lib/floatundidf.c

   1 /* ===-- floatundidf.c - Implement __floatundidf ---------------------------===
   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 __floatundidf for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 /* Returns: convert a to a double, rounding toward even. */
  16
  17 /* Assumption: double is a IEEE 64 bit floating point type
  18  *             du_int is a 64 bit integral type
  19  */
  20
  21 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
  22
  23 #include "int_lib.h"
  24
  25 ARM_EABI_FNALIAS(ul2d, floatundidf);
  26
  27 #ifndef __SOFT_FP__
  28 /* Support for systems that have hardware floating-point; we'll set the inexact flag
  29  * as a side-effect of this computation.
  30  */
  31
  32
  33 COMPILER_RT_ABI double
  34 __floatundidf(du_int a)
  35 {
  36         static const double twop52 = 0x1.0p52;
  37         static const double twop84 = 0x1.0p84;
  38         static const double twop84_plus_twop52 = 0x1.00000001p84;
  39
  40         union { uint64_t x; double d; } high = { .d = twop84 };
  41         union { uint64_t x; double d; } low = { .d = twop52 };
  42
  43         high.x |= a >> 32;
  44         low.x |= a & UINT64_C(0x00000000ffffffff);
  45
  46         const double result = (high.d - twop84_plus_twop52) + low.d;
  47         return result;
  48 }
  49
  50 #else
  51 /* Support for systems that don't have hardware floating-point; there are no flags to
  52  * set, and we don't want to code-gen to an unknown soft-float implementation.
  53  */
  54
  55 COMPILER_RT_ABI double
  56 __floatundidf(du_int a)
  57 {
  58     if (a == 0)
  59         return 0.0;
  60     const unsigned N = sizeof(du_int) * CHAR_BIT;
  61     int sd = N - __builtin_clzll(a);  /* number of significant digits */
  62     int e = sd - 1;             /* exponent */
  63     if (sd > DBL_MANT_DIG)
  64     {
  65         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  66          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  67          *                                                12345678901234567890123456
  68          *  1 = msb 1 bit
  69          *  P = bit DBL_MANT_DIG-1 bits to the right of 1
  70          *  Q = bit DBL_MANT_DIG bits to the right of 1
  71          *  R = "or" of all bits to the right of Q
  72          */
  73         switch (sd)
  74         {
  75         case DBL_MANT_DIG + 1:
  76             a <<= 1;
  77             break;
  78         case DBL_MANT_DIG + 2:
  79             break;
  80         default:
  81             a = (a >> (sd - (DBL_MANT_DIG+2))) |
  82                 ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
  83         };
  84         /* finish: */
  85         a |= (a & 4) != 0;  /* Or P into R */
  86         ++a;  /* round - this step may add a significant bit */
  87         a >>= 2;  /* dump Q and R */
  88         /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
  89         if (a & ((du_int)1 << DBL_MANT_DIG))
  90         {
  91             a >>= 1;
  92             ++e;
  93         }
  94         /* a is now rounded to DBL_MANT_DIG bits */
  95     }
  96     else
  97     {
  98         a <<= (DBL_MANT_DIG - sd);
  99         /* a is now rounded to DBL_MANT_DIG bits */
 100     }
 101     double_bits fb;
 102     fb.u.high = ((e + 1023) << 20)      |        /* exponent */
 103                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
 104     fb.u.low = (su_int)a;                         /* mantissa-low  */
 105     return fb.f;
 106 }
 107 #endif