Re: Emacs-devel Digest, Vol 166, Issue 137

[Stefan Monnier]

> If you mean always keep the source code around in the bytecode file, I'm
> all for that!

No, I'm thinking rather of not keeping the bytecode file around at all,
or to store it in an "internal cache" (which could be kept in the
file-system but could be erased at any point and without any command to
"load a bytecode file").

>> > Maybe this could be incorporated into a "safe-load-file" function.
>> Define "safe"
> Okay. Let me call it then "safer" then. And I will define that: detecting
> problems that can be reasonably detected in advance of hitting them instead
> of giving a ¯\_(ツ)_/¯ traceback.

As long as we want to support the case of using a .elc file without the
corresponding .el file, some users would be very annoyed if Emacs-27
prevents them from using a file compiled with Emacs-23 just because
there's a risk that you might hit an error later.

> Recently have come to learn it can be worse because checks are not done on
> bytecode...
> Want to crash emacs immediately without a traceback?  Run
> emacs -batch -Q --eval '(print (#[0 "\300\207" [] 0]))'

Yes, BYTE_CODE_SAFE is not safe enough, agreed.  Patches welcome.

> The bytecode file for cl-lib no doubt had in it "Hey, I'm emacs 24."
> and I probably ran that on Emacs 25 where there was an incompatibility
> that can happen between major releases.

I doubt that's the problem [ cl-lib has rather unusual problems in
this respect, and I suspect that you hit one of those, which don't have
anything to do with the bytecode but with the fact that the cl-lib on
GNU ELPA only works on Emacsen that don't come with cl-lib.
It includes hacks to try and avoid the problem, but maybe you've seen
people using odd configurations or older versions of GNU ELPA's cl-lib
where those hacks weren't sufficiently refined.  ]

> If that were the case (and although probably it is not the *only*
> scenario case)  how much nicer would it have been if a safer-load-file
> warned me about running version 24 bytecode.

Do you have actual byte-code generated by Emacs-24's bytecomp.el which
causes a serious error when used on more recent Emacsen?

For cl-lib, the problem is not that the cl-lib compiled with Emacs-24.1
contains invalid bytecode for Emacs-27, but that for some reason that
cl-lib ended up earlier in the load-path than the cl-lib that comes with
Emacs-27 and it's the bundled cl-lib which should be loaded.

> There are free opcode space available. "apply" could be added is someone
> chooses to add it.

I'm just pointing out differences that illustrate the fact that
tradeoffs were quite different.

> There may be some glibness here. The benefits of using a lower-level
> general-purpose intermediate language like LLVM IR or vmgen is that because
> it a lower level, working with registers and pointers, understands some
> structure layouts, and is more statically typed. So efficiency can be
> gained by specialization.  But if one doesn't break down Lisp_Object and
> uses that in the same way the C interpreter currently does, then I don't
> see why vmgen will be any faster than the current interpreter. (Other than
> the benefit that would also be had by rewriting the interpreter without the
> bloat and compatibility overhead)

The idea is not to rewrite the same thing with vmgen, indeed.  It's to
design a new bytecode (which would hopefully streamline some use cases,
e.g. allow processing function calls more efficiently, maybe also
`apply` more efficiently, which would allow allocating closures with
a single malloc of an object of size proportional to the number of free
variables, ...), and while we're at it, use something like vmgen, so
that we benefit from other people's efforts at improving the efficiency
of the dispatch.

> Thanks for the kind words. It's not something I feel all that knowledgeable
> or qualified to do.

That makes you a good person to write it.
Feel free to ask questions about parts you don't understand or aren't sure.


        Stefan