The constant AO_UNUSED_MBZ in jdk.pack's constants.h is initialized using the expression (-1 << 13). In C89/C++98 the semantics of left shifts of negative values are not described. C99/C++11 clarified that this is undefined behavior.

gcc -Wshift-negative-value warns about this. That warning is implicitly enabled by -Wextra when requesting builds using recent versions of the language standards.

Note that the jdk.pack module is deprecated.