[project @ 1998-06-05 14:37:50 by simonm]

Initial revision

ghc/rts/gmp/mpn/generic/divmod_1.c

+/* mpn_divmod_1(quot_ptr, dividend_ptr, dividend_size, divisor_limb) --
+   Divide (DIVIDEND_PTR,,DIVIDEND_SIZE) by DIVISOR_LIMB.
+   Write DIVIDEND_SIZE limbs of quotient at QUOT_PTR.
+   Return the single-limb remainder.
+   There are no constraints on the value of the divisor.
+
+   QUOT_PTR and DIVIDEND_PTR might point to the same limb.
+
+Copyright (C) 1991, 1993, 1994, 1996 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 Library General Public License as published by
+the Free Software Foundation; either version 2 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 Library General Public
+License for more details.
+
+You should have received a copy of the GNU Library General Public License
+along with the GNU MP Library; see the file COPYING.LIB.  If not, write to
+the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+MA 02111-1307, USA. */
+
+#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
+#if __STDC__
+mpn_divmod_1 (mp_ptr quot_ptr,
+	      mp_srcptr dividend_ptr, mp_size_t dividend_size,
+	      mp_limb_t divisor_limb)
+#else
+mpn_divmod_1 (quot_ptr, dividend_ptr, dividend_size, divisor_limb)
+     mp_ptr quot_ptr;
+     mp_srcptr dividend_ptr;
+     mp_size_t dividend_size;
+     mp_limb_t divisor_limb;
+#endif
+{
+  mp_size_t i;
+  mp_limb_t n1, n0, r;
+  int dummy;
+
+  /* ??? 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 (quot_ptr[i + 1], r, r,
+				 ((n1 << normalization_steps)
+				  | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
+				 divisor_limb, divisor_limb_inverted);
+	      n1 = n0;
+	    }
+	  udiv_qrnnd_preinv (quot_ptr[0], 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
+	    {
+	      quot_ptr[i] = 0;
+	      i--;
+	    }
+
+	  for (; i >= 0; i--)
+	    {
+	      n0 = dividend_ptr[i];
+	      udiv_qrnnd_preinv (quot_ptr[i], 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 (quot_ptr[i + 1], r, r,
+			      ((n1 << normalization_steps)
+			       | (n0 >> (BITS_PER_MP_LIMB - normalization_steps))),
+			      divisor_limb);
+		  n1 = n0;
+		}
+	      udiv_qrnnd (quot_ptr[0], 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
+	{
+	  quot_ptr[i] = 0;
+	  i--;
+	}
+
+      for (; i >= 0; i--)
+	{
+	  n0 = dividend_ptr[i];
+	  udiv_qrnnd (quot_ptr[i], r, r, n0, divisor_limb);
+	}
+      return r;
+    }
+}