compiler-rt/lib/builtins/floatuntidf.c

   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
  25 COMPILER_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