On Thu, Mar 19, 2020 at 4:35 PM Alan Mackenzie <acm@muc.de> wrote:

> Hello, Stefan.
>
> On Thu, Mar 19, 2020 at 13:35:08 -0400, Stefan Monnier wrote:
>
> [ .... ]
>
> > It should be easy (much smaller than a summer project) to change the C
> > code so that a bytecode offset can be extracted from the backtrace.
>
> > The harder and more interesting part is how to propagate source
> > information (line numbers and/or lexical variable names and location)
> > to byte-code.  There are many parts to this, so it's definitely
> > possible to get some summer project(s) out of it.  E.g. one such
> > project is to change the reader so it outputs "fat cons cells" (i.e.
> > cons-cells with line-num info), then arrange for that info to survive
> > `macroexpand-all` and `cconv.el`.  That could already be used to give
> > more precise line numbers in bytecompiler warnings.
>
> "More precise line numbers" is a misconstruction, even though I've used
> such language myself in the past.  Line numbers don't come from a
> physical instrument which measures with, say +-1% accuracy.  CORRECT
> line (and column) numbers are what we need.
>

A bytecode offset is exact and accurate.  Right now this information
unavailable. I think the interpreter uses C pointers stored in a register.
So just recording the bytecode offset is a little bit of a slowdown, but
not that much. I doubt it would even register as %1 slower.

But just that would open the way for improvements. This is doable by a
Summer student - Stefan thinks it trivial. But tas you point out there is
overhead in getting it accepted and into GNU Emacs.

Having access to the bytecode offset in a traceback there next are several
options. At the lowest level there is just showing that along with a
disassembly of the bytecode.
And that I believe that is also doable by a summer student.

Going further are a number of options that folks have mentioned so I won't
expand on that.


> You will recall that the output of correct line/column numbers for byte
> compiler messages is a solved problem.  I solved it and presented the
> fix in December 2018.  This fix was rejected because it made Emacs
> slightly slower.
>
> In the 3½ years I've been grappling with this problem, I've tried all
> sorts of things like "fat cons cells".  They don't work, and can't work.
> They can't work because large chunks of our software chew up and spit
> out cons cells with gay abandon (I'm talking about the byte compiler and
> things like cconv.el here).  More to the point, users' macros chew up and
> spit out cons cells, and we have no control over them.  So whilst we
> could, with a lot of tedious effort, clean up our own software to
> preserve cons cells (believe me, I've tried), this would fail in users'
> macros.
>
> Since then I've worked a fair bit on creating a "double" Emacs core, one
> core being for normal use, the other for byte compiling.  There's a fair
> amount of work still to do on this, but I know how to do it.  The problem
> is that I have been discouraged by the prospect of having this solution
> vetoed too, since it will make Emacs quite a bit bigger.
>
> I don't think it is fair to give this problem to a group of summer
> coders.  It is too hard a problem, both technically and politically.
>


Ok. So do you have a suggestion for what a summer student might do?


>
> [ .... ]
>
> >         Stefan
>
> --
> Alan Mackenzie (Nuremberg, Germany).
>