diff options
Diffstat (limited to 'REORG.TODO/stdlib/mod_1.c')
| -rw-r--r-- | REORG.TODO/stdlib/mod_1.c | 189 |
1 files changed, 189 insertions, 0 deletions
diff --git a/REORG.TODO/stdlib/mod_1.c b/REORG.TODO/stdlib/mod_1.c new file mode 100644 index 0000000000..33ca193ce2 --- /dev/null +++ b/REORG.TODO/stdlib/mod_1.c @@ -0,0 +1,189 @@ +/* mpn_mod_1(dividend_ptr, dividend_size, divisor_limb) -- + Divide (DIVIDEND_PTR,,DIVIDEND_SIZE) by DIVISOR_LIMB. + Return the single-limb remainder. + There are no constraints on the value of the divisor. + +Copyright (C) 1991-2017 Free Software Foundation, Inc. + +This file is part of the GNU MP Library. + +The GNU MP Library is free software; you can redistribute it and/or modify +it under the terms of the GNU Lesser General Public License as published by +the Free Software Foundation; either version 2.1 of the License, or (at your +option) any later version. + +The GNU MP Library is distributed in the hope that it will be useful, but +WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY +or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public +License for more details. + +You should have received a copy of the GNU Lesser General Public License +along with the GNU MP Library; see the file COPYING.LIB. If not, see +<http://www.gnu.org/licenses/>. */ + +#include <gmp.h> +#include "gmp-impl.h" +#include "longlong.h" + +#ifndef UMUL_TIME +#define UMUL_TIME 1 +#endif + +#ifndef UDIV_TIME +#define UDIV_TIME UMUL_TIME +#endif + +/* FIXME: We should be using invert_limb (or invert_normalized_limb) + here (not udiv_qrnnd). */ + +mp_limb_t +mpn_mod_1 (mp_srcptr dividend_ptr, mp_size_t dividend_size, + mp_limb_t divisor_limb) +{ + mp_size_t i; + mp_limb_t n1, n0, r; + mp_limb_t dummy __attribute__ ((unused)); + + /* Botch: Should this be handled at all? Rely on callers? */ + if (dividend_size == 0) + return 0; + + /* If multiplication is much faster than division, and the + dividend is large, pre-invert the divisor, and use + only multiplications in the inner loop. */ + + /* This test should be read: + Does it ever help to use udiv_qrnnd_preinv? + && Does what we save compensate for the inversion overhead? */ + if (UDIV_TIME > (2 * UMUL_TIME + 6) + && (UDIV_TIME - (2 * UMUL_TIME + 6)) * dividend_size > UDIV_TIME) + { + int normalization_steps; + + count_leading_zeros (normalization_steps, divisor_limb); + if (normalization_steps != 0) + { + mp_limb_t divisor_limb_inverted; + + divisor_limb <<= normalization_steps; + + /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The + result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the + most significant bit (with weight 2**N) implicit. */ + + /* Special case for DIVISOR_LIMB == 100...000. */ + if (divisor_limb << 1 == 0) + divisor_limb_inverted = ~(mp_limb_t) 0; + else + udiv_qrnnd (divisor_limb_inverted, dummy, + -divisor_limb, 0, divisor_limb); + + n1 = dividend_ptr[dividend_size - 1]; + r = n1 >> (BITS_PER_MP_LIMB - normalization_steps); + + /* Possible optimization: + if (r == 0 + && divisor_limb > ((n1 << normalization_steps) + | (dividend_ptr[dividend_size - 2] >> ...))) + ...one division less... */ + + for (i = dividend_size - 2; i >= 0; i--) + { + n0 = dividend_ptr[i]; + udiv_qrnnd_preinv (dummy, r, r, + ((n1 << normalization_steps) + | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))), + divisor_limb, divisor_limb_inverted); + n1 = n0; + } + udiv_qrnnd_preinv (dummy, r, r, + n1 << normalization_steps, + divisor_limb, divisor_limb_inverted); + return r >> normalization_steps; + } + else + { + mp_limb_t divisor_limb_inverted; + + /* Compute (2**2N - 2**N * DIVISOR_LIMB) / DIVISOR_LIMB. The + result is a (N+1)-bit approximation to 1/DIVISOR_LIMB, with the + most significant bit (with weight 2**N) implicit. */ + + /* Special case for DIVISOR_LIMB == 100...000. */ + if (divisor_limb << 1 == 0) + divisor_limb_inverted = ~(mp_limb_t) 0; + else + udiv_qrnnd (divisor_limb_inverted, dummy, + -divisor_limb, 0, divisor_limb); + + i = dividend_size - 1; + r = dividend_ptr[i]; + + if (r >= divisor_limb) + r = 0; + else + i--; + + for (; i >= 0; i--) + { + n0 = dividend_ptr[i]; + udiv_qrnnd_preinv (dummy, r, r, + n0, divisor_limb, divisor_limb_inverted); + } + return r; + } + } + else + { + if (UDIV_NEEDS_NORMALIZATION) + { + int normalization_steps; + + count_leading_zeros (normalization_steps, divisor_limb); + if (normalization_steps != 0) + { + divisor_limb <<= normalization_steps; + + n1 = dividend_ptr[dividend_size - 1]; + r = n1 >> (BITS_PER_MP_LIMB - normalization_steps); + + /* Possible optimization: + if (r == 0 + && divisor_limb > ((n1 << normalization_steps) + | (dividend_ptr[dividend_size - 2] >> ...))) + ...one division less... */ + + for (i = dividend_size - 2; i >= 0; i--) + { + n0 = dividend_ptr[i]; + udiv_qrnnd (dummy, r, r, + ((n1 << normalization_steps) + | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))), + divisor_limb); + n1 = n0; + } + udiv_qrnnd (dummy, r, r, + n1 << normalization_steps, + divisor_limb); + return r >> normalization_steps; + } + } + /* No normalization needed, either because udiv_qrnnd doesn't require + it, or because DIVISOR_LIMB is already normalized. */ + + i = dividend_size - 1; + r = dividend_ptr[i]; + + if (r >= divisor_limb) + r = 0; + else + i--; + + for (; i >= 0; i--) + { + n0 = dividend_ptr[i]; + udiv_qrnnd (dummy, r, r, n0, divisor_limb); + } + return r; + } +} |