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.

4 files changed, 73 insertions, 0 deletions

diff --git a/ChangeLog b/ChangeLog
index 7cd1a45ea3..cfbaf86faa 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,15 @@
+2014-12-10  Joseph Myers  <joseph@codesourcery.com>
+
+	* 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.
+
 2014-12-09  Torvald Riegel  <triegel@redhat.com>
 
 	* sysdeps/unix/sysv/linux/sh/pthread_once.S: Remove file.
diff --git a/include/libc-internal.h b/include/libc-internal.h
index 2ced1c17d3..bca59a4614 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  */
diff --git a/stdio-common/bug21.c b/stdio-common/bug21.c
index d22b9c1a97..ca27272ba1 100644
--- a/stdio-common/bug21.c
+++ b/stdio-common/bug21.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <libc-internal.h>
 
 static int
 do_test (void)
@@ -6,7 +7,15 @@ do_test (void)
   static const char buf[] = " ";
   char *str;
 
+  /* GCC in C99 mode treats %a as the C99 format expecting float *,
+     but glibc with _GNU_SOURCE treats %as as the GNU allocation
+     extension, so resulting in "warning: format '%a' expects argument
+     of type 'float *', but argument 3 has type 'char **'".  This
+     applies to the other %as, %aS and %a[] formats below as well.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   int r = sscanf (buf, "%as", &str);
+  DIAG_POP_NEEDS_COMMENT;
   printf ("%d %p\n", r, str);
 
   return r != -1 || str != NULL;
diff --git a/stdio-common/scanf14.c b/stdio-common/scanf14.c
index 6ca5c7c567..cffccb0b19 100644
--- a/stdio-common/scanf14.c
+++ b/stdio-common/scanf14.c
@@ -2,6 +2,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <wchar.h>
+#include <libc-internal.h>
 
 #define FAIL() \
   do {							\
@@ -23,6 +24,13 @@ main (void)
     FAIL ();
   else if (f != 0.25 || memcmp (c, "s x", 3) != 0)
     FAIL ();
+  /* GCC in C99 mode treats %a as the C99 format expecting float *,
+     but glibc with _GNU_SOURCE treats %as as the GNU allocation
+     extension, so resulting in "warning: format '%a' expects argument
+     of type 'float *', but argument 3 has type 'char **'".  This
+     applies to the other %as, %aS and %a[] formats below as well.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   if (sscanf (" 1.25s x", "%as%2c", &sp, c) != 2)
     FAIL ();
   else
@@ -32,10 +40,14 @@ main (void)
       memset (sp, 'x', sizeof "1.25s");
       free (sp);
     }
+  DIAG_POP_NEEDS_COMMENT;
   if (sscanf (" 2.25s x", "%las%2c", &d, c) != 2)
     FAIL ();
   else if (d != 2.25 || memcmp (c, " x", 2) != 0)
     FAIL ();
+  /* See explanation above.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   if (sscanf (" 3.25S x", "%4aS%3c", &lsp, c) != 2)
     FAIL ();
   else
@@ -54,6 +66,7 @@ main (void)
       memset (sp, 'x', sizeof "4.25");
       free (sp);
     }
+  DIAG_POP_NEEDS_COMMENT;
   if (sscanf ("5.25[0-9.] x", "%la[0-9.]%2c", &d, c) != 2)
     FAIL ();
   else if (d != 5.25 || memcmp (c, " x", 2) != 0)
@@ -82,6 +95,9 @@ main (void)
 	FAIL ();
       if (fseek (fp, 0, SEEK_SET) != 0)
 	FAIL ();
+      /* See explanation above.  */
+      DIAG_PUSH_NEEDS_COMMENT;
+      DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
       if (fscanf (fp, "%as%2c", &sp, c) != 2)
 	FAIL ();
       else
@@ -91,11 +107,15 @@ main (void)
 	  memset (sp, 'x', sizeof "1.25s");
 	  free (sp);
 	}
+      DIAG_POP_NEEDS_COMMENT;
 
       if (freopen (fname, "r", stdin) == NULL)
 	FAIL ();
       else
 	{
+	  /* See explanation above.  */
+	  DIAG_PUSH_NEEDS_COMMENT;
+	  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
 	  if (scanf ("%as%2c", &sp, c) != 2)
 	    FAIL ();
 	  else
@@ -105,6 +125,7 @@ main (void)
 	      memset (sp, 'x', sizeof "1.25s");
 	      free (sp);
 	    }
+	  DIAG_POP_NEEDS_COMMENT;
 	}
 
       fclose (fp);