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.
| • 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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https://gcc.gnu.org/onlinedocs/gcc-7.3.0/gcc/Constraints.html