Linux: GCC static data is 16byte aligned, messes up param

GCC 4.8 and higher implement 16 byte static data alignment on 64-bit.
This means that the 24-byte param_info_s variables are 16 byte aligned
by GCC and that messes up the assumption that the address of the second
parameter is at &param[0]+sizeof(param[0]).
When compiled with clang it is true, with gcc is is not true.

See https://llvm.org/bugs/show_bug.cgi?format=multiple&id=18006

The fix is needed for GCC >=4.8 only. Clang works fine without this.

Added __attribute__((aligned(16))) to first member of param_info_s.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/modules/systemlib/param/param.h

@@ -374,6 +374,7 @@ union param_value_u {
 	void		*p;
 	int32_t		i;
 	float		f;
+	long long	x;
 };
 
 /**
@@ -383,7 +384,23 @@ union param_value_u {
  * instead.
  */
 struct param_info_s {
-	const char	*name;
+	const char	*name
+
+// GCC 4.8 and higher don't implement proper alignment of static data on 
+// 64-bit. This means that the 24-byte param_info_s variables are
+// 16 byte aligned by GCC and that messes up the assumption that
+// sequential items in the __param segment can be addressed as an array.
+// The assumption is that the address of the second parameter is at
+// &param[0]+sizeof(param[0]). When compiled with clang it is
+// true, with gcc is is not true.
+// See https://llvm.org/bugs/show_bug.cgi?format=multiple&id=18006
+// The following hack is for GCC >=4.8 only. Clang works fine without
+// this.
+#ifdef __PX4_LINUX
+				__attribute__((aligned(16)));
+#else
+				;
+#endif
 	param_type_t	type;
 	union param_value_u val;
 };