src/compiler-rt/lib/builtins/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 COMPILER_RT_ABI double
  33 __floatundidf(du_int a)
  34 {
  35         static const double twop52 = 4503599627370496.0; // 0x1.0p52
  36         static const double twop84 = 19342813113834066795298816.0; // 0x1.0p84
  37         static const double twop84_plus_twop52 = 19342813118337666422669312.0; // 0x1.00000001p84
  38
  39         union { uint64_t x; double d; } high = { .d = twop84 };
  40         union { uint64_t x; double d; } low = { .d = twop52 };
  41
  42         high.x |= a >> 32;
  43         low.x |= a & UINT64_C(0x00000000ffffffff);
  44
  45         const double result = (high.d - twop84_plus_twop52) + low.d;
  46         return result;
  47 }
  48
  49 #else
  50 /* Support for systems that don't have hardware floating-point; there are no flags to
  51  * set, and we don't want to code-gen to an unknown soft-float implementation.
  52  */
  53
  54 COMPILER_RT_ABI double
  55 __floatundidf(du_int a)
  56 {
  57     if (a == 0)
  58         return 0.0;
  59     const unsigned N = sizeof(du_int) * CHAR_BIT;
  60     int sd = N - __builtin_clzll(a);  /* number of significant digits */
  61     int e = sd - 1;             /* exponent */
  62     if (sd > DBL_MANT_DIG)
  63     {
  64         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  65          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  66          *                                                12345678901234567890123456
  67          *  1 = msb 1 bit
  68          *  P = bit DBL_MANT_DIG-1 bits to the right of 1
  69          *  Q = bit DBL_MANT_DIG bits to the right of 1
  70          *  R = "or" of all bits to the right of Q
  71          */
  72         switch (sd)
  73         {
  74         case DBL_MANT_DIG + 1:
  75             a <<= 1;
  76             break;
  77         case DBL_MANT_DIG + 2:
  78             break;
  79         default:
  80             a = (a >> (sd - (DBL_MANT_DIG+2))) |
  81                 ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
  82         };
  83         /* finish: */
  84         a |= (a & 4) != 0;  /* Or P into R */
  85         ++a;  /* round - this step may add a significant bit */
  86         a >>= 2;  /* dump Q and R */
  87         /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
  88         if (a & ((du_int)1 << DBL_MANT_DIG))
  89         {
  90             a >>= 1;
  91             ++e;
  92         }
  93         /* a is now rounded to DBL_MANT_DIG bits */
  94     }
  95     else
  96     {
  97         a <<= (DBL_MANT_DIG - sd);
  98         /* a is now rounded to DBL_MANT_DIG bits */
  99     }
 100     double_bits fb;
 101     fb.u.high = ((e + 1023) << 20)      |        /* exponent */
 102                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
 103     fb.u.low = (su_int)a;                         /* mantissa-low  */
 104     return fb.f;
 105 }
 106 #endif