Fix iszero for excess precision.

Floating-point classification macros are supposed to remove any excess
range or precision from their arguments.  This patch fixes the
non-sNaN version of iszero to do so, by casting the argument to its
own type.  (This will of course work only for standard-conforming
excess precision, not for what GCC does on 32-bit x86 by default where
the back end hides excess precision from the front end; the same
applies to most of the classification macros in that case, as showed
up when we made them use GCC built-in functions.)

(iseqsig will have the reverse issue, needing to ensure that when an
underlying function is used it's for a type wide enough not to remove
any excess precision, since comparison macros must not remove excess
precision.)

Tested for x86_64 and x86.

	* math/math.h
	[__GLIBC_USE (IEC_60559_BFP_EXT) && !__SUPPORT_SNAN__] (iszero):
	Cast argument to its own type.
	* math/test-iszero-excess-precision.c: New file.
	* math/Makefile (tests): Add test-iszero-excess-precision.
	(CFLAGS-test-iszero-excess-precision.c): New variable.

1 files changed, 49 insertions, 0 deletions

diff --git a/math/test-iszero-excess-precision.c b/math/test-iszero-excess-precision.c
new file mode 100644
index 0000000000..52abc5aae4
--- /dev/null
+++ b/math/test-iszero-excess-precision.c
@@ -0,0 +1,49 @@
+/* Test iszero with excess precision.
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <float.h>
+#include <math.h>
+#include <stdio.h>
+
+#define TEST(TYPE, TRUE_MIN)						\
+  do									\
+    {									\
+      if (iszero (TRUE_MIN / 2))					\
+	puts ("iszero removes excess precision for " #TYPE);		\
+      else								\
+	{								\
+	  puts ("iszero fails to remove excess precision for " #TYPE);	\
+	  result = 1;							\
+	}								\
+    }									\
+  while (0)
+
+static int
+do_test (void)
+{
+  int result = 0;
+
+  TEST (float, FLT_TRUE_MIN);
+  TEST (double, DBL_TRUE_MIN);
+  TEST (long double, LDBL_TRUE_MIN);
+
+  return result;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"