Add macros for diagnostic control, use for scanf %a tests.

In <https://sourceware.org/ml/libc-alpha/2014-11/msg00326.html>,
Roland requested internal macros for use of "#pragma GCC diagnostic".

This patch adds such macros and uses them to disable -Wformat warnings
for some code testing GNU scanf %as where GCC expects C99 scanf %a
(several other stdio tests currently use -Wno-format to disable
warnings).  Limitations in GCC's diagnostic pragmas require separate
macros before and after the code generating the warnings, rather than
a single macro taking that code as an argument.

The macros are named DIAG_*_NEEDS_COMMENT to emphasise to reviewers
the need for a comment accompanying any use of them (such comments may
however just appear once for several uses of the macros for the same
issue in the same file).  I put a GCC version in the arguments to
DIAG_IGNORE_NEEDS_COMMENT, as that seems something useful to grep for
when obsoleting support for an old GCC version and needing to decide
if warning-disabling code is still relevant.

These macros should be usable for replacing existing -Wno-* use in
makefiles (as also suggested by Roland), though I have no plans to
work on that (only on use of the macros in cases where warnings are
currently present that need disabling to use -Werror).

Tested for x86_64.

	* include/libc-internal.h (DIAG_PUSH_NEEDS_COMMENT): New macro.
	(DIAG_POP_NEEDS_COMMENT): Likewise.
	(_DIAG_STR1): Likewise.
	(_DIAG_STR): Likewise.
	(DIAG_IGNORE_NEEDS_COMMENT): Likewise.
	* stdio-common/bug21.c: Include <libc-internal.h>.
	(do_test): Disable -Wformat around call to sscanf.
	* stdio-common/scanf14.c: Include <libc-internal.h>.
	(main): Disable -Wformat around some calls to scanf functions.

1 files changed, 31 insertions, 0 deletions

diff --git a/include/libc-internal.h b/include/libc-internal.h
index 2ced1c1..bca59a4 100644
--- a/include/libc-internal.h
+++ b/include/libc-internal.h
@@ -76,4 +76,35 @@ extern void __init_misc (int, char **, char **);
 #define ignore_value(x) \
   ({ __typeof__ (x) __ignored_value = (x); (void) __ignored_value; })
 
+/* The macros to control diagnostics are structured like this, rather
+   than a single macro that both pushes and pops diagnostic state and
+   takes the affected code as an argument, because the GCC pragmas
+   work by disabling the diagnostic for a range of source locations
+   and do not work when all the pragmas and the affected code are in a
+   single macro expansion.  */
+
+/* Push diagnostic state.  */
+#define DIAG_PUSH_NEEDS_COMMENT _Pragma ("GCC diagnostic push")
+
+/* Pop diagnostic state.  */
+#define DIAG_POP_NEEDS_COMMENT _Pragma ("GCC diagnostic pop")
+
+#define _DIAG_STR1(s) #s
+#define _DIAG_STR(s) _DIAG_STR1(s)
+
+/* Ignore the diagnostic OPTION.  VERSION is the most recent GCC
+   version for which the diagnostic has been confirmed to appear in
+   the absence of the pragma (in the form MAJOR.MINOR for GCC 4.x,
+   just MAJOR for GCC 5 and later).  Uses of this pragma should be
+   reviewed when the GCC version given is no longer supported for
+   building glibc; the version number should always be on the same
+   source line as the macro name, so such uses can be found with grep.
+   Uses should come with a comment giving more details of the
+   diagnostic, and an architecture on which it is seen if possibly
+   optimization-related and not in architecture-specific code.  This
+   macro should only be used if the diagnostic seems hard to fix (for
+   example, optimization-related false positives).  */
+#define DIAG_IGNORE_NEEDS_COMMENT(version, option)	\
+  _Pragma (_DIAG_STR (GCC diagnostic ignored option))
+
 #endif /* _LIBC_INTERNAL  */