compiler-rt/lib/builtins/floatuntitf.c

   1 //===-- lib/floatuntitf.c - uint128 -> quad-precision conversion --*- C -*-===//
   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 tu_int to quad-precision conversion for the
  10 // compiler-rt library in the IEEE-754 default round-to-nearest, ties-to-even
  11 // mode.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #define QUAD_PRECISION
  16 #include "fp_lib.h"
  17 #include "int_lib.h"
  18
  19 // Returns: convert a tu_int to a fp_t, rounding toward even.
  20
  21 // Assumption: fp_t is a IEEE 128 bit floating point type
  22 //             tu_int is a 128 bit integral type
  23
  24 // seee eeee eeee eeee mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm
  25 // mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm
  26 // mmmm mmmm mmmm
  27
  28 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
  29 COMPILER_RT_ABI fp_t __floatuntitf(tu_int a) {
  30   if (a == 0)
  31     return 0.0;
  32   const unsigned N = sizeof(tu_int) * CHAR_BIT;
  33   int sd = N - __clzti2(a); // number of significant digits
  34   int e = sd - 1;           // exponent
  35   if (sd > LDBL_MANT_DIG) {
  36     //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  37     //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  38     //                                                12345678901234567890123456
  39     //  1 = msb 1 bit
  40     //  P = bit LDBL_MANT_DIG-1 bits to the right of 1
  41     //  Q = bit LDBL_MANT_DIG bits to the right of 1
  42     //  R = "or" of all bits to the right of Q
  43     switch (sd) {
  44     case LDBL_MANT_DIG + 1:
  45       a <<= 1;
  46       break;
  47     case LDBL_MANT_DIG + 2:
  48       break;
  49     default:
  50       a = (a >> (sd - (LDBL_MANT_DIG + 2))) |
  51           ((a & ((tu_int)(-1) >> ((N + LDBL_MANT_DIG + 2) - sd))) != 0);
  52     };
  53     // finish:
  54     a |= (a & 4) != 0; // Or P into R
  55     ++a;               // round - this step may add a significant bit
  56     a >>= 2;           // dump Q and R
  57     // a is now rounded to LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits
  58     if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
  59       a >>= 1;
  60       ++e;
  61     }
  62     // a is now rounded to LDBL_MANT_DIG bits
  63   } else {
  64     a <<= (LDBL_MANT_DIG - sd);
  65     // a is now rounded to LDBL_MANT_DIG bits
  66   }
  67
  68   long_double_bits fb;
  69   fb.u.high.all = (du_int)(e + 16383) << 48             // exponent
  70                   | ((a >> 64) & 0x0000ffffffffffffLL); // significand
  71   fb.u.low.all = (du_int)(a);
  72   return fb.f;
  73 }
  74
  75 #endif