12.8 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.

• 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.