1. A computer program product for providing diagnostic information on transaction aborts, the computer program product comprising: a non-transitory 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 storing diagnostic information in the transaction diagnostic block comprising: determining a reason for the abort of the transaction: based on the reason for the abort, determining an address of an instruction corresponding to the transaction that aborted, wherein the address depends on the reason for the abort: and based on determining the address of the instruction corresponding to the transaction that aborted, storing the address of the instruction corresponding to the transaction that aborted in the transaction diagnostic block.