208 lines
5.6 KiB
C
208 lines
5.6 KiB
C
/* mpn_sb_divrem_mn -- Divide natural numbers, producing both remainder and
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quotient.
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THE FUNCTIONS IN THIS FILE ARE INTERNAL FUNCTIONS WITH MUTABLE
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INTERFACES. IT IS ONLY SAFE TO REACH THEM THROUGH DOCUMENTED INTERFACES.
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IN FACT, IT IS ALMOST GUARANTEED THAT THEY'LL CHANGE OR DISAPPEAR IN A
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FUTURE GNU MP RELEASE.
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Copyright 1993, 1994, 1995, 1996, 2000, 2001, 2002 Free Software Foundation,
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Inc.
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This file is part of the GNU MP Library.
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The GNU MP Library 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 published by
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the Free Software Foundation; either version 2.1 of the License, or (at your
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option) any later version.
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The GNU MP Library is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
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or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
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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 the GNU MP Library; see the file COPYING.LIB. If not, write to
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the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
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MA 02110-1301, USA. */
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#include "gmp.h"
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#include "gmp-impl.h"
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#include "longlong.h"
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/* The size where udiv_qrnnd_preinv should be used rather than udiv_qrnnd,
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meaning the quotient size where that should happen, the quotient size
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being how many udiv divisions will be done.
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The default is to use preinv always, CPUs where this doesn't suit have
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tuned thresholds. Note in particular that preinv should certainly be
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used if that's the only division available (USE_PREINV_ALWAYS). */
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#ifndef DIV_SB_PREINV_THRESHOLD
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#define DIV_SB_PREINV_THRESHOLD 0
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#endif
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/* Divide num (NP/NSIZE) by den (DP/DSIZE) and write
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the NSIZE-DSIZE least significant quotient limbs at QP
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and the DSIZE long remainder at NP.
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Return the most significant limb of the quotient, this is always 0 or 1.
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Preconditions:
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0. NSIZE >= DSIZE.
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1. The most significant bit of the divisor must be set.
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2. QP must either not overlap with the input operands at all, or
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QP + DSIZE >= NP must hold true. (This means that it's
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possible to put the quotient in the high part of NUM, right after the
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remainder in NUM.
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3. NSIZE >= DSIZE.
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4. DSIZE > 2. */
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mp_limb_t
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mpn_sb_divrem_mn (mp_ptr qp,
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mp_ptr np, mp_size_t nn,
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mp_srcptr dp, mp_size_t dn)
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{
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mp_limb_t most_significant_q_limb = 0;
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mp_size_t qn = nn - dn;
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mp_size_t i;
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mp_limb_t dx, d1, n0;
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mp_limb_t dxinv;
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int use_preinv;
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ASSERT (dn > 2);
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ASSERT (nn >= dn);
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ASSERT (dp[dn-1] & GMP_NUMB_HIGHBIT);
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ASSERT (! MPN_OVERLAP_P (np, nn, dp, dn));
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ASSERT (! MPN_OVERLAP_P (qp, nn-dn, dp, dn));
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ASSERT (! MPN_OVERLAP_P (qp, nn-dn, np, nn) || qp+dn >= np);
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ASSERT_MPN (np, nn);
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ASSERT_MPN (dp, dn);
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np += qn;
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dx = dp[dn - 1];
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d1 = dp[dn - 2];
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n0 = np[dn - 1];
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if (n0 >= dx)
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{
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if (n0 > dx || mpn_cmp (np, dp, dn - 1) >= 0)
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{
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mpn_sub_n (np, np, dp, dn);
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most_significant_q_limb = 1;
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}
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}
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use_preinv = ABOVE_THRESHOLD (qn, DIV_SB_PREINV_THRESHOLD);
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if (use_preinv)
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invert_limb (dxinv, dx);
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for (i = qn - 1; i >= 0; i--)
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{
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mp_limb_t q;
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mp_limb_t nx;
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mp_limb_t cy_limb;
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nx = np[dn - 1]; /* FIXME: could get value from r1 */
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np--;
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if (nx == dx)
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{
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/* This might over-estimate q, but it's probably not worth
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the extra code here to find out. */
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q = GMP_NUMB_MASK;
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#if 1
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cy_limb = mpn_submul_1 (np, dp, dn, q);
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#else
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/* This should be faster on many machines */
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cy_limb = mpn_sub_n (np + 1, np + 1, dp, dn);
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cy = mpn_add_n (np, np, dp, dn);
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np[dn] += cy;
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#endif
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if (nx != cy_limb)
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{
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mpn_add_n (np, np, dp, dn);
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q--;
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}
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qp[i] = q;
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}
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else
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{
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mp_limb_t rx, r1, r0, p1, p0;
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/* "workaround" avoids a problem with gcc 2.7.2.3 i386 register usage
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when np[dn-1] is used in an asm statement like umul_ppmm in
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udiv_qrnnd_preinv. The symptom is seg faults due to registers
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being clobbered. gcc 2.95 i386 doesn't have the problem. */
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{
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mp_limb_t workaround = np[dn - 1];
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if (CACHED_ABOVE_THRESHOLD (use_preinv, DIV_SB_PREINV_THRESHOLD))
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udiv_qrnnd_preinv (q, r1, nx, workaround, dx, dxinv);
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else
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{
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udiv_qrnnd (q, r1, nx, workaround << GMP_NAIL_BITS,
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dx << GMP_NAIL_BITS);
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r1 >>= GMP_NAIL_BITS;
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}
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}
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umul_ppmm (p1, p0, d1, q << GMP_NAIL_BITS);
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p0 >>= GMP_NAIL_BITS;
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r0 = np[dn - 2];
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rx = 0;
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if (r1 < p1 || (r1 == p1 && r0 < p0))
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{
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p1 -= p0 < d1;
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p0 = (p0 - d1) & GMP_NUMB_MASK;
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q--;
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r1 = (r1 + dx) & GMP_NUMB_MASK;
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rx = r1 < dx;
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}
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p1 += r0 < p0; /* cannot carry! */
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rx -= r1 < p1; /* may become 11..1 if q is still too large */
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r1 = (r1 - p1) & GMP_NUMB_MASK;
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r0 = (r0 - p0) & GMP_NUMB_MASK;
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cy_limb = mpn_submul_1 (np, dp, dn - 2, q);
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/* Check if we've over-estimated q, and adjust as needed. */
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{
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mp_limb_t cy1, cy2;
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cy1 = r0 < cy_limb;
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r0 = (r0 - cy_limb) & GMP_NUMB_MASK;
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cy2 = r1 < cy1;
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r1 -= cy1;
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np[dn - 1] = r1;
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np[dn - 2] = r0;
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if (cy2 != rx)
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{
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mpn_add_n (np, np, dp, dn);
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q--;
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}
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}
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qp[i] = q;
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}
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}
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/* ______ ______ ______
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|__rx__|__r1__|__r0__| partial remainder
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______ ______
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- |__p1__|__p0__| partial product to subtract
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______ ______
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- |______|cylimb|
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rx is -1, 0 or 1. If rx=1, then q is correct (it should match
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carry out). If rx=-1 then q is too large. If rx=0, then q might
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be too large, but it is most likely correct.
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*/
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return most_significant_q_limb;
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}
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