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Source at commit 1321 created 12 years 8 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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