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(How) can I get position information backtraces?

From: Rocky Bernstein
Subject: (How) can I get position information backtraces?
Date: Mon, 18 Sep 2017 19:05:03 -0400

On Mon, Sep 18, 2017 at 5:53 PM, Helmut Eller <address@hidden> wrote:
The following message is a courtesy copy of an article
that has been posted to gmane.emacs.devel as well.

On Mon, Sep 18 2017, Rocky Bernstein wrote:

> I've been looking at Emacs Lisp code and the C source. I don't see
> how/where execution position is stored.

I suppose you want to know how Emacs stores the source position
(filename and line number) of Lisp code

Not really file name and line number. I meant it in the broadest possible way. 
Position could be a pointer to a structure, an offset in some bytecode, or something else.

and in particular how the
debugger can take you from a stack frame in the backtrace to the source

 I see that edebug munges function definitions for its own tracking. The calls
to enter and leave pass the position, I think a byte offset. 

That's okay too, but even there I don't see something that stacks prior
positions. It doesn't take much effort to do that, so I might consider doing it.

Emacs doesn't store such information in a nice uniform table -- sorry to
disappoint you -- instead Emacs has a bunch of heuristics to
guess/search the source position.  One heuristic goes like this: first
determine the filename by searching the name of the function in the list
`load-history' or for subrs use the file etc/DOC.  The line number is
not stored anywhere and is usually determined by some regexp search in
the source file.

Those heuristics work quite well for "normal" code, but are too limited
if lambdas or complex macros are involved.

Right. Although it is a long ways from being usable. bytecode deparsing would
handle all of the the complex cases (for bytecode) very naturally without hackery.
It only needs information about where the interpreter is.

There is also no nice API for this.  E.g. look at
`elisp--xref-find-definitions' for the messy code that is needed for
this kind of task.

Ideally, the compiler/intepreter would generate source maps and attach
them to functions/lambdas in some way.  But it seems that the motivation
for doing that (I guess it would be quite a big project) is limited,
given that the current heuristics work good enough for the simple cases.
Actually, the current approach works suprisingly well, considering how
little information is kept around.

And I think with just a little more information, position information, I think the situation would be 
much better. 



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