John Coomes recently pushed 6888954 which extends HotSpot's assert / guarantee macros to allow format strings so that we can print out the contents of variables, fields, etc. when an assert / guarantee fails.

We should put some effort into G1 to extend all our asserts / guarantees to print out all the values involved in the asserts / guarantees, when they fail, to facilitate our debugging and reduce the need to have to essentially add that code manually every time we chase such a failure.