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floatdidf.c revision 1.4 
      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 #ifndef __SOFT_FP__
     26 /* Support for systems that have hardware floating-point; we'll set the inexact flag
     27  * as a side-effect of this computation.
     28  */
     29 
     30 COMPILER_RT_ABI double
     31 __floatdidf(di_int a)
     32 {
     33 	static const double twop52 = 4503599627370496.0; // 0x1.0p52
     34 	static const double twop32 = 4294967296.0; // 0x1.0p32
     35 
     36 	union { int64_t x; double d; } low = { .d = twop52 };
     37 
     38 	const double high = (int32_t)(a >> 32) * twop32;
     39 	low.x |= a & INT64_C(0x00000000ffffffff);
     40 
     41 	const double result = (high - twop52) + low.d;
     42 	return result;
     43 }
     44 
     45 #else
     46 /* Support for systems that don't have hardware floating-point; there are no flags to
     47  * set, and we don't want to code-gen to an unknown soft-float implementation.
     48  */
     49 
     50 COMPILER_RT_ABI double
     51 __floatdidf(di_int a)
     52 {
     53     if (a == 0)
     54         return 0.0;
     55     const unsigned N = sizeof(di_int) * CHAR_BIT;
     56     const di_int s = a >> (N-1);
     57     a = (a ^ s) - s;
     58     int sd = N - __builtin_clzll(a);  /* number of significant digits */
     59     int e = sd - 1;             /* exponent */
     60     if (sd > DBL_MANT_DIG)
     61     {
     62         /*  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
     63          *  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
     64          *                                                12345678901234567890123456
     65          *  1 = msb 1 bit
     66          *  P = bit DBL_MANT_DIG-1 bits to the right of 1
     67          * Q = bit DBL_MANT_DIG bits to the right of 1
     68          *  R = "or" of all bits to the right of Q
     69         */
     70         switch (sd)
     71         {
     72         case DBL_MANT_DIG + 1:
     73             a <<= 1;
     74             break;
     75         case DBL_MANT_DIG + 2:
     76             break;
     77         default:
     78             a = ((du_int)a >> (sd - (DBL_MANT_DIG+2))) |
     79                 ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0);
     80         };
     81         /* finish: */
     82         a |= (a & 4) != 0;  /* Or P into R */
     83         ++a;  /* round - this step may add a significant bit */
     84         a >>= 2;  /* dump Q and R */
     85         /* a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits */
     86         if (a & ((du_int)1 << DBL_MANT_DIG))
     87         {
     88             a >>= 1;
     89             ++e;
     90         }
     91         /* a is now rounded to DBL_MANT_DIG bits */
     92     }
     93     else
     94     {
     95         a <<= (DBL_MANT_DIG - sd);
     96         /* a is now rounded to DBL_MANT_DIG bits */
     97     }
     98     double_bits fb;
     99     fb.u.s.high = ((su_int)s & 0x80000000) |        /* sign */
    100                 ((e + 1023) << 20)      |        /* exponent */
    101                 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */
    102     fb.u.s.low = (su_int)a;                         /* mantissa-low */
    103     return fb.f;
    104 }
    105 #endif
    106 
    107 #if defined(__ARM_EABI__)
    108 AEABI_RTABI double __aeabi_l2d(di_int a) {
    109   return __floatdidf(a);
    110 }
    111 #endif
    112 
    113