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Source at commit 1321 created 3 years 2 months ago.
By meklort, Add __extendsfdf2 from LLVM
1//===-- lib/extendsfdf2.c - single -> double conversion -----------*- C -*-===//
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 a fairly generic conversion from a narrower to a wider
11// IEEE-754 floating-point type. The constants and types defined following the
12// includes below parameterize the conversion.
13//
14// This routine can be trivially adapted to support conversions from
15// half-precision or to quad-precision. It does not support types that don't
16// use the usual IEEE-754 interchange formats; specifically, some work would be
17// needed to adapt it to (for example) the Intel 80-bit format or PowerPC
18// double-double format.
19//
20// Note please, however, that this implementation is only intended to support
21// *widening* operations; if you need to convert to a *narrower* floating-point
22// type (e.g. double -> float), then this routine will not do what you want it
23// to.
24//
25// It also requires that integer types at least as large as both formats
26// are available on the target platform; this may pose a problem when trying
27// to add support for quad on some 32-bit systems, for example. You also may
28// run into trouble finding an appropriate CLZ function for wide source types;
29// you will likely need to roll your own on some platforms.
30//
31// Finally, the following assumptions are made:
32//
33// 1. floating-point types and integer types have the same endianness on the
34// target platform
35//
36// 2. quiet NaNs, if supported, are indicated by the leading bit of the
37// significand field being set
38//
39//===----------------------------------------------------------------------===//
40
41#include <stdint.h>
42#include <limits.h>
43
44//#include "abi.h"
45
46typedef float src_t;
47typedef uint32_t src_rep_t;
48#define SRC_REP_C UINT32_C
49static const int srcSigBits = 23;
50#define src_rep_t_clz __builtin_clz
51
52typedef double dst_t;
53typedef uint64_t dst_rep_t;
54#define DST_REP_C UINT64_C
55static const int dstSigBits = 52;
56
57// End of specialization parameters. Two helper routines for conversion to and
58// from the representation of floating-point data as integer values follow.
59
60static inline src_rep_t srcToRep(src_t x) {
61 const union { src_t f; src_rep_t i; } rep = {.f = x};
62 return rep.i;
63}
64
65static inline dst_t dstFromRep(dst_rep_t x) {
66 const union { dst_t f; dst_rep_t i; } rep = {.i = x};
67 return rep.f;
68}
69
70// End helper routines. Conversion implementation follows.
71
72dst_t __extendsfdf2(src_t a) {
73
74 // Various constants whose values follow from the type parameters.
75 // Any reasonable optimizer will fold and propagate all of these.
76 const int srcBits = sizeof(src_t)*CHAR_BIT;
77 const int srcExpBits = srcBits - srcSigBits - 1;
78 const int srcInfExp = (1 << srcExpBits) - 1;
79 const int srcExpBias = srcInfExp >> 1;
80
81 const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
82 const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
83 const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
84 const src_rep_t srcAbsMask = srcSignMask - 1;
85 const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
86 const src_rep_t srcNaNCode = srcQNaN - 1;
87
88 const int dstBits = sizeof(dst_t)*CHAR_BIT;
89 const int dstExpBits = dstBits - dstSigBits - 1;
90 const int dstInfExp = (1 << dstExpBits) - 1;
91 const int dstExpBias = dstInfExp >> 1;
92
93 const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits;
94
95 // Break a into a sign and representation of the absolute value
96 const src_rep_t aRep = srcToRep(a);
97 const src_rep_t aAbs = aRep & srcAbsMask;
98 const src_rep_t sign = aRep & srcSignMask;
99 dst_rep_t absResult;
100
101 if (aAbs - srcMinNormal < srcInfinity - srcMinNormal) {
102 // a is a normal number.
103 // Extend to the destination type by shifting the significand and
104 // exponent into the proper position and rebiasing the exponent.
105 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits);
106 absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits;
107 }
108
109 else if (aAbs >= srcInfinity) {
110 // a is NaN or infinity.
111 // Conjure the result by beginning with infinity, then setting the qNaN
112 // bit (if needed) and right-aligning the rest of the trailing NaN
113 // payload field.
114 absResult = (dst_rep_t)dstInfExp << dstSigBits;
115 absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits);
116 absResult |= aAbs & srcNaNCode;
117 }
118
119 else if (aAbs) {
120 // a is denormal.
121 // renormalize the significand and clear the leading bit, then insert
122 // the correct adjusted exponent in the destination type.
123 const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal);
124 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale);
125 absResult ^= dstMinNormal;
126 const int resultExponent = dstExpBias - srcExpBias - scale + 1;
127 absResult |= (dst_rep_t)resultExponent << dstSigBits;
128 }
129
130 else {
131 // a is zero.
132 absResult = 0;
133 }
134
135 // Apply the signbit to (dst_t)abs(a).
136 const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits);
137 return dstFromRep(result);
138}

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