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match_operand in an instruction pattern can specify a
constraint for the type of operands allowed.
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.
20.7.1 Simple Constraints Basic use of constraints. 20.7.2 Multiple Alternative Constraints When an insn has two alternative constraint-patterns. 20.7.3 Register Class Preferences Constraints guide which hard register to put things in. 20.7.4 Constraint Modifier Characters More precise control over effects of constraints. 20.7.5 Constraints for Particular Machines Existing constraints for some particular machines.