Constraints

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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.
No Constraints: Describing a clean machine without constraints.

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