Imported gcc-4.4.3

[msp430-gcc.git] / gmp / mpf / mul_2exp.c

diff --git a/gmp/mpf/mul_2exp.c b/gmp/mpf/mul_2exp.c
new file mode 100644 (file)
index 0000000..bd7df9c
--- /dev/null
+++ b/gmp/mpf/mul_2exp.c
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+/* mpf_mul_2exp -- Multiply 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 added to EXP(u)
+   to set EXP(r).  The remainder exp%GMP_NUMB_BITS is then a left 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_mul_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_lshift (PTR(r), PTR(u)+k, size, ...), where k is the number of
+   low limbs dropped from u, and the carry out is stored to PTR(r)[size].
+
+   It may be noted that the low limb PTR(r)[0] doesn't incorporate bits from
+   PTR(u)[k-1] (when k>=1 makes that limb available).  Taking just prec
+   limbs from the input (with the high non-zero) is enough bits for the
+   application requested precision, there's no need for extra work.
+
+   If r==u the shift will have overlapping operands.  When k==0 (ie. when
+   usize <= prec), the overlap is supported by lshift (ie. dst == src).
+
+   But when r==u and k>=1 (ie. usize > prec), we would have an invalid
+   overlap (ie. mpn_lshift (rp, rp+k, ...)).  In this case we must instead
+   use mpn_rshift (PTR(r)+1, PTR(u)+k, size, NUMB-shift) with the carry out
+   stored to PTR(r)[0].  An rshift by NUMB-shift bits like this gives
+   identical data, it's just its overlap restrictions which differ.
+
+   Enhancements:
+
+   The way mpn_lshift is used means successive mpf_mul_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_mul_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,
+                               GMP_NUMB_BITS - exp % GMP_NUMB_BITS);
+         rp[0] = cy_limb;
+         adj = rp[abs_usize] != 0;
+       }
+      else
+       {
+         cy_limb = mpn_lshift (rp, up, abs_usize, exp % GMP_NUMB_BITS);
+         rp[abs_usize] = cy_limb;
+         adj = cy_limb != 0;
+       }
+
+      abs_usize += adj;
+      r->_mp_exp = uexp + exp / GMP_NUMB_BITS + adj;
+    }
+  r->_mp_size = usize >= 0 ? abs_usize : -abs_usize;
+}