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Operand Constraints

Each 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.

• Simple Constraints: Basic use of constraints.
• Multi-Alternative: When an insn has two alternative constraint-patterns.
• Class Preferences: Constraints guide which hard register to put things in.
• Modifiers: More precise control over effects of constraints.
• Machine Constraints: Existing constraints for some particular machines.