src/libcompiler_builtins/compiler-rt/lib/builtins/floatuntitf.c

   1 //===-- lib/floatuntitf.c - uint128 -> quad-precision conversion --*- C -*-===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is dual licensed under the MIT and the University of Illinois Open
   6 // Source Licenses. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This file implements tu_int to quad-precision conversion for the
  11 // compiler-rt library in the IEEE-754 default round-to-nearest, ties-to-even
  12 // mode.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #define QUAD_PRECISION
  17 #include "fp_lib.h"
  18 #include "int_lib.h"
  19
  20 /* Returns: convert a tu_int to a fp_t, rounding toward even. */
  21
  22 /* Assumption: fp_t is a IEEE 128 bit floating point type
  23  *             tu_int is a 128 bit integral type
  24  */
  25
  26 /* seee eeee eeee eeee mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm |
  27  * mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm
  28  */
  29
  30 #if defined(CRT_HAS_128BIT) && defined(CRT_LDBL_128BIT)
  31 COMPILER_RT_ABI fp_t
  32 __floatuntitf(tu_int a) {
  33     if (a == 0)
  34         return 0.0;
  35     const unsigned N = sizeof(tu_int) * CHAR_BIT;
  36     int sd = N - __clzti2(a);  /* number of significant digits */
  37     int e = sd - 1;            /* exponent */
  38     if (sd > LDBL_MANT_DIG) {
  39         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  40          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  41          *                                                12345678901234567890123456
  42          *  1 = msb 1 bit
  43          *  P = bit LDBL_MANT_DIG-1 bits to the right of 1
  44          *  Q = bit LDBL_MANT_DIG bits to the right of 1
  45          *  R = "or" of all bits to the right of Q
  46          */
  47         switch (sd) {
  48         case LDBL_MANT_DIG + 1:
  49             a <<= 1;
  50             break;
  51         case LDBL_MANT_DIG + 2:
  52             break;
  53         default:
  54             a = (a >> (sd - (LDBL_MANT_DIG+2))) |
  55                 ((a & ((tu_int)(-1) >> ((N + LDBL_MANT_DIG+2) - sd))) != 0);
  56         };
  57         /* finish: */
  58         a |= (a & 4) != 0;  /* Or P into R */
  59         ++a;  /* round - this step may add a significant bit */
  60         a >>= 2;  /* dump Q and R */
  61         /* a is now rounded to LDBL_MANT_DIG or LDBL_MANT_DIG+1 bits */
  62         if (a & ((tu_int)1 << LDBL_MANT_DIG)) {
  63             a >>= 1;
  64             ++e;
  65         }
  66         /* a is now rounded to LDBL_MANT_DIG bits */
  67     } else {
  68         a <<= (LDBL_MANT_DIG - sd);
  69         /* a is now rounded to LDBL_MANT_DIG bits */
  70     }
  71
  72     long_double_bits fb;
  73     fb.u.high.all = (du_int)(e + 16383) << 48            /* exponent */
  74                   | ((a >> 64) & 0x0000ffffffffffffLL);  /* significand */
  75     fb.u.low.all = (du_int)(a);
  76     return fb.f;
  77 }
  78
  79 #endif