src/compiler-rt/lib/builtins/floatdidf.c

   1 /*===-- floatdidf.c - Implement __floatdidf -------------------------------===
   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 __floatdidf for the compiler_rt library.
  11  *
  12  *===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 /* Returns: convert a to a double, rounding toward even. */
  18
  19 /* Assumption: double is a IEEE 64 bit floating point type
  20  *             di_int is a 64 bit integral type
  21  */
  22
  23 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */
  24
  25 ARM_EABI_FNALIAS(l2d, floatdidf)
  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 __floatdidf(di_int a)
  34 {
  35         static const double twop52 = 4503599627370496.0; // 0x1.0p52
  36         static const double twop32 = 4294967296.0; // 0x1.0p32
  37
  38         union { int64_t x; double d; } low = { .d = twop52 };
  39
  40         const double high = (int32_t)(a >> 32) * twop32;
  41         low.x |= a & INT64_C(0x00000000ffffffff);
  42
  43         const double result = (high - twop52) + low.d;
  44         return result;
  45 }
  46
  47 #else
  48 /* Support for systems that don't have hardware floating-point; there are no flags to
  49  * set, and we don't want to code-gen to an unknown soft-float implementation.
  50  */
  51
  52 COMPILER_RT_ABI double
  53 __floatdidf(di_int a)
  54 {
  55     if (a == 0)
  56         return 0.0;
  57     const unsigned N = sizeof(di_int) * CHAR_BIT;
  58     const di_int s = a >> (N-1);
  59     a = (a ^ s) - s;
  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 = ((du_int)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 = ((su_int)s & 0x80000000) |        /* sign */
 102                 ((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