(gcc.info.gz) Constraints

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 5.38 Constraints for `asm' Operands
 ===================================
 
 Here are specific details on what constraint letters you can use with
 `asm' operands.  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.
 

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* Simple Constraints  Basic use of constraints.
* Multi-Alternative   When an insn has two alternative constraint-patterns.
* Modifiers           More precise control over effects of constraints.
* Machine Constraints Special constraints for some particular machines.
 

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