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

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

[Alan Mackenzie]

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).

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

[Andreas Schwab]

The arguments are not consumed.  Each byte-code execution has a private
value stack, which is just discarded at the end, no matter what's left.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

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

[Alan Mackenzie]

Hello, Andreas.

On Sat, Jan 01, 2022 at 20:43:52 +0100, Andreas Schwab wrote:
> The arguments are not consumed.  Each byte-code execution has a private
> value stack, which is just discarded at the end, no matter what's left.

Thanks, I understand it now.

> -- 
> Andreas Schwab, schwab@linux-m68k.org
> GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
> "And now for something completely different."

-- 
Alan Mackenzie (Nuremberg, Germany).