| GNU Compiler Collection (GCC) Internals: Constraints |
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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.
Side-effects aren’t allowed in operands of inline asm, unless
‘<’ or ‘>’ constraints are used, because there is no guarantee
that the side-effects will happen exactly once in an instruction that can update
the addressing register.
| • 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. | |
| • Disable Insn Alternatives: | Disable insn alternatives using attributes. | |
| • Define Constraints: | How to define machine-specific constraints. | |
| • C Constraint Interface: | How to test constraints from C code. |