[src/get_ld.c] Fixed bug for little-endian x86 extended precision.

[tests/tset_ld.c] Added test case.
(merged changesets r10807-10809,10811-10812 from the trunk)

git-svn-id: svn://scm.gforge.inria.fr/svn/mpfr/branches/3.1@10815 280ebfd0-de03-0410-8827-d642c229c3f4

2 files changed, 59 insertions, 2 deletions

diff --git a/src/get_ld.c b/src/get_ld.c
index 1c1efedbc..3abacefb0 100644
--- a/src/get_ld.c
+++ b/src/get_ld.c
@@ -108,7 +108,6 @@ mpfr_get_ld (mpfr_srcptr x, mpfr_rnd_t rnd_mode)
 }
 
 #else
-
 long double
 mpfr_get_ld (mpfr_srcptr x, mpfr_rnd_t rnd_mode)
 {
@@ -122,7 +121,7 @@ mpfr_get_ld (mpfr_srcptr x, mpfr_rnd_t rnd_mode)
   mpfr_init2 (tmp, MPFR_LDBL_MANT_DIG);
   inex = mpfr_set (tmp, x, rnd_mode);
 
-  mpfr_set_emin (-16382-63);
+  mpfr_set_emin (-16381-63); /* emin=-16444, see below */
   mpfr_set_emax (16384);
   mpfr_subnormalize (tmp, mpfr_check_range (tmp, inex, rnd_mode), rnd_mode);
   mpfr_prec_round (tmp, 64, MPFR_RNDZ); /* exact */
diff --git a/tests/tset_ld.c b/tests/tset_ld.c
index 6e2b931f5..46791052d 100644
--- a/tests/tset_ld.c
+++ b/tests/tset_ld.c
@@ -193,6 +193,62 @@ test_fixed_bugs (void)
   mpfr_clear (x);
 }
 
+/* bug reported by Walter Mascarenhas
+   https://sympa.inria.fr/sympa/arc/mpfr/2016-09/msg00005.html */
+static void
+bug_20160907 (void)
+{
+#if HAVE_LDOUBLE_IEEE_EXT_LITTLE
+  long double dn = 1.0, ld;
+  mpfr_t mp;
+  mpfr_exp_t e = 0;
+
+  while (dn * (long double) 0.5 != 0)
+    {
+      dn = dn * (long double) 0.5;
+      e --;
+    }
+  /* dn=2^e is now the smallest subnormal */
+
+  mpfr_init2 (mp, 64);
+  mpfr_set_ui_2exp (mp, 1, e - 1, MPFR_RNDN);
+  ld = mpfr_get_ld (mp, MPFR_RNDU);
+  /* since mp = 2^(e-1) and ld is rounded upwards, we should have
+     ld = 2^e */
+  if (ld != dn)
+    {
+      printf ("Error, ld = %Le <> dn = %Le\n", ld, dn);
+      printf ("mp=");
+      mpfr_out_str (stdout, 10, 0, mp, MPFR_RNDN);
+      printf ("\n");
+      exit (1);
+    }
+
+  /* check a few more numbers */
+  for (e = -16446; e <= -16381; e++)
+    {
+      mpfr_set_ui_2exp (mp, 1, e, MPFR_RNDN);
+      ld = mpfr_get_ld (mp, MPFR_RNDU);
+      mpfr_set_ld (mp, ld, MPFR_RNDU);
+      /* mp is 2^e rounded up, thus should be >= 2^e */
+      MPFR_ASSERTN(mpfr_cmp_ui_2exp (mp, 1, e) >= 0);
+
+      mpfr_set_ui_2exp (mp, 1, e, MPFR_RNDN);
+      ld = mpfr_get_ld (mp, MPFR_RNDD);
+      mpfr_set_ld (mp, ld, MPFR_RNDD);
+      /* mp is 2^e rounded down, thus should be <= 2^e */
+      if (mpfr_cmp_ui_2exp (mp, 3, e) > 0)
+        {
+          printf ("Error, expected value <= 2^%ld\n", e);
+          printf ("got "); mpfr_dump (mp);
+          exit (1);
+        }
+    }
+
+  mpfr_clear (mp);
+#endif
+}
+
 int
 main (int argc, char *argv[])
 {
@@ -309,6 +365,8 @@ main (int argc, char *argv[])
 
   test_small ();
 
+  bug_20160907 ();
+
   tests_end_mpfr ();
 
   return 0;