Help with the disassembly of a very short function, please.

[Alan Mackenzie <acm@muc.de>]

Hello, Emacs.

The following function comes from .../test/src/comp-tests.el, more or
less:

;; -*- lexical-binding: t -*-
(defun comp-test-55 (x)
  (unless (integerp x)
      x))

I byte compile it with C-x C-e then dump out its byte code with M-x
disassemble.  I get this:

byte code for comp-test-55:
  doc:   ...
    args: (arg1)
    0       dup
    1       integerp
    2       not
    3       goto-if-nil-else-pop 1
    6       dup
    7:1     return

..  In the case where x is not an integer, it seems that the dup at 6
will leave two many copies of x on the stack for the return.

In the case where x is an integer, it seems that the
goto-if-nil-else-pop will leave a nil on the stack over the original
x, returning the nil but leaving a superfluous copy of the integer x
on the stack.

So, either way, there is an unwanted copy of x left on the stack at the
end of the function.

I can't work this out.  What am I missing?

-- 
Alan Mackenzie (Nuremberg, Germany).