byte-defop-compiler is a Lisp macro in `bytecomp.el'.

(byte-defop-compiler FUNCTION &optional COMPILE-HANDLER)

Add a compiler-form for FUNCTION.
If function is a symbol, then the variable "byte-SYMBOL" must name
the opcode to be used. If function is a list, the first element
is the function and the second element is the bytecode-symbol.
The second element may be nil, meaning there is no opcode.
COMPILE-HANDLER is the function to use to compile this byte-op, or
may be the abbreviations 0, 1, 2, 3, 0-1, or 1-2.
If it is nil, then the handler is "byte-compile-SYMBOL."