Apparatus and method of executing thread groups

A method of executing thread groups on an apparatus having scheduling circuitry to select for execution a first thread group from a plurality of thread groups, and thread processing circuitry, responsive to the scheduling circuitry, to execute active threads of the first thread group in dependence on a common program counter shared between the active threads, the method comprising:

• determining, responsive to an exit event from the thread processing circuitry occurring for the first thread group, whether a program counter check condition is present;
• providing the thread processing circuitry with register storage to store program counter information for each thread of the first thread group;
• employing program counter checking circuitry, in the presence of said program counter check condition, to perform a program counter check operation to update the common program counter and an active thread indication for the first thread group, the program counter checking circuitry having access to said register storage during performance of the program counter check operation;
• and
• causing the scheduling circuitry to select, for execution by the thread processing circuitry, a second thread group from said plurality of thread groups other than said first thread group, whilst awaiting performance of the program counter check operation by the program counter checking circuitry for the first thread group,
• wherein said register storage comprises at least one register for each thread group in said plurality of thread groups, to allow program counter information for each thread to be maintained for each of the plurality of thread groups.