[src/gamma.c] Fixed bug in the underflow detection code of mpfr_gamma,

  found by Giridhar Tammana: some results may incorrectly be regarded
  as underflow.
[tests/tgamma.c] Added testcase.
(merged changeset r8159 from the trunk)

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/branches/3.1@8160 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 32 insertions, 1 deletions

diff --git a/src/gamma.c b/src/gamma.c
index cd60041f0..a346f8b1b 100644
--- a/src/gamma.c
+++ b/src/gamma.c
@@ -296,7 +296,7 @@ mpfr_gamma (mpfr_ptr gamma, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
       /* we want an upper bound for x * [log(2-x)-1].
          since x < 0, we need a lower bound on log(2-x) */
       mpfr_ui_sub (xp, 2, x, MPFR_RNDD);
-      mpfr_log2 (xp, xp, MPFR_RNDD);
+      mpfr_log (xp, xp, MPFR_RNDD);
       mpfr_sub_ui (xp, xp, 1, MPFR_RNDD);
       mpfr_mul (xp, xp, x, MPFR_RNDU);
 
diff --git a/tests/tgamma.c b/tests/tgamma.c
index 9100c7c45..62fec4b63 100644
--- a/tests/tgamma.c
+++ b/tests/tgamma.c
@@ -478,6 +478,36 @@ test20100709 (void)
   mpfr_clear (x);
 }
 
+/* bug found by Giridhar Tammana */
+static void
+test20120426 (void)
+{
+  mpfr_t xa, xb;
+  int i;
+  mpfr_exp_t emin;
+
+  mpfr_init2 (xa, 53);
+  mpfr_init2 (xb, 53);
+  mpfr_set_d (xb, -168.5, MPFR_RNDN);
+  emin = mpfr_get_emin ();
+  mpfr_set_emin (-1073);
+  i = mpfr_gamma (xa, xb, MPFR_RNDN);
+  i = mpfr_subnormalize (xa, i, MPFR_RNDN); /* new ternary value */
+  mpfr_set_str (xb, "-9.5737343987585366746184749943e-304", 10, MPFR_RNDN);
+  if (!((i > 0) && (mpfr_cmp (xa, xb) == 0)))
+    {
+      printf ("Error in test20120426, i=%d\n", i);
+      printf ("expected ");
+      mpfr_print_binary (xb); putchar ('\n');
+      printf ("got      ");
+      mpfr_print_binary (xa); putchar ('\n');
+      exit (1);
+    }
+  mpfr_set_emin (emin);
+  mpfr_clear (xa);
+  mpfr_clear (xb);
+}
+
 static void
 exprange (void)
 {
@@ -821,6 +851,7 @@ main (int argc, char *argv[])
   gamma_integer ();
   test20071231 ();
   test20100709 ();
+  test20120426 ();
 
   data_check ("data/gamma", mpfr_gamma, "mpfr_gamma");