1. A computer program product for providing diagnostic information on transaction aborts, the computer program product comprising: a computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: detecting, by a processor, an abort of a transaction, the transaction comprising one or more instructions, and the transaction effectively delaying committing transactional stores to main memory until completion of a selected transaction; determining, by the processor, whether diagnostic information is to be stored into a transaction diagnostic block (TDB) based on the abort; and based on the determining indicating diagnostic information is to be stored, storing diagnostic information in the transaction diagnostic block, the diagnostic information including an address of an instruction corresponding to the transaction that aborted, the address of the instruction depending on a reason for the abort of the transaction.