--- /dev/null
+/* mpf_div_2exp -- Divide a float by 2^n.
+
+Copyright 1993, 1994, 1996, 2000, 2001, 2002, 2004 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 3 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. If not, see http://www.gnu.org/licenses/. */
+
+#include "gmp.h"
+#include "gmp-impl.h"
+
+
+/* Multiples of GMP_NUMB_BITS in exp simply mean an amount subtracted from
+ EXP(u) to set EXP(r). The remainder exp%GMP_NUMB_BITS is then a right
+ shift for the limb data.
+
+ If exp%GMP_NUMB_BITS == 0 then there's no shifting, we effectively just
+ do an mpz_set with changed EXP(r). Like mpz_set we take prec+1 limbs in
+ this case. Although just prec would suffice, it's nice to have
+ mpf_div_2exp with exp==0 come out the same as mpz_set.
+
+ When shifting we take up to prec many limbs from the input. Our shift is
+ cy = mpn_rshift (PTR(r)+1, PTR(u)+k, ...), where k is the number of low
+ limbs dropped from u, and the carry out is stored to PTR(r)[0]. We don't
+ try to work extra bits from PTR(u)[k-1] (when k>=1 makes it available)
+ into that low carry limb. Just prec limbs (with the high non-zero) from
+ the input is enough bits for the application requested precision, no need
+ to do extra work.
+
+ If r==u the shift will have overlapping operands. When k>=1 (ie. when
+ usize > prec), the overlap is in the style supported by rshift (ie. dst
+ <= src).
+
+ But when r==u and k==0 (ie. usize <= prec), we would have an invalid
+ overlap (mpn_rshift (rp+1, rp, ...)). In this case we must instead use
+ mpn_lshift (PTR(r), PTR(u), size, NUMB-shift). An lshift by NUMB-shift
+ bits gives identical data of course, it's just its overlap restrictions
+ which differ.
+
+ In both shift cases, the resulting data is abs_usize+1 limbs. "adj" is
+ used to add +1 to that size if the high is non-zero (it may of course
+ have become zero by the shifting). EXP(u) is the exponent just above
+ those abs_usize+1 limbs, so it gets -1+adj, which means -1 if the high is
+ zero, or no change if the high is non-zero.
+
+ Enhancements:
+
+ The way mpn_lshift is used means successive mpf_div_2exp calls on the
+ same operand will accumulate low zero limbs, until prec+1 limbs is
+ reached. This is wasteful for subsequent operations. When abs_usize <=
+ prec, we should test the low exp%GMP_NUMB_BITS many bits of PTR(u)[0],
+ ie. those which would be shifted out by an mpn_rshift. If they're zero
+ then use that mpn_rshift. */
+
+void
+mpf_div_2exp (mpf_ptr r, mpf_srcptr u, unsigned long int exp)
+{
+ mp_srcptr up;
+ mp_ptr rp = r->_mp_d;
+ mp_size_t usize;
+ mp_size_t abs_usize;
+ mp_size_t prec = r->_mp_prec;
+ mp_exp_t uexp = u->_mp_exp;
+
+ usize = u->_mp_size;
+
+ if (UNLIKELY (usize == 0))
+ {
+ r->_mp_size = 0;
+ r->_mp_exp = 0;
+ return;
+ }
+
+ abs_usize = ABS (usize);
+ up = u->_mp_d;
+
+ if (exp % GMP_NUMB_BITS == 0)
+ {
+ prec++; /* retain more precision here as we don't need
+ to account for carry-out here */
+ if (abs_usize > prec)
+ {
+ up += abs_usize - prec;
+ abs_usize = prec;
+ }
+ if (rp != up)
+ MPN_COPY_INCR (rp, up, abs_usize);
+ r->_mp_exp = uexp - exp / GMP_NUMB_BITS;
+ }
+ else
+ {
+ mp_limb_t cy_limb;
+ mp_size_t adj;
+ if (abs_usize > prec)
+ {
+ up += abs_usize - prec;
+ abs_usize = prec;
+ /* Use mpn_rshift since mpn_lshift operates downwards, and we
+ therefore would clobber part of U before using that part, in case
+ R is the same variable as U. */
+ cy_limb = mpn_rshift (rp + 1, up, abs_usize, exp % GMP_NUMB_BITS);
+ rp[0] = cy_limb;
+ adj = rp[abs_usize] != 0;
+ }
+ else
+ {
+ cy_limb = mpn_lshift (rp, up, abs_usize,
+ GMP_NUMB_BITS - exp % GMP_NUMB_BITS);
+ rp[abs_usize] = cy_limb;
+ adj = cy_limb != 0;
+ }
+
+ abs_usize += adj;
+ r->_mp_exp = uexp - exp / GMP_NUMB_BITS - 1 + adj;
+ }
+ r->_mp_size = usize >= 0 ? abs_usize : -abs_usize;
+}
projects / msp430-gcc.git / blobdiff