speedup of mpfr_div when divisor has one limb

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@9086 280ebfd0-de03-0410-8827-d642c229c3f4

1 files changed, 22 insertions, 0 deletions

diff --git a/src/div.c b/src/div.c
index a5c12036b..6e30268e9 100644
--- a/src/div.c
+++ b/src/div.c
@@ -240,6 +240,28 @@ mpfr_div (mpfr_ptr q, mpfr_srcptr u, mpfr_srcptr v, mpfr_rnd_t rnd_mode)
    *                                                                        *
    **************************************************************************/
 
+  /* when the divisor has one limb, we can use mpfr_div_ui, which should be
+     faster, assuming there is no intermediate overflow or underflow.
+     The divisor interpreted as an integer satisfies
+     2^(GMP_NUMB_BITS-1) <= vm < 2^GMP_NUMB_BITS, thus the quotient
+     satisfies 2^(EXP(u)-1-GMP_NUMB_BITS) < u/vm < 2^(EXP(u)-GMP_NUMB_BITS+1)
+     and its exponent is either EXP(u)-GMP_NUMB_BITS or one more. */
+  if (vsize <= 1 && __gmpfr_emin <= MPFR_EXP(u) - GMP_NUMB_BITS
+      && MPFR_EXP(u) - GMP_NUMB_BITS + 1 <= __gmpfr_emax)
+    {
+      mpfr_exp_t exp_v = MPFR_EXP(v); /* save it in case q=v */
+      if (MPFR_SIGN(v) > 0)
+        inex = mpfr_div_ui (q, u, vp[0], rnd_mode);
+      else
+        {
+          inex = -mpfr_div_ui (q, u, vp[0], MPFR_INVERT_RND(rnd_mode));
+          MPFR_CHANGE_SIGN(q);
+        }
+      /* q did not under/overflow */
+      MPFR_EXP(q) -= exp_v - GMP_NUMB_BITS;
+      return mpfr_check_range (q, inex, rnd_mode);
+    }
+
   MPFR_TMP_MARK(marker);
 
   /* set sign */