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GNU Compiler Collection (GCC) Internals

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9.7 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.

9.7.1 Simple Constraints  Basic use of constraints.
9.7.2 Multiple Alternative Constraints  When an insn has two alternative constraint-patterns.
9.7.3 Register Class Preferences  Constraints guide which hard register to put things in.
9.7.4 Constraint Modifier Characters  More precise control over effects of constraints.
9.7.5 Constraints for Particular Machines  Existing constraints for some particular machines.

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