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.

4 files changed, 62 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 6d6e668..1f9538c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2016-09-28  Joseph Myers  <joseph@codesourcery.com>
+
+	* 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.
+
 2016-09-28  Rasmus Villemoes <rv@rasmusvillemoes.dk>
 
 	* sysdeps/unix/sysv/linux/spawni.c (posix_spawn_args): Remove pipe
diff --git a/math/Makefile b/math/Makefile
index 6c58b97..7ccd59a 100644
--- a/math/Makefile
+++ b/math/Makefile
@@ -156,7 +156,7 @@ tests = test-matherr test-fenv atest-exp atest-sincos atest-exp2 basic-test \
 	test-signgam-ullong-init test-nan-overflow test-nan-payload \
 	test-fexcept test-fexcept-traps test-fesetexcept \
 	test-fesetexcept-traps test-fetestexceptflag test-femode \
-	test-femode-traps $(tests-static)
+	test-femode-traps test-iszero-excess-precision $(tests-static)
 tests-static = test-fpucw-static test-fpucw-ieee-static \
 	       test-signgam-uchar-static test-signgam-uchar-init-static \
 	       test-signgam-uint-static test-signgam-uint-init-static \
@@ -266,6 +266,8 @@ CFLAGS-test-signgam-ullong-init-static.c = -std=c99
 
 CFLAGS-test-math-isinff.cc = -std=gnu++11
 
+CFLAGS-test-iszero-excess-precision.c = -fexcess-precision=standard
+
 # The -lieee module sets the _LIB_VERSION_ switch to IEEE mode
 # for error handling in the -lm functions.
 install-lib += libieee.a
diff --git a/math/math.h b/math/math.h
index 0a09959..1382baa 100644
--- a/math/math.h
+++ b/math/math.h
@@ -336,7 +336,7 @@ enum
 # ifdef __SUPPORT_SNAN__
 #  define iszero(x) (fpclassify (x) == FP_ZERO)
 # else
-#  define iszero(x) ((x) == 0)
+#  define iszero(x) (((__typeof (x)) (x)) == 0)
 # endif
 #endif /* Use IEC_60559_BFP_EXT.  */
 
diff --git a/math/test-iszero-excess-precision.c b/math/test-iszero-excess-precision.c
new file mode 100644
index 0000000..52abc5a
--- /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"