1//===-- floatuntidf.c - Implement __floatuntidf ---------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements __floatuntidf for the compiler_rt library.
10//
11//===----------------------------------------------------------------------===//
12
13#include "int_lib.h"
14
15#ifdef CRT_HAS_128BIT
16
17// Returns: convert a to a double, rounding toward even.
18
19// Assumption: double is a IEEE 64 bit floating point type
20//             tu_int is a 128 bit integral type
21
22// seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm
23// mmmm
24
25COMPILER_RT_ABI double __floatuntidf(tu_int a) {
26  if (a == 0)
27    return 0.0;
28  const unsigned N = sizeof(tu_int) * CHAR_BIT;
29  int sd = N - __clzti2(a); // number of significant digits
30  int e = sd - 1;           // exponent
31  if (sd > DBL_MANT_DIG) {
32    //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
33    //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
34    //                                                12345678901234567890123456
35    //  1 = msb 1 bit
36    //  P = bit DBL_MANT_DIG-1 bits to the right of 1
37    //  Q = bit DBL_MANT_DIG bits to the right of 1
38    //  R = "or" of all bits to the right of Q
39    switch (sd) {
40    case DBL_MANT_DIG + 1:
41      a <<= 1;
42      break;
43    case DBL_MANT_DIG + 2:
44      break;
45    default:
46      a = (a >> (sd - (DBL_MANT_DIG + 2))) |
47          ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG + 2) - sd))) != 0);
48    };
49    // finish:
50    a |= (a & 4) != 0; // Or P into R
51    ++a;               // round - this step may add a significant bit
52    a >>= 2;           // dump Q and R
53    // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
54    if (a & ((tu_int)1 << DBL_MANT_DIG)) {
55      a >>= 1;
56      ++e;
57    }
58    // a is now rounded to DBL_MANT_DIG bits
59  } else {
60    a <<= (DBL_MANT_DIG - sd);
61    // a is now rounded to DBL_MANT_DIG bits
62  }
63  double_bits fb;
64  fb.u.s.high = ((e + 1023) << 20) |              // exponent
65                ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
66  fb.u.s.low = (su_int)a;                         // mantissa-low
67  return fb.f;
68}
69
70#endif // CRT_HAS_128BIT
71