17.8 Operand Constraints

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. Side-effects aren’t allowed in operands of inline asm, unless ‘<’ or ‘>’ constraints are used, because there is no guarantee that the side effects will happen exactly once in an instruction that can update the addressing register.