Processing circuitry for performing data processing operations includes issue control circuitry to control issue of the processing operations. Validity marking circuitry marks when input operands are valid and available within an issue queue, and is responsive to a first input operand of the plurality of input operands having a predetermined value to mark a second input operand of the plurality of input operands as meeting its validity condition (i.e. it is possible to determine from the first input operand that the result of the processing operation concerned will be independent of the value of the second input operand and accordingly there is no need to wait for it to actually be available). In order to resolve ordering constraint problems which may be associated with the use of the early valid marking technique separate ordering valid flags may be provided and monitored in respect of at least order-constrained processing operations.