[src/digamma.c] Fixed bug in mpfr_digamma_positive when t-u is zero.

[tests/tdigamma.c] Testcase.
(merged changesets r14399,14401-14404 from the trunk)

git-svn-id: https://scm.gforge.inria.fr/anonscm/svn/mpfr/branches/4.1@14425 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 41 insertions, 15 deletions

diff --git a/src/digamma.c b/src/digamma.c
index 6dba748e6..b971930fa 100644
--- a/src/digamma.c
+++ b/src/digamma.c
@@ -296,21 +296,26 @@ mpfr_digamma_positive (mpfr_ptr y, mpfr_srcptr x, mpfr_rnd_t rnd_mode)
       errt = mpfr_digamma_approx (t, x_plus_j);
       expt = MPFR_GET_EXP (t);
       mpfr_sub (t, t, u, MPFR_RNDN);
-      if (MPFR_GET_EXP (t) < expt)
-        errt += expt - MPFR_EXP(t);
-      /* Warning: if u is zero (which happens when x_plus_j >= min at the
-         beginning of the while loop above), EXP(u) is not defined.
-         In this case we have no error from u. */
-      if (MPFR_NOTZERO(u) && MPFR_GET_EXP (t) < MPFR_GET_EXP (u))
-        erru += MPFR_EXP(u) - MPFR_EXP(t);
-      if (errt > erru)
-        errt = errt + 1;
-      else if (errt == erru)
-        errt = errt + 2;
-      else
-        errt = erru + 1;
-      if (MPFR_CAN_ROUND (t, p - errt, MPFR_PREC(y), rnd_mode))
-        break;
+      /* Warning! t may be zero (more likely in small precision). Note
+         that in this case, this is an exact zero, not an underflow. */
+      if (MPFR_NOTZERO(t))
+        {
+          if (MPFR_GET_EXP (t) < expt)
+            errt += expt - MPFR_EXP(t);
+          /* Warning: if u is zero (which happens when x_plus_j >= min at the
+             beginning of the while loop above), EXP(u) is not defined.
+             In this case we have no error from u. */
+          if (MPFR_NOTZERO(u) && MPFR_GET_EXP (t) < MPFR_GET_EXP (u))
+            erru += MPFR_EXP(u) - MPFR_EXP(t);
+          if (errt > erru)
+            errt = errt + 1;
+          else if (errt == erru)
+            errt = errt + 2;
+          else
+            errt = erru + 1;
+          if (MPFR_CAN_ROUND (t, p - errt, MPFR_PREC(y), rnd_mode))
+            break;
+        }
       MPFR_ZIV_NEXT (loop, p);
       mpfr_set_prec (t, p);
       mpfr_set_prec (u, p);
diff --git a/tests/tdigamma.c b/tests/tdigamma.c
index 6841220b0..612e5e098 100644
--- a/tests/tdigamma.c
+++ b/tests/tdigamma.c
@@ -90,6 +90,26 @@ bug20210206 (void)
   set_emax (emax);
 }
 
+/* another test that fails with GMP_CHECK_RANDOMIZE=1612741376857003
+   on revision 14398 */
+static void
+bug20210208 (void)
+{
+  mpfr_t x, y;
+  int inex;
+
+  mpfr_init2 (x, 73);
+  mpfr_init2 (y, 1);
+  mpfr_set_str (x, "1.4613470547060071827450", 10, MPFR_RNDN);
+  mpfr_clear_flags ();
+  inex = mpfr_digamma (y, x, MPFR_RNDU);
+  MPFR_ASSERTN (mpfr_cmp_si_2exp (y, -1, -12) == 0);
+  MPFR_ASSERTN (inex > 0);
+  MPFR_ASSERTN (__gmpfr_flags == MPFR_FLAGS_INEXACT);
+  mpfr_clear (x);
+  mpfr_clear (y);
+}
+
 int
 main (int argc, char *argv[])
 {
@@ -97,6 +117,7 @@ main (int argc, char *argv[])
 
   special ();
   bug20210206 ();
+  bug20210208 ();
 
   test_generic (MPFR_PREC_MIN, 200, 20);