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(gcc-300.info)Constraints

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Operand Constraints
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   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.