private int pop() {
        if (outputStackTop > 0) {
            return outputStack[--outputStackTop];
        } else {
            // If the output frame stack is empty, pop from the input stack.
            return STACK_KIND | -(--outputStackStart);
        }
    }
Pops an abstract type from the output frame stack and returns its value.

@return the abstract type that has been popped from the output frame stack.
core/src/main/java/com/alibaba/fastjson2/internal/asm/Frame.java