gccint: Constraints

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1 17.8 Operand Constraints
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1 Each 'match_operand' in an instruction pattern can specify constraints
1 for the operands allowed.  The constraints allow you to fine-tune
1 matching within the set of operands allowed by the predicate.
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1  Constraints can say whether an operand may be in a register, and which
1 kinds of register; whether the operand can be a memory reference, and
1 which kinds of address; whether the operand may be an immediate
1 constant, and which possible values it may have.  Constraints can also
1 require two operands to match.  Side-effects aren't allowed in operands
1 of inline 'asm', unless '<' or '>' constraints are used, because there
1 is no guarantee that the side effects will happen exactly once in an
1 instruction that can update the addressing register.
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Menu

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· 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.
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