1 /* ===-- floatuntidf.c - Implement __floatuntidf ---------------------------=== 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 __floatuntidf for the compiler_rt library. 11 * 12 * ===----------------------------------------------------------------------=== 13 */ 14 15 #include "int_lib.h" 16 17 #if __x86_64 18 19 /* Returns: convert a to a double, rounding toward even. */ 20 21 /* Assumption: double is a IEEE 64 bit floating point type 22 * tu_int is a 128 bit integral type 23 */ 24 25 /* seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm */ 26 27 si_int __clzti2(ti_int a); 28 29 double 30 __floatuntidf(tu_int a) 31 { 32 if (a == 0) 33 return 0.0; 34 const unsigned N = sizeof(tu_int) * CHAR_BIT; 35 int sd = N - __clzti2(a); /* number of significant digits */ 36 int e = sd - 1; /* exponent */ 37 if (sd > DBL_MANT_DIG) 38 { 39 /* start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx 40 * finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR 41 * 12345678901234567890123456 42 * 1 = msb 1 bit 43 * P = bit DBL_MANT_DIG-1 bits to the right of 1 44 * Q = bit DBL_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 DBL_MANT_DIG + 1: 50 a <<= 1; 51 break; 52 case DBL_MANT_DIG + 2: 53 break; 54 default: 55 a = (a >> (sd - (DBL_MANT_DIG+2))) | 56 ((a & ((tu_int)(-1) >> ((N + DBL_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 DBL_MANT_DIG or DBL_MANT_DIG+1 bits */ 63 if (a & ((tu_int)1 << DBL_MANT_DIG)) 64 { 65 a >>= 1; 66 ++e; 67 } 68 /* a is now rounded to DBL_MANT_DIG bits */ 69 } 70 else 71 { 72 a <<= (DBL_MANT_DIG - sd); 73 /* a is now rounded to DBL_MANT_DIG bits */ 74 } 75 double_bits fb; 76 fb.u.s.high = ((e + 1023) << 20) | /* exponent */ 77 ((su_int)(a >> 32) & 0x000FFFFF); /* mantissa-high */ 78 fb.u.s.low = (su_int)a; /* mantissa-low */ 79 return fb.f; 80 } 81 82 #endif 83
