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Eachmatch_operand in an instruction pattern can specifyconstraints for the operands allowed. The constraints allow you tofine-tune matching within the set of operands allowed by thepredicate.
Constraints can say whetheran operand may be in a register, and which kinds of register; whether theoperand can be a memory reference, and which kinds of address; whether theoperand may be an immediate constant, and which possible values it mayhave. Constraints can also require two operands to match.Side-effects aren’t allowed in operands of inlineasm, unless‘<’ or ‘>’ constraints are used, because there is no guaranteethat the side effects will happen exactly once in an instruction that can updatethe addressing register.