1);
  a = (a ^ s) - s;
  int sd = N - __clzti2(a); // number of significant digits
  int e = sd - 1;           // exponent
  if (sd > LDBL_MANT_DIG) {
    //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
    //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
    //                                                12345678901234567890123456
    //  1 = msb 1 bit
    //  P = bit LDBL_MANT_DIG-1 bits to the right of 1
    //  Q = bit LDBL_MANT_DIG bits to the right of 1
    //  R = "or" of all bits to the right of Q
    switch (sd) {
    case LDBL_MANT_DIG + 1:
      a <<= 1;
      break;
    case LDBL_MANT_DIG + 2:
      break;
    default:
      a = ((tu_int)a >> (sd - (LDBL_MANT_DIG + 2))) |
          ((a & ((tu_int)(-1) >> ((N + LDBL_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 LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits
    if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
      a >>= 1;
      ++e;
    }
    // a is now rounded to LDBL_MANT_DIG bits
  } else {
    a <<= (LDBL_MANT_DIG - sd);
    // a is now rounded to LDBL_MANT_DIG bits
  }

  long_double_bits fb;
  fb.u.high.all = (s & 0x8000000000000000LL)            // sign
                  | (du_int)(e + 16383) << 48           // exponent
                  | ((a >> 64) & 0x0000ffffffffffffLL); // significand
  fb.u.low.all = (du_int)(a);
  return fb.f;
}

#endif