Each match_operand in an instruction pattern can specify
constraints for the operands allowed. The constraints allow you to
fine-tune matching within the set of operands allowed by the
predicate.
Constraints can say whether an operand may be in a register, and which kinds of register; whether the operand can be a memory reference, and which kinds of address; whether the operand may be an immediate constant, and which possible values it may have. Constraints can also require two operands to match.