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75ad83f564
C23 adds various <math.h> function families originally defined in TS 18661-4. Add the pown functions, which are like pow but with an integer exponent. That exponent has type long long int in C23; it was intmax_t in TS 18661-4, and as with other interfaces changed after their initial appearance in the TS, I don't think we need to support the original version of the interface. The test inputs are based on the subset of test inputs for pow that use integer exponents that fit in long long. As the first such template implementation that saves and restores the rounding mode internally (to avoid possible issues with directed rounding and intermediate overflows or underflows in the wrong rounding mode), support also needed to be added for using SET_RESTORE_ROUND* in such template function implementations. This required math-type-macros-float128.h to include <fenv_private.h>, so it can tell whether SET_RESTORE_ROUNDF128 is defined. In turn, the include order with <fenv_private.h> included before <math_private.h> broke loongarch builds, showing up that sysdeps/loongarch/math_private.h is really a fenv_private.h file (maybe implemented internally before the consistent split of those headers in 2018?) and needed to be renamed to fenv_private.h to avoid errors with duplicate macro definitions if <math_private.h> is included after <fenv_private.h>. The underlying implementation uses __ieee754_pow functions (called more than once in some cases, where the exponent does not fit in the floating type). I expect a custom implementation for a given format, that only handles integer exponents but handles larger exponents directly, could be faster and more accurate in some cases. I encourage searching for worst cases for ulps error for these implementations (necessarily non-exhaustively, given the size of the input space). Tested for x86_64 and x86, and with build-many-glibcs.py.
92 lines
2.6 KiB
C
92 lines
2.6 KiB
C
/* Return X^Y for integer Y.
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Copyright (C) 2025 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library 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 GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#include <errno.h>
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#include <fenv_private.h>
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#include <limits.h>
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#include <math.h>
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#include <math-barriers.h>
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#include <math-narrow-eval.h>
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#include <math_private.h>
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#include <stdlib.h>
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FLOAT
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M_DECL_FUNC (__pown) (FLOAT x, long long int y)
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{
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FLOAT ret;
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#if M_MANT_DIG >= LLONG_WIDTH - 1
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ret = M_SUF (__ieee754_pow) (x, y);
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#else
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if ((y <= 1LL << M_MANT_DIG
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&& y >= -(1LL << M_MANT_DIG))
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|| y == LLONG_MIN)
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ret = M_SUF (__ieee754_pow) (x, y);
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else
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{
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{
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/* To avoid problems with overflow and underflow from
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intermediate computations occurring in the wrong directed
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rounding mode (when X is negative and Y is odd), do these
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computations in round-to-nearest mode and correct
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overflowing and underflowing results afterwards. */
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M_SET_RESTORE_ROUND (FE_TONEAREST);
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ret = M_LIT (1.0);
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while (y != 0)
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{
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long long int absy = llabs (y);
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int bits = LLONG_WIDTH - __builtin_clzll (absy);
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if (bits <= M_MANT_DIG)
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{
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ret *= M_SUF (__ieee754_pow) (x, y);
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y = 0;
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}
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else
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{
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long long int absy_high
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= absy & (((1ULL << M_MANT_DIG) - 1) << (bits - M_MANT_DIG));
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long long int absy_low = absy - absy_high;
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long long int y_high = y < 0 ? -absy_high : absy_high;
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ret *= M_SUF (__ieee754_pow) (x, y_high);
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y = y < 0 ? -absy_low : absy_low;
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}
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}
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ret = math_narrow_eval (ret);
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math_force_eval (ret);
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}
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if (isfinite (x) && x != M_LIT (0.0))
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{
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if (isinf (ret))
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ret = math_narrow_eval (M_COPYSIGN (M_MAX, ret) * M_MAX);
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else if (ret == M_LIT (0.0))
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ret = math_narrow_eval (M_COPYSIGN (M_MIN, ret) * M_MIN);
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}
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}
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#endif
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if (!isfinite (ret))
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{
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if (isfinite (x))
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__set_errno (ERANGE);
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}
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else if (ret == 0
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&& isfinite (x)
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&& x != 0)
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__set_errno (ERANGE);
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return ret;
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}
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declare_mgen_alias (__pown, pown);
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