Compiler.h: add gcc_returns_nonnull, gcc_returns_twice

src/db/plugins/simple/Directory.hxx

@@ -106,7 +106,7 @@ public:
 	/**
 	 * Create a new root #Directory object.
 	 */
-	gcc_malloc
+	gcc_malloc gcc_returns_nonnull
 	static Directory *NewRoot() {
 		return new Directory(std::string(), nullptr);
 	}

src/db/plugins/simple/Song.hxx

@@ -95,11 +95,11 @@ struct Song {
 	Song(const char *_uri, size_t uri_length, Directory &parent);
 	~Song();
 
-	gcc_malloc
+	gcc_malloc gcc_returns_nonnull
 	static Song *NewFrom(DetachedSong &&other, Directory &parent);
 
 	/** allocate a new song with a local file name */
-	gcc_malloc
+	gcc_malloc gcc_returns_nonnull
 	static Song *NewFile(const char *path_utf8, Directory &parent);
 
 	/**