Re: Compiled code in Emacs-26 will fail in Emacs-25 if use pcase [was: Add new bytecode op `switch' for implementing branch tables.]

[Vibhav Pant <vibhavp@gmail.com>]

On Thu, Feb 23, 2017 at 2:06 PM Richard Copley <rcopley@gmail.com> wrote:
>
> On 23 February 2017 at 04:35, Tino Calancha <tino.calancha@gmail.com> wrote:
> >
> > Sorry if this is obvious, then just ignore.
> >
> > I have a custom lib using `pcase' somewhere.  That lib is
> > compatible with Emacs25.
> > Usually i compile this lib against the master branch.  Then, i load
> > that very same resulting .elc in Emacs-25 and runs same as in Emacs-26.
> > (Maybe this is just wrong and i must keep separated .elc)
> >
> > After the new bytecode those parts of the lib using pcase fail.
> > For instance, put the following in a file:
> > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> > (defun test-case ()
> >   (let ((val "foo"))
> >     (pcase val
> >       ("foo" "foo")
> >       ("bar" "bar")
> >       (_ (error "Neither 'foo' nor 'bar'")))))
> > ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> > I) Compile under master branch.
> > II) Load the resultant .elc with Emacs-25
> > II) M-: (test-case)
> > ;; Signal error: Invalid byte opcode: op=183, ptr=4
> >
> > So, in conclusion:
> > From now on, we need to keep two separated .elc in such custom libs, right?
>
> Yes, ideally.
>
> You need to arrange not to run byte-compiled libs in an earlier
> version of Emacs than where they were compiled. This isn't a new
> requirement. There has never been a guarantee that newer versions'
> bytecode will run on older versions, but in reality incompatible
> changes don't happen every day so at times you could get away with
> ignoring the issue.
>
> There are various options that avoid the problem.
> * keep your `load-path' entries version specific
> * don't byte compile libs that are shared between versions
> * byte compile them with the earliest Emacs you'll use
> * don't use different versions of Emacs
>
Alternatively, in this case, you could compile your code with
`byte-compile-cond-use-jump-table' to nil.

> Eli recommended the first option (the same one you suggested) to me,
> see #14513. In `emacs -Q' by default all `load-path' entries are in a
> version-specific subdirectory.



-- 
Vibhav Pant
vibhavp@gmail.com
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