frac.c, tests/tfrac.c: fixed bug in mpfr_frac (the non-significant bits

in low limb were not cleared) and added testcase (this corresponds to
changesets 6453 to 6456 in trunk).

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/branches/2.4@6457 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 32 insertions, 0 deletions

diff --git a/frac.c b/frac.c
index f43e94215..cd7f934d1 100644
--- a/frac.c
+++ b/frac.c
@@ -112,6 +112,14 @@ mpfr_frac (mpfr_ptr r, mpfr_srcptr u, mp_rnd_t rnd_mode)
     tp[tn] = k | ((un) ? mpn_lshift (tp + t0, up, un, sh) : (mp_limb_t) 0);
   if (t0 > 0)
     MPN_ZERO(tp, t0);
+  else
+    {
+      int tsh;
+
+      /* non-significant bits in low limb */
+      tsh = (mp_prec_t) tn * BITS_PER_MP_LIMB - MPFR_PREC(t);
+      tp[0] &= ~ MPFR_LIMB_MASK (tsh);
+    }
 
   if (t != r)
     { /* t is tmp */
diff --git a/tests/tfrac.c b/tests/tfrac.c
index a0d7e9f2f..7f0e25c79 100644
--- a/tests/tfrac.c
+++ b/tests/tfrac.c
@@ -172,6 +172,28 @@ special (void)
   mpfr_clear (t);
 }
 
+static void
+bug20090918 (void)
+{
+  mpfr_t x, y;
+  mp_limb_t y0;
+
+  mpfr_init2 (x, 32);
+  mpfr_init2 (y, 13);
+  mpfr_set_str (x, "61680.352935791015625", 10, GMP_RNDN);
+  mpfr_frac (y, x, GMP_RNDZ);
+  y0 = MPFR_MANT(y)[0];
+  while ((y0 >> 1) << 1 == y0)
+    y0 >>= 1;
+  if (y0 > 0x2000)
+    {
+      printf ("Error in bug20090918 (significand has more than 13 bits).\n");
+      exit (1);
+    }
+  mpfr_clear (x);
+  mpfr_clear (y);
+}
+
 #define TEST_FUNCTION mpfr_frac
 #include "tgeneric.c"
 
@@ -227,6 +249,8 @@ main (void)
   mpfr_clear (ip);
   mpfr_clear (fp);
 
+  bug20090918 ();
+
   test_generic (2, 1000, 10);
 
   tests_end_mpfr ();