1 //===-- floatuntidf.c - Implement __floatuntidf ---------------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This file implements __floatuntidf for the compiler_rt library. 10 // 11 //===----------------------------------------------------------------------===// 12 13 #include "int_lib.h" 14 15 #ifdef CRT_HAS_128BIT 16 17 // Returns: convert a to a double, rounding toward even. 18 19 // Assumption: double is a IEEE 64 bit floating point type 20 // tu_int is a 128 bit integral type 21 22 // seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm 23 // mmmm 24 25 COMPILER_RT_ABI double __floatuntidf(tu_int a) { 26 if (a == 0) 27 return 0.0; 28 const unsigned N = sizeof(tu_int) * CHAR_BIT; 29 int sd = N - __clzti2(a); // number of significant digits 30 si_int e = sd - 1; // exponent 31 if (sd > DBL_MANT_DIG) { 32 // start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx 33 // finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR 34 // 12345678901234567890123456 35 // 1 = msb 1 bit 36 // P = bit DBL_MANT_DIG-1 bits to the right of 1 37 // Q = bit DBL_MANT_DIG bits to the right of 1 38 // R = "or" of all bits to the right of Q 39 switch (sd) { 40 case DBL_MANT_DIG + 1: 41 a <<= 1; 42 break; 43 case DBL_MANT_DIG + 2: 44 break; 45 default: 46 a = (a >> (sd - (DBL_MANT_DIG + 2))) | 47 ((a & ((tu_int)(-1) >> ((N + DBL_MANT_DIG + 2) - sd))) != 0); 48 }; 49 // finish: 50 a |= (a & 4) != 0; // Or P into R 51 ++a; // round - this step may add a significant bit 52 a >>= 2; // dump Q and R 53 // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits 54 if (a & ((tu_int)1 << DBL_MANT_DIG)) { 55 a >>= 1; 56 ++e; 57 } 58 // a is now rounded to DBL_MANT_DIG bits 59 } else { 60 a <<= (DBL_MANT_DIG - sd); 61 // a is now rounded to DBL_MANT_DIG bits 62 } 63 double_bits fb; 64 fb.u.s.high = ((e + 1023) << 20) | // exponent 65 ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high 66 fb.u.s.low = (su_int)a; // mantissa-low 67 return fb.f; 68 } 69 70 #endif // CRT_HAS_128BIT 71