Circuitry and method for controlling a generated association of a physical register with a predicated processing operation based on predicate data state

Instruction decoder circuitry decodes processing instructions each generating an output multi-bit data item in a destination architectural register by applying a processing operation to source data item(s) in respective source architectural register(s). The decoder circuitry detects whether an instruction defines a predicated merge operation that propagates a set of zero or more portions of the prevailing contents of the destination architectural register as respective portions of the output multi-bit data item. The portions are defined by predicate data. Register allocation circuitry associates physical registers with the destination architectural register and the source architectural register(s). When detector circuitry detects that an instruction defines a predicated merge operation, the register allocation circuitry associates a further physical register with that instruction to store a copy of the prevailing contents. In response to a state of generated predicate data, predicate detector circuitry controls association of the further physical register with the instruction.