1 //===-- lib/builtins/ppc/fixunstfti.c - Convert long double->int128 *-C -*-===//
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 converting the 128bit IBM/PowerPC long double (double-
10 // double) data type to an unsigned 128 bit integer.
11 //
12 //===----------------------------------------------------------------------===//
13
14 #include "../int_math.h"
15 #define BIAS 1023
16
17 // Convert long double into an unsigned 128-bit integer.
__fixunstfti(long double input)18 __uint128_t __fixunstfti(long double input) {
19
20 // If we are trying to convert a NaN, return the NaN bit pattern.
21 if (crt_isnan(input)) {
22 return ((__uint128_t)0x7FF8000000000000ll) << 64 |
23 (__uint128_t)0x0000000000000000ll;
24 }
25
26 __uint128_t result, hiResult, loResult;
27 int hiExponent, loExponent, shift;
28 // The long double representation, with the high and low portions of
29 // the long double, and the corresponding bit patterns of each double.
30 union {
31 long double ld;
32 double d[2]; // [0] is the high double, [1] is the low double.
33 unsigned long long ull[2]; // High and low doubles as 64-bit integers.
34 } ldUnion;
35
36 // If the long double is less than 1.0 or negative,
37 // return 0.
38 if (input < 1.0)
39 return 0;
40
41 // Retrieve the 64-bit patterns of high and low doubles.
42 // Compute the unbiased exponent of both high and low doubles by
43 // removing the signs, isolating the exponent, and subtracting
44 // the bias from it.
45 ldUnion.ld = input;
46 hiExponent = ((ldUnion.ull[0] & 0x7FFFFFFFFFFFFFFFll) >> 52) - BIAS;
47 loExponent = ((ldUnion.ull[1] & 0x7FFFFFFFFFFFFFFFll) >> 52) - BIAS;
48
49 // Convert each double into int64; they will be added to the int128 result.
50 // CASE 1: High or low double fits in int64
51 // - Convert the each double normally into int64.
52 //
53 // CASE 2: High or low double does not fit in int64
54 // - Scale the double to fit within a 64-bit integer
55 // - Calculate the shift (amount to scale the double by in the int128)
56 // - Clear all the bits of the exponent (with 0x800FFFFFFFFFFFFF)
57 // - Add BIAS+53 (0x4350000000000000) to exponent to correct the value
58 // - Scale (move) the double to the correct place in the int128
59 // (Move it by 2^53 places)
60 //
61 // Note: If the high double is assumed to be positive, an unsigned conversion
62 // from long double to 64-bit integer is needed. The low double can be either
63 // positive or negative, so a signed conversion is needed to retain the result
64 // of the low double and to ensure it does not simply get converted to 0.
65
66 // CASE 1 - High double fits in int64.
67 if (hiExponent < 63) {
68 hiResult = (unsigned long long)ldUnion.d[0];
69 } else if (hiExponent < 128) {
70 // CASE 2 - High double does not fit in int64, scale and convert it.
71 shift = hiExponent - 54;
72 ldUnion.ull[0] &= 0x800FFFFFFFFFFFFFll;
73 ldUnion.ull[0] |= 0x4350000000000000ll;
74 hiResult = (unsigned long long)ldUnion.d[0];
75 hiResult <<= shift;
76 } else {
77 // Detect cases for overflow. When the exponent of the high
78 // double is greater than 128 bits and when the long double
79 // input is positive, return the max 128-bit integer.
80 // For negative inputs with exponents > 128, return 1, like gcc.
81 if (ldUnion.d[0] > 0) {
82 return ((__uint128_t)0xFFFFFFFFFFFFFFFFll) << 64 |
83 (__uint128_t)0xFFFFFFFFFFFFFFFFll;
84 } else {
85 return ((__uint128_t)0x0000000000000000ll) << 64 |
86 (__uint128_t)0x0000000000000001ll;
87 }
88 }
89
90 // CASE 1 - Low double fits in int64.
91 if (loExponent < 63) {
92 loResult = (long long)ldUnion.d[1];
93 } else {
94 // CASE 2 - Low double does not fit in int64, scale and convert it.
95 shift = loExponent - 54;
96 ldUnion.ull[1] &= 0x800FFFFFFFFFFFFFll;
97 ldUnion.ull[1] |= 0x4350000000000000ll;
98 loResult = (long long)ldUnion.d[1];
99 loResult <<= shift;
100 }
101
102 // If the low double is negative, it may change the integer value of the
103 // whole number if the absolute value of its fractional part is bigger than
104 // the fractional part of the high double. Because both doubles cannot
105 // overlap, this situation only occurs when the high double has no
106 // fractional part.
107 ldUnion.ld = input;
108 if ((ldUnion.d[0] == (double)hiResult) &&
109 (ldUnion.d[1] < (double)((__int128_t)loResult)))
110 loResult--;
111
112 // Add the high and low doublewords together to form a 128 bit integer.
113 result = loResult + hiResult;
114 return result;
115 }
116