compiler-rt/lib/builtins/floattidf.c

   1 //===-- floattidf.c - Implement __floattidf -------------------------------===//
   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 __floattidf 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 //            ti_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 __floattidf(ti_int a) {
  26   if (a == 0)
  27     return 0.0;
  28   const unsigned N = sizeof(ti_int) * CHAR_BIT;
  29   const ti_int s = a >> (N - 1);
  30   a = (a ^ s) - s;
  31   int sd = N - __clzti2(a); // number of significant digits
  32   int e = sd - 1;           // exponent
  33   if (sd > DBL_MANT_DIG) {
  34     // start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  35     //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  36     //                                               12345678901234567890123456
  37     // 1 = msb 1 bit
  38     // P = bit DBL_MANT_DIG-1 bits to the right of 1
  39     // Q = bit DBL_MANT_DIG bits to the right of 1
  40     // R = "or" of all bits to the right of Q
  41     switch (sd) {
  42     case DBL_MANT_DIG + 1:
  43       a <<= 1;
  44       break;
  45     case DBL_MANT_DIG + 2:
  46       break;
  47     default:
  48       a = ((tu_int)a >> (sd - (DBL_MANT_DIG + 2))) |
  49           ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG + 2) - sd))) != 0);
  50     };
  51     // finish:
  52     a |= (a & 4) != 0; // Or P into R
  53     ++a;               // round - this step may add a significant bit
  54     a >>= 2;           // dump Q and R
  55     // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits
  56     if (a & ((tu_int)1 << DBL_MANT_DIG)) {
  57       a >>= 1;
  58       ++e;
  59     }
  60     // a is now rounded to DBL_MANT_DIG bits
  61   } else {
  62     a <<= (DBL_MANT_DIG - sd);
  63     // a is now rounded to DBL_MANT_DIG bits
  64   }
  65   double_bits fb;
  66   fb.u.s.high = ((su_int)s & 0x80000000) |        // sign
  67                 ((e + 1023) << 20) |              // exponent
  68                 ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high
  69   fb.u.s.low = (su_int)a;                         // mantissa-low
  70   return fb.f;
  71 }
  72
  73 #endif // CRT_HAS_128BIT