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      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
    108