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 40static __inline dst_t __extendXfYf2__(src_t a) { 41 // Various constants whose values follow from the type parameters. 42 // Any reasonable optimizer will fold and propagate all of these. 43 const int srcBits = sizeof(src_t) * CHAR_BIT; 44 const int srcExpBits = srcBits - srcSigBits - 1; 45 const int srcInfExp = (1 << srcExpBits) - 1; 46 const int srcExpBias = srcInfExp >> 1; 47 48 const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits; 49 const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits; 50 const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits); 51 const src_rep_t srcAbsMask = srcSignMask - 1; 52 const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1); 53 const src_rep_t srcNaNCode = srcQNaN - 1; 54 55 const int dstBits = sizeof(dst_t) * CHAR_BIT; 56 const int dstExpBits = dstBits - dstSigBits - 1; 57 const int dstInfExp = (1 << dstExpBits) - 1; 58 const int dstExpBias = dstInfExp >> 1; 59 60 const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits; 61 62 // Break a into a sign and representation of the absolute value. 63 const src_rep_t aRep = srcToRep(a); 64 const src_rep_t aAbs = aRep & srcAbsMask; 65 const src_rep_t sign = aRep & srcSignMask; 66 dst_rep_t absResult; 67 68 // If sizeof(src_rep_t) < sizeof(int), the subtraction result is promoted 69 // to (signed) int. To avoid that, explicitly cast to src_rep_t. 70 if ((src_rep_t)(aAbs - srcMinNormal) < srcInfinity - srcMinNormal) { 71 // a is a normal number. 72 // Extend to the destination type by shifting the significand and 73 // exponent into the proper position and rebiasing the exponent. 74 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits); 75 absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits; 76 } 77 78 else if (aAbs >= srcInfinity) { 79 // a is NaN or infinity. 80 // Conjure the result by beginning with infinity, then setting the qNaN 81 // bit (if needed) and right-aligning the rest of the trailing NaN 82 // payload field. 83 absResult = (dst_rep_t)dstInfExp << dstSigBits; 84 absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits); 85 absResult |= (dst_rep_t)(aAbs & srcNaNCode) << (dstSigBits - srcSigBits); 86 } 87 88 else if (aAbs) { 89 // a is denormal. 90 // renormalize the significand and clear the leading bit, then insert 91 // the correct adjusted exponent in the destination type. 92 const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal); 93 absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale); 94 absResult ^= dstMinNormal; 95 const int resultExponent = dstExpBias - srcExpBias - scale + 1; 96 absResult |= (dst_rep_t)resultExponent << dstSigBits; 97 } 98 99 else { 100 // a is zero. 101 absResult = 0; 102 } 103 104 // Apply the signbit to the absolute value. 105 const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits); 106 return dstFromRep(result); 107} 108