Home | History | Annotate | Download | only in builtins
      1 /* ===-- floatuntisf.c - Implement __floatuntisf ---------------------------===
      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 __floatuntisf for the compiler_rt library.
     11  *
     12  * ===----------------------------------------------------------------------===
     13  */
     14 
     15 #include "int_lib.h"
     16 
     17 #ifdef CRT_HAS_128BIT
     18 
     19 /* Returns: convert a to a float, rounding toward even. */
     20 
     21 /* Assumption: float is a IEEE 32 bit floating point type
     22  *             tu_int is a 128 bit integral type
     23  */
     24 
     25 /* seee eeee emmm mmmm mmmm mmmm mmmm mmmm */
     26 
     27 COMPILER_RT_ABI float
     28 __floatuntisf(tu_int a)
     29 {
     30     if (a == 0)
     31         return 0.0F;
     32     const unsigned N = sizeof(tu_int) * CHAR_BIT;
     33     int sd = N - __clzti2(a);  /* number of significant digits */
     34     int e = sd - 1;             /* exponent */
     35     if (sd > FLT_MANT_DIG)
     36     {
     37         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
     38          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
     39          *                                                12345678901234567890123456
     40          *  1 = msb 1 bit
     41          *  P = bit FLT_MANT_DIG-1 bits to the right of 1
     42          *  Q = bit FLT_MANT_DIG bits to the right of 1
     43          *  R = "or" of all bits to the right of Q
     44 	 */
     45         switch (sd)
     46         {
     47         case FLT_MANT_DIG + 1:
     48             a <<= 1;
     49             break;
     50         case FLT_MANT_DIG + 2:
     51             break;
     52         default:
     53             a = (a >> (sd - (FLT_MANT_DIG+2))) |
     54                 ((a & ((tu_int)(-1) >> ((N + FLT_MANT_DIG+2) - sd))) != 0);
     55         };
     56         /* finish: */
     57         a |= (a & 4) != 0;  /* Or P into R */
     58         ++a;  /* round - this step may add a significant bit */
     59         a >>= 2;  /* dump Q and R */
     60         /* a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits */
     61         if (a & ((tu_int)1 << FLT_MANT_DIG))
     62         {
     63             a >>= 1;
     64             ++e;
     65         }
     66         /* a is now rounded to FLT_MANT_DIG bits */
     67     }
     68     else
     69     {
     70         a <<= (FLT_MANT_DIG - sd);
     71         /* a is now rounded to FLT_MANT_DIG bits */
     72     }
     73     float_bits fb;
     74     fb.u = ((e + 127) << 23)       |  /* exponent */
     75            ((su_int)a & 0x007FFFFF);  /* mantissa */
     76     return fb.f;
     77 }
     78 
     79 #endif /* CRT_HAS_128BIT */
     80