lib/builtins/floatdisf.c

   1 /*===-- floatdisf.c - Implement __floatdisf -------------------------------===
   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 __floatdisf for the compiler_rt library.
  11  *
  12  *===----------------------------------------------------------------------===
  13  */
  14
  15 /* Returns: convert a to a float, rounding toward even.*/
  16
  17 /* Assumption: float is a IEEE 32 bit floating point type
  18  *             di_int is a 64 bit integral type
  19  */
  20
  21 /* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */
  22
  23 #include "int_lib.h"
  24
  25 ARM_EABI_FNALIAS(l2f, floatdisf)
  26
  27 COMPILER_RT_ABI float
  28 __floatdisf(di_int a)
  29 {
  30     if (a == 0)
  31         return 0.0F;
  32     const unsigned N = sizeof(di_int) * CHAR_BIT;
  33     const di_int s = a >> (N-1);
  34     a = (a ^ s) - s;
  35     int sd = N - __builtin_clzll(a);  /* number of significant digits */
  36     int e = sd - 1;             /* exponent */
  37     if (sd > FLT_MANT_DIG)
  38     {
  39         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
  40          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
  41          *                                                12345678901234567890123456
  42          *  1 = msb 1 bit
  43          *  P = bit FLT_MANT_DIG-1 bits to the right of 1
  44          *  Q = bit FLT_MANT_DIG bits to the right of 1
  45          *  R = "or" of all bits to the right of Q
  46          */
  47         switch (sd)
  48         {
  49         case FLT_MANT_DIG + 1:
  50             a <<= 1;
  51             break;
  52         case FLT_MANT_DIG + 2:
  53             break;
  54         default:
  55             a = ((du_int)a >> (sd - (FLT_MANT_DIG+2))) |
  56                 ((a & ((du_int)(-1) >> ((N + FLT_MANT_DIG+2) - sd))) != 0);
  57         };
  58         /* finish: */
  59         a |= (a & 4) != 0;  /* Or P into R */
  60         ++a;  /* round - this step may add a significant bit */
  61         a >>= 2;  /* dump Q and R */
  62         /* a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
  63         if (a & ((du_int)1 << FLT_MANT_DIG))
  64         {
  65             a >>= 1;
  66             ++e;
  67         }
  68         /* a is now rounded to FLT_MANT_DIG bits */
  69     }
  70     else
  71     {
  72         a <<= (FLT_MANT_DIG - sd);
  73         /* a is now rounded to FLT_MANT_DIG bits */
  74     }
  75     float_bits fb;
  76     fb.u = ((su_int)s & 0x80000000) |  /* sign */
  77            ((e + 127) << 23)       |  /* exponent */
  78            ((su_int)a & 0x007FFFFF);   /* mantissa */
  79     return fb.f;
  80 }