PATCH, boehm-gc: silence warning

[Ben Elliston <bje@au1.ibm.com>]

When compiling mark_rts.c, GCC warns about taking the address of a local
variable.  This is not a bug, but an explicit hack to get the
approximate address of a new stack frame, to calculate the limits of the
current frame.  There is a cleaner way to do this with GCC: use
__builtin_frame_address.  My reading of the code suggests that this will
work just as well.

Tested with a bootstrap on x86_64-linux and a full regression testsuite
run, including make check-target-boehm-gc. Okay for mainline?

2009-11-30  Ben Elliston  <bje@au.ibm.com>

	* mark_rts.c (GC_approx_sp): Use __builtin_frame_address when
	compiling with GCC rather than taking the address of a local
	variable.

Index: mark_rts.c
===================================================================
--- mark_rts.c	(revision 154749)
+++ mark_rts.c	(working copy)
@@ -376,7 +376,13 @@ ptr_t GC_approx_sp()
 #   ifdef _MSC_VER
 #     pragma warning(disable:4172)
 #   endif
-    return((ptr_t)(&dummy));
+#ifdef __GNUC__
+    /* Eliminate a warning from GCC about taking the address of a
+       local variable.  */
+    return __builtin_frame_address (0);
+#else
+    return ((ptr_t)(&dummy));
+#endif /* __GNUC__ */
 #   ifdef _MSC_VER
 #     pragma warning(default:4172)
 #   endif