Fix bug due to wrong estimation of (get_d (x) / LOG2).

TODO: Rexamine the algorithm of mpfr_exp_2!


git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/trunk@3362 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 16 insertions, 1 deletions

diff --git a/exp_2.c b/exp_2.c
index e9175d16b..f1cf47495 100644
--- a/exp_2.c
+++ b/exp_2.c
@@ -144,7 +144,7 @@ mpfr_exp_2 (mpfr_ptr y, mpfr_srcptr x, mp_rnd_t rnd_mode)
       mpfr_sub (r, x, r, GMP_RNDU);
       /* possible cancellation here: the error on r is at most
 	 3*2^(EXP(old_r)-EXP(new_r)) */
-      if (MPFR_IS_NEG (r))
+      while (MPFR_IS_NEG (r))
 	{ /* initial approximation n was too large */
 	  n--;
 	  mpfr_add (r, r, s, GMP_RNDU);
diff --git a/tests/texp.c b/tests/texp.c
index a7140792d..3057652f8 100644
--- a/tests/texp.c
+++ b/tests/texp.c
@@ -345,6 +345,21 @@ check_special ()
   mpfr_mul_2ui (x, x, 32, GMP_RNDN);
   test_exp (y, x, GMP_RNDN); /* Can't test return value: May overflow or not*/
 
+  /* Bug due to wrong approximation of (x)/log2 */
+  mpfr_set_prec (x, 163);
+
+  mpfr_set_str (x, "-4.28ac8fceeadcda06bb56359017b1c81b85b392e7", 16,
+		GMP_RNDN);
+  mpfr_exp (x, x, GMP_RNDN);
+  if (mpfr_cmp_str (x, "3.fffffffffffffffffffffffffffffffffffffffe8@-2",
+		    16, GMP_RNDN))
+    {
+      printf ("Error for x= -4.28ac8fceeadcda06bb56359017b1c81b85b392e7");
+      printf ("expected  3.fffffffffffffffffffffffffffffffffffffffe8@-2");
+      printf ("Got       ");
+      mpfr_out_str (stdout, 16, 0, x, GMP_RNDN); putchar ('\n');
+    }
+
   mpfr_clear (x);
   mpfr_clear (y);
   mpfr_clear (z);