RTEMS GNU Tools On-Line Library

Constraints

Using and Porting the GNU Compiler Collection (GCC)

16.6: 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.

Using and Porting the GNU Compiler Collection (GCC)