xref: /freebsd/contrib/llvm-project/compiler-rt/lib/builtins/fp_extend_impl.inc (revision f5f40dd63bc7acbb5312b26ac1ea1103c12352a6)
1//=-lib/fp_extend_impl.inc - low precision -> high precision conversion -*-- -//
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 a fairly generic conversion from a narrower to a wider
10// IEEE-754 floating-point type.  The constants and types defined following the
11// includes below parameterize the conversion.
12//
13// It does not support types that don't use the usual IEEE-754 interchange
14// formats; specifically, some work would be needed to adapt it to
15// (for example) the Intel 80-bit format or PowerPC double-double format.
16//
17// Note please, however, that this implementation is only intended to support
18// *widening* operations; if you need to convert to a *narrower* floating-point
19// type (e.g. double -> float), then this routine will not do what you want it
20// to.
21//
22// It also requires that integer types at least as large as both formats
23// are available on the target platform; this may pose a problem when trying
24// to add support for quad on some 32-bit systems, for example.  You also may
25// run into trouble finding an appropriate CLZ function for wide source types;
26// you will likely need to roll your own on some platforms.
27//
28// Finally, the following assumptions are made:
29//
30// 1. Floating-point types and integer types have the same endianness on the
31//    target platform.
32//
33// 2. Quiet NaNs, if supported, are indicated by the leading bit of the
34//    significand field being set.
35//
36//===----------------------------------------------------------------------===//
37
38#include "fp_extend.h"
39
40// The source type may use a usual IEEE-754 interchange format or Intel 80-bit
41// format. In particular, for the source type srcSigFracBits may be not equal to
42// srcSigBits. The destination type is assumed to be one of IEEE-754 standard
43// types.
44static __inline dst_t __extendXfYf2__(src_t a) {
45  // Various constants whose values follow from the type parameters.
46  // Any reasonable optimizer will fold and propagate all of these.
47  const int srcInfExp = (1 << srcExpBits) - 1;
48  const int srcExpBias = srcInfExp >> 1;
49
50  const int dstInfExp = (1 << dstExpBits) - 1;
51  const int dstExpBias = dstInfExp >> 1;
52
53  // Break a into a sign and representation of the absolute value.
54  const src_rep_t aRep = srcToRep(a);
55  const src_rep_t srcSign = extract_sign_from_src(aRep);
56  const src_rep_t srcExp = extract_exp_from_src(aRep);
57  const src_rep_t srcSigFrac = extract_sig_frac_from_src(aRep);
58
59  dst_rep_t dstSign = srcSign;
60  dst_rep_t dstExp;
61  dst_rep_t dstSigFrac;
62
63  if (srcExp >= 1 && srcExp < (src_rep_t)srcInfExp) {
64    // a is a normal number.
65    dstExp = (dst_rep_t)srcExp + (dst_rep_t)(dstExpBias - srcExpBias);
66    dstSigFrac = (dst_rep_t)srcSigFrac << (dstSigFracBits - srcSigFracBits);
67  }
68
69  else if (srcExp == srcInfExp) {
70    // a is NaN or infinity.
71    dstExp = dstInfExp;
72    dstSigFrac = (dst_rep_t)srcSigFrac << (dstSigFracBits - srcSigFracBits);
73  }
74
75  else if (srcSigFrac) {
76    // a is denormal.
77    if (srcExpBits == dstExpBits) {
78      // The exponent fields are identical and this is a denormal number, so all
79      // the non-significand bits are zero. In particular, this branch is always
80      // taken when we extend a denormal F80 to F128.
81      dstExp = 0;
82      dstSigFrac = ((dst_rep_t)srcSigFrac) << (dstSigFracBits - srcSigFracBits);
83    } else {
84#ifndef src_rep_t_clz
85      // If src_rep_t_clz is not defined this branch must be unreachable.
86      __builtin_unreachable();
87#else
88      // Renormalize the significand and clear the leading bit.
89      // For F80 -> F128 this codepath is unused.
90      const int scale = clz_in_sig_frac(srcSigFrac) + 1;
91      dstExp = dstExpBias - srcExpBias - scale + 1;
92      dstSigFrac = (dst_rep_t)srcSigFrac
93                   << (dstSigFracBits - srcSigFracBits + scale);
94      const dst_rep_t dstMinNormal = DST_REP_C(1) << (dstBits - 1 - dstExpBits);
95      dstSigFrac ^= dstMinNormal;
96#endif
97    }
98  }
99
100  else {
101    // a is zero.
102    dstExp = 0;
103    dstSigFrac = 0;
104  }
105
106  const dst_rep_t result = construct_dst_rep(dstSign, dstExp, dstSigFrac);
107  return dstFromRep(result);
108}
109