A data processing system has an execution pipeline with programmable execution stages which execute instructions to perform data processing operations provided by a host processor and in which execution threads are grouped together into groups in which the threads are executed in lockstep. The system also includes a compiler that compiles programs to generate instructions for the execution stages. The compiler is configured to, for an operation that comprises a memory transaction: issue to the execution stage instructions for executing the operation for the thread group to: perform the operation for the thread group as a whole; and provide the result of the operation to all the active threads of the group. At least one execution stage is configured to, in response to the instructions: perform the operation for the thread group as a whole; and provide the result of the operation to all the active threads of the group.