Re: GSoC project - Improving ELisp Traceback and Debugging Information

[Zach Shaftel <zshaftel@gmail.com>]

> I don't see this branch on Savannah; there's a
> feature/soc-bytecode-in-traceback from 2020-04-27, but 
> apparently
> doesn't contain all this.

Ah my mistake, Rocky Bernstein had pushed that branch, I'm still 
waiting
to hear back from copyright-clerk@fsf.org so I don't know if I can 
push
to Savannah just yet. The repo is available at
https://github.com/SwiftLawnGnome/emacs-gsoc/tree/feature/soc-bytecode-in-traceback-specbinding
if you'd like to take a look.

> Anyway, just wanted to say, that it would nice if bytecode to 
> bytecode
> calls would not leave the exec_byte_code function. Those calls 
> should
> push the necessary frames and continue the interpreter loop. 
> That way
> the bytecoe PC doesn't need to be saved redundantly on the C 
> stack and
> the specbinding stack.

That's an excellent idea. That would make the logic cleaner and 
should
speed up the interpreter to boot. I'll get to work on that right 
away.

> Instead of annotating symbols I would annotate cons cells.  The 
> reader
> could keep a hash table on the side an record the source 
> position of
> cons cells.

That was also mentioned in this thread
https://lists.gnu.org/archive/html/emacs-devel/2020-03/msg00444.html
discussing this project. I'll be looking into this option but as 
Alan
Mackenzie mentioned in that thread it might not be plausible, 
largely
due to the sheer number of cons cells created during compilation 
and
macroexpansion. Keeping that information across all those source
transformations seems nigh impossible without some very convoluted
logic.

I'm also not so keen on the symbols approach because it splits 
symbols
into two different types, annotated and bare, which to me just 
seems
unnecessarily complicated. But this could be changed so that it 
isn't
transparent to the user like it is in that branch.

I'll be looking at how other Lisp compilers record source code
locations. SBCL is what I'm most familiar with but that compiler 
is very
complex, and uses an intermediate code representation during 
compilation
that makes recording this type of information easier. Ideally I 
would
teach the byte compiler to do something as advanced as this as 
well, but
that would probably entail a complete overhaul that wouldn't fit 
into
the span of my project. Perhaps, once the offset is readily 
available, I
could start this undertaking and continue work on it after GSoC 
ends.

-Zach