126 lines
4.3 KiB
C
126 lines
4.3 KiB
C
/* mpf_mul_2exp -- Multiply a float by 2^n.
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Copyright 1993, 1994, 1996, 2000, 2001, 2002, 2004 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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/* Multiples of GMP_NUMB_BITS in exp simply mean an amount added to EXP(u)
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to set EXP(r). The remainder exp%GMP_NUMB_BITS is then a left shift for
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the limb data.
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If exp%GMP_NUMB_BITS == 0 then there's no shifting, we effectively just
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do an mpz_set with changed EXP(r). Like mpz_set we take prec+1 limbs in
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this case. Although just prec would suffice, it's nice to have
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mpf_mul_2exp with exp==0 come out the same as mpz_set.
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When shifting we take up to prec many limbs from the input. Our shift is
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cy = mpn_lshift (PTR(r), PTR(u)+k, size, ...), where k is the number of
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low limbs dropped from u, and the carry out is stored to PTR(r)[size].
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It may be noted that the low limb PTR(r)[0] doesn't incorporate bits from
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PTR(u)[k-1] (when k>=1 makes that limb available). Taking just prec
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limbs from the input (with the high non-zero) is enough bits for the
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application requested precision, there's no need for extra work.
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If r==u the shift will have overlapping operands. When k==0 (ie. when
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usize <= prec), the overlap is supported by lshift (ie. dst == src).
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But when r==u and k>=1 (ie. usize > prec), we would have an invalid
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overlap (ie. mpn_lshift (rp, rp+k, ...)). In this case we must instead
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use mpn_rshift (PTR(r)+1, PTR(u)+k, size, NUMB-shift) with the carry out
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stored to PTR(r)[0]. An rshift by NUMB-shift bits like this gives
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identical data, it's just its overlap restrictions which differ.
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Enhancements:
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The way mpn_lshift is used means successive mpf_mul_2exp calls on the
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same operand will accumulate low zero limbs, until prec+1 limbs is
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reached. This is wasteful for subsequent operations. When abs_usize <=
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prec, we should test the low exp%GMP_NUMB_BITS many bits of PTR(u)[0],
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ie. those which would be shifted out by an mpn_rshift. If they're zero
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then use that mpn_rshift. */
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void
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mpf_mul_2exp (mpf_ptr r, mpf_srcptr u, unsigned long int exp)
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{
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mp_srcptr up;
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mp_ptr rp = r->_mp_d;
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mp_size_t usize;
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mp_size_t abs_usize;
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mp_size_t prec = r->_mp_prec;
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mp_exp_t uexp = u->_mp_exp;
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usize = u->_mp_size;
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if (UNLIKELY (usize == 0))
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{
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r->_mp_size = 0;
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r->_mp_exp = 0;
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return;
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}
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abs_usize = ABS (usize);
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up = u->_mp_d;
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if (exp % GMP_NUMB_BITS == 0)
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{
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prec++; /* retain more precision here as we don't need
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to account for carry-out here */
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if (abs_usize > prec)
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{
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up += abs_usize - prec;
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abs_usize = prec;
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}
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if (rp != up)
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MPN_COPY_INCR (rp, up, abs_usize);
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r->_mp_exp = uexp + exp / GMP_NUMB_BITS;
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}
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else
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{
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mp_limb_t cy_limb;
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mp_size_t adj;
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if (abs_usize > prec)
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{
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up += abs_usize - prec;
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abs_usize = prec;
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/* Use mpn_rshift since mpn_lshift operates downwards, and we
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therefore would clobber part of U before using that part, in case
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R is the same variable as U. */
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cy_limb = mpn_rshift (rp + 1, up, abs_usize,
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GMP_NUMB_BITS - exp % GMP_NUMB_BITS);
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rp[0] = cy_limb;
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adj = rp[abs_usize] != 0;
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}
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else
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{
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cy_limb = mpn_lshift (rp, up, abs_usize, exp % GMP_NUMB_BITS);
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rp[abs_usize] = cy_limb;
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adj = cy_limb != 0;
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}
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abs_usize += adj;
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r->_mp_exp = uexp + exp / GMP_NUMB_BITS + adj;
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}
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r->_mp_size = usize >= 0 ? abs_usize : -abs_usize;
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}
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