Shenandoah and G1 are using CardTable for most of its infrastructure, but flip the card tables as they go, and maintain the actual card table reference in TLS. As such, accessing card table base from assembler and compilers runs into risk of accidentally encoding the wrong card table base in generated code.

Shenandoah added a few asserts for its own use:
 SHENANDOAHGC_ONLY(assert(!UseShenandoahGC, "Shenandoah byte_map_base is not constant.");)

...but G1 would also benefit from the similar safety mechanism.