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Collin Funk
77 lines
2.7 KiB
C
77 lines
2.7 KiB
C
/* Total order for 'double'
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Copyright 2023-2026 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation, either version 3 of the
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License, or (at your option) any later version.
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This file is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert. */
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#include <config.h>
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/* Specification. */
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#include <math.h>
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#include <string.h>
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int
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totalorder (double const *x, double const *y)
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{
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/* If the sign bits of *X and *Y differ, the one with non-zero sign bit
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is "smaller" than the one with sign bit == 0. */
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int xs = signbit (*x);
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int ys = signbit (*y);
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if (!xs != !ys)
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return xs;
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/* If one of *X, *Y is a NaN and the other isn't, the answer is easy
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as well: the negative NaN is "smaller", the positive NaN is "greater"
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than the other argument. */
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int xn = isnand (*x);
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int yn = isnand (*y);
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if (!xn != !yn)
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return !xn == !xs;
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/* If none of *X, *Y is a NaN, the '<=' operator does the job, including
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for -Infinity and +Infinity. */
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if (!xn)
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return *x <= *y;
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/* At this point, *X and *Y are NaNs with the same sign bit. */
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unsigned long long extended_sign = -!!xs;
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#if defined __hppa || (defined __mips__ && !MIPS_NAN2008_DOUBLE) || defined __sh__
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/* Invert the most significant bit of the mantissa field. Cf. snan.h. */
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extended_sign ^= (1ULL << 51);
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#endif
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union { unsigned long long i; double f; } xu = {0}, yu = {0};
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#if 0
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xu.f = *x;
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yu.f = *y;
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#else
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# if defined __GNUC__ || defined __clang__
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/* Prevent gcc and clang from reusing the values of *x and *y (fetched above)
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in optimized inlined memcpy expansions.
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Seen with gcc <https://gcc.gnu.org/PR114659>
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and with clang 16.0.6 on OpenBSD 7.5. */
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__asm__ __volatile__ ("" : : : "memory");
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# endif
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/* On 32-bit x86 processors, as well as on x86_64 processors with
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CC="gcc -mfpmath=387", the evaluation of *x and *y above is done through
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an 'fldl' instruction, which converts a signalling NaN to a quiet NaN. See
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<https://lists.gnu.org/archive/html/bug-gnulib/2023-10/msg00060.html>
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for details. Use memcpy to avoid this. */
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memcpy (&xu.f, x, sizeof (double));
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memcpy (&yu.f, y, sizeof (double));
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#endif
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return (xu.i ^ extended_sign) <= (yu.i ^ extended_sign);
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}
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