Questions for extending llvm back-end instruction

Dear llvm developers :

I am doing a project about GPU,

and need to add a new instruction to the generated PTX file by llvm back-end,

this instruction should be able to specifically manipulate(load) the constant variables.

My question is,

I am not sure if there are any existing llvm elements that could archive my requirements,

since according to your documentation,

adding a new instruction to llvm is very time consuming.

Thanks a lot for your help