Ignore -Wmaybe-uninitialized in stdlib/bug-getcontext.c

[Joseph Myers <joseph@codesourcery.com>]

Doing all-ABIs compile testing produces a compiler warning in
stdlib/bug-getcontext.c on nios2 and tilepro (with GCC 5 branch):

bug-getcontext.c: In function 'do_test':
bug-getcontext.c:53:6: error: 'except_mask' may be used uninitialized in this function [-Werror=maybe-uninitialized]
   if (mask != except_mask)
      ^

This warning appears nonsensical; except_mask is initialized where
it's declared.  I think what must be happening here is that the
compiler is confused by the returns-twice nature of getcontext: if
there were a call to setcontext, local variables could indeed have
lost their values on the second return from getcontext.  This patch
duly uses the DIAG_* macros to disable the warning here.

Tested for nios2 and tilepro (compilation only; after this patch all
the tests compile, though there are other failures) and x86_64 (full
testsuite run).

2016-11-04  Joseph Myers  <joseph@codesourcery.com>

	* stdlib/bug-getcontext.c: Include <libc-internal.h>.
	(do_test): Disable -Wmaybe-uninitialized around uses of
	except_mask.

diff --git a/stdlib/bug-getcontext.c b/stdlib/bug-getcontext.c
index 133ee91..c407212 100644
--- a/stdlib/bug-getcontext.c
+++ b/stdlib/bug-getcontext.c
@@ -5,6 +5,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <ucontext.h>
+#include <libc-internal.h>
 
 static int
 do_test (void)
@@ -49,6 +50,13 @@ do_test (void)
   printf ("\nDone with getcontext()!\n");
   fflush (NULL);
 
+  /* On nios2 and tilepro, GCC 5 warns that except_mask may be used
+     uninitialized.  Because it is always initialized and nothing in
+     this test ever calls setcontext (a setcontext call could result
+     in local variables being clobbered on the second return from
+     getcontext), in fact an uninitialized use is not possible.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (5, "-Wmaybe-uninitialized");
   int mask = fegetexcept ();
   if (mask != except_mask)
     {
@@ -59,6 +67,7 @@ do_test (void)
 
   printf("\nAt end fegetexcept() returned %d, expected: %d.\n",
 	 mask, except_mask);
+  DIAG_POP_NEEDS_COMMENT;
 
   return 0;
 }

-- 
Joseph S. Myers
joseph@codesourcery.com