How to write a plugin to implement the function of adding a new assembly instruction in GCC

I want to add an assembly instruction to GCC that detects array boundarie.The form of instruction is "check rd,rs1,rs2". Now, I am writing GCC plugin, I have obtained the information of the array in gimple layer, but I am not clear about how to implement the detection function of check instruction in gimple layer. Could somebody please tell me what I should do next