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108 lines
4.2 KiB
C++
108 lines
4.2 KiB
C++
//=-lib/fp_extend_impl.inc - low precision -> high precision conversion -*-- -//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements a fairly generic conversion from a narrower to a wider
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// IEEE-754 floating-point type. The constants and types defined following the
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// includes below parameterize the conversion.
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//
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// It does not support types that don't use the usual IEEE-754 interchange
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// formats; specifically, some work would be needed to adapt it to
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// (for example) the Intel 80-bit format or PowerPC double-double format.
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//
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// Note please, however, that this implementation is only intended to support
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// *widening* operations; if you need to convert to a *narrower* floating-point
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// type (e.g. double -> float), then this routine will not do what you want it
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// to.
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//
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// It also requires that integer types at least as large as both formats
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// are available on the target platform; this may pose a problem when trying
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// to add support for quad on some 32-bit systems, for example. You also may
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// run into trouble finding an appropriate CLZ function for wide source types;
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// you will likely need to roll your own on some platforms.
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//
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// Finally, the following assumptions are made:
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//
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// 1. Floating-point types and integer types have the same endianness on the
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// target platform.
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//
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// 2. Quiet NaNs, if supported, are indicated by the leading bit of the
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// significand field being set.
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//
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//===----------------------------------------------------------------------===//
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#include "fp16_extend.inc"
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// The source type may use a usual IEEE-754 interchange format or Intel 80-bit
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// format. In particular, for the source type srcSigFracBits may be not equal to
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// srcSigBits. The destination type is assumed to be one of IEEE-754 standard
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// types.
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static __inline dst_t __extendXfYf2__(src_t a) {
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// Various constants whose values follow from the type parameters.
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// Any reasonable optimizer will fold and propagate all of these.
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const int srcInfExp = (1 << srcExpBits) - 1;
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const int srcExpBias = srcInfExp >> 1;
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const int dstInfExp = (1 << dstExpBits) - 1;
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const int dstExpBias = dstInfExp >> 1;
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// Break a into a sign and representation of the absolute value.
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const src_rep_t aRep = srcToRep(a);
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const src_rep_t srcSign = extract_sign_from_src(aRep);
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const src_rep_t srcExp = extract_exp_from_src(aRep);
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const src_rep_t srcSigFrac = extract_sig_frac_from_src(aRep);
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dst_rep_t dstSign = srcSign;
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dst_rep_t dstExp;
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dst_rep_t dstSigFrac;
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if (srcExp >= 1 && srcExp < (src_rep_t)srcInfExp) {
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// a is a normal number.
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dstExp = (dst_rep_t)srcExp + (dst_rep_t)(dstExpBias - srcExpBias);
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dstSigFrac = (dst_rep_t)srcSigFrac << (dstSigFracBits - srcSigFracBits);
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}
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else if (srcExp == srcInfExp) {
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// a is NaN or infinity.
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dstExp = dstInfExp;
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dstSigFrac = (dst_rep_t)srcSigFrac << (dstSigFracBits - srcSigFracBits);
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}
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else if (srcSigFrac) {
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// a is denormal.
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if (srcExpBits == dstExpBits) {
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// The exponent fields are identical and this is a denormal number, so all
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// the non-significand bits are zero. In particular, this branch is always
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// taken when we extend a denormal F80 to F128.
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dstExp = 0;
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dstSigFrac = ((dst_rep_t)srcSigFrac) << (dstSigFracBits - srcSigFracBits);
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} else {
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#ifndef src_rep_t_clz
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// If src_rep_t_clz is not defined this branch must be unreachable.
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__builtin_unreachable();
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#else
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// Renormalize the significand and clear the leading bit.
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// For F80 -> F128 this codepath is unused.
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const int scale = clz_in_sig_frac(srcSigFrac) + 1;
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dstExp = dstExpBias - srcExpBias - scale + 1;
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dstSigFrac = (dst_rep_t)srcSigFrac
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<< (dstSigFracBits - srcSigFracBits + scale);
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const dst_rep_t dstMinNormal = DST_REP_C(1) << (dstBits - 1 - dstExpBits);
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dstSigFrac ^= dstMinNormal;
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#endif
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}
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}
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else {
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// a is zero.
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dstExp = 0;
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dstSigFrac = 0;
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}
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const dst_rep_t result = construct_dst_rep(dstSign, dstExp, dstSigFrac);
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return dstFromRep(result);
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}
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