Daniel Dunbar | b3a6901 | 2009-06-26 16:47:03 +0000 | [diff] [blame^] | 1 | //===-- floatundidf.c - Implement __floatundidf ---------------------------===// |
| 2 | // |
| 3 | // The LLVM Compiler Infrastructure |
| 4 | // |
| 5 | // This file is distributed under the University of Illinois Open Source |
| 6 | // License. See LICENSE.TXT for details. |
| 7 | // |
| 8 | //===----------------------------------------------------------------------===// |
| 9 | // |
| 10 | // This file implements __floatundidf for the compiler_rt library. |
| 11 | // |
| 12 | //===----------------------------------------------------------------------===// |
| 13 | |
| 14 | #include "int_lib.h" |
| 15 | #include <float.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 | // du_int is a 64 bit integral type |
| 21 | |
| 22 | // seee eeee eeee mmmm mmmm mmmm mmmm mmmm | mmmm mmmm mmmm mmmm mmmm mmmm mmmm mmmm |
| 23 | |
| 24 | #ifndef __SOFT_FP__ |
| 25 | // Support for systems that have hardware floating-point; we'll set the inexact flag |
| 26 | // as a side-effect of this computation. |
| 27 | #include <stdint.h> |
| 28 | |
| 29 | double |
| 30 | __floatundidf(du_int a) |
| 31 | { |
| 32 | static const double twop52 = 0x1.0p52; |
| 33 | static const double twop84 = 0x1.0p84; |
| 34 | static const double twop84_plus_twop52 = 0x1.00000001p84; |
| 35 | |
| 36 | union { uint64_t x; double d; } high = { .d = twop84 }; |
| 37 | union { uint64_t x; double d; } low = { .d = twop52 }; |
| 38 | |
| 39 | high.x |= a >> 32; |
| 40 | low.x |= a & UINT64_C(0x00000000ffffffff); |
| 41 | |
| 42 | const double result = (high.d - twop84_plus_twop52) + low.d; |
| 43 | return result; |
| 44 | } |
| 45 | |
| 46 | #else |
| 47 | // Support for systems that don't have hardware floating-point; there are no flags to |
| 48 | // set, and we don't want to code-gen to an unknown soft-float implementation. |
| 49 | |
| 50 | double |
| 51 | __floatundidf(du_int a) |
| 52 | { |
| 53 | if (a == 0) |
| 54 | return 0.0; |
| 55 | const unsigned N = sizeof(du_int) * CHAR_BIT; |
| 56 | int sd = N - __builtin_clzll(a); // number of significant digits |
| 57 | int e = sd - 1; // exponent |
| 58 | if (sd > DBL_MANT_DIG) |
| 59 | { |
| 60 | // start: 0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx |
| 61 | // finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR |
| 62 | // 12345678901234567890123456 |
| 63 | // 1 = msb 1 bit |
| 64 | // P = bit DBL_MANT_DIG-1 bits to the right of 1 |
| 65 | // Q = bit DBL_MANT_DIG bits to the right of 1 |
| 66 | // R = "or" of all bits to the right of Q |
| 67 | switch (sd) |
| 68 | { |
| 69 | case DBL_MANT_DIG + 1: |
| 70 | a <<= 1; |
| 71 | break; |
| 72 | case DBL_MANT_DIG + 2: |
| 73 | break; |
| 74 | default: |
| 75 | a = (a >> (sd - (DBL_MANT_DIG+2))) | |
| 76 | ((a & ((du_int)(-1) >> ((N + DBL_MANT_DIG+2) - sd))) != 0); |
| 77 | }; |
| 78 | // finish: |
| 79 | a |= (a & 4) != 0; // Or P into R |
| 80 | ++a; // round - this step may add a significant bit |
| 81 | a >>= 2; // dump Q and R |
| 82 | // a is now rounded to DBL_MANT_DIG or DBL_MANT_DIG+1 bits |
| 83 | if (a & ((du_int)1 << DBL_MANT_DIG)) |
| 84 | { |
| 85 | a >>= 1; |
| 86 | ++e; |
| 87 | } |
| 88 | // a is now rounded to DBL_MANT_DIG bits |
| 89 | } |
| 90 | else |
| 91 | { |
| 92 | a <<= (DBL_MANT_DIG - sd); |
| 93 | // a is now rounded to DBL_MANT_DIG bits |
| 94 | } |
| 95 | double_bits fb; |
| 96 | fb.u.high = ((e + 1023) << 20) | // exponent |
| 97 | ((su_int)(a >> 32) & 0x000FFFFF); // mantissa-high |
| 98 | fb.u.low = (su_int)a; // mantissa-low |
| 99 | return fb.f; |
| 100 | } |
| 101 | #endif |