forked from OSchip/llvm-project
108 lines
4.5 KiB
C++
108 lines
4.5 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 "fp_extend.h"
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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 srcBits = sizeof(src_t) * CHAR_BIT;
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const int srcExpBits = srcBits - srcSigBits - 1;
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const int srcInfExp = (1 << srcExpBits) - 1;
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const int srcExpBias = srcInfExp >> 1;
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const src_rep_t srcMinNormal = SRC_REP_C(1) << srcSigBits;
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const src_rep_t srcInfinity = (src_rep_t)srcInfExp << srcSigBits;
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const src_rep_t srcSignMask = SRC_REP_C(1) << (srcSigBits + srcExpBits);
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const src_rep_t srcAbsMask = srcSignMask - 1;
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const src_rep_t srcQNaN = SRC_REP_C(1) << (srcSigBits - 1);
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const src_rep_t srcNaNCode = srcQNaN - 1;
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const int dstBits = sizeof(dst_t) * CHAR_BIT;
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const int dstExpBits = dstBits - dstSigBits - 1;
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const int dstInfExp = (1 << dstExpBits) - 1;
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const int dstExpBias = dstInfExp >> 1;
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const dst_rep_t dstMinNormal = DST_REP_C(1) << dstSigBits;
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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 aAbs = aRep & srcAbsMask;
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const src_rep_t sign = aRep & srcSignMask;
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dst_rep_t absResult;
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// If sizeof(src_rep_t) < sizeof(int), the subtraction result is promoted
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// to (signed) int. To avoid that, explicitly cast to src_rep_t.
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if ((src_rep_t)(aAbs - srcMinNormal) < srcInfinity - srcMinNormal) {
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// a is a normal number.
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// Extend to the destination type by shifting the significand and
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// exponent into the proper position and rebiasing the exponent.
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absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits);
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absResult += (dst_rep_t)(dstExpBias - srcExpBias) << dstSigBits;
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}
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else if (aAbs >= srcInfinity) {
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// a is NaN or infinity.
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// Conjure the result by beginning with infinity, then setting the qNaN
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// bit (if needed) and right-aligning the rest of the trailing NaN
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// payload field.
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absResult = (dst_rep_t)dstInfExp << dstSigBits;
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absResult |= (dst_rep_t)(aAbs & srcQNaN) << (dstSigBits - srcSigBits);
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absResult |= (dst_rep_t)(aAbs & srcNaNCode) << (dstSigBits - srcSigBits);
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}
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else if (aAbs) {
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// a is denormal.
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// renormalize the significand and clear the leading bit, then insert
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// the correct adjusted exponent in the destination type.
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const int scale = src_rep_t_clz(aAbs) - src_rep_t_clz(srcMinNormal);
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absResult = (dst_rep_t)aAbs << (dstSigBits - srcSigBits + scale);
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absResult ^= dstMinNormal;
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const int resultExponent = dstExpBias - srcExpBias - scale + 1;
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absResult |= (dst_rep_t)resultExponent << dstSigBits;
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}
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else {
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// a is zero.
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absResult = 0;
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}
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// Apply the signbit to the absolute value.
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const dst_rep_t result = absResult | (dst_rep_t)sign << (dstBits - srcBits);
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return dstFromRep(result);
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}
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