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Re: Update 1 on Bytecode Offset tracking

From: Zach Shaftel
Subject: Re: Update 1 on Bytecode Offset tracking
Date: Thu, 16 Jul 2020 18:45:00 -0400
User-agent: mu4e 1.4.10; emacs 28.0.50

Hi Stefan,

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> The second branch saves the offset only before a call. Therefore, the
>> traceback on all of the functions other than the current one are
>> accurate, but the current one is not accurate if the error happens in
>> a byte op.
> IIUC this has a negligible performance impact.  The info it provides in
> not 100% accurate, but I think it's a "sweet spot": it does provide the
> byte-offset info and is cheap enough to be acceptable into `master` with
> no real downside.

Great! I just followed up on my copyright assignment as I still haven't
finished that process. I don't know whether this could be exempt or if
Rocky's assignment is sufficient, but hopefully I will hear back from
copyright-clerk soon.

> I'd look at it as a "step" along the way: subsequent steps can be to
> make use of that info, or to improve the accuracy of that info.

Absolutely, it would be great to have that in place as a basis for
further improvement.

>> The third branch bypasses invoking Ffuncall from within
>> exec_byte_code, and instead does essentially the same thing that
>> Ffuncall does, right in the Bcall ops.
> This would be useful in its own right.
> So I suggest you try and get this code into shape for `master` as well.

I will definitely continue work on this.

> I expect this will tend to suffer from some amount of code duplication.
> Maybe we can avoid it via refactoring, or maybe by "clever" macro
> tricks, but if the speedup is important enough, we can probably live
> with some amount of duplication.

That seems to be the case. I'll keep looking to see if there's any low
hanging fruit in terms of splitting up the funcall logic without slowing
things down. More testing is necessary, but if a moderate chunk of
duplicated code is acceptable then there may not be as much work needed
on that branch as I had thought.

>> All of them print the offset next to function names in the backtrace like 
>> this:
>> Debugger entered--Lisp error: (wrong-type-argument stringp t)
>>        string-match(t t nil)
>>     13 test-condition-case()
>>        load("/home/zach/.repos/bench-compare.el/test/test-debug...")
>>     78 
>> byte-recompile-file("/home/zach/.repos/bench-compare.el/test/test-debug..." 
>> nil 0 t)
>>     35 emacs-lisp-byte-compile-and-load()
>>        funcall-interactively(emacs-lisp-byte-compile-and-load)
>>        call-interactively(emacs-lisp-byte-compile-and-load record nil)
>>    101 command-execute(emacs-lisp-byte-compile-and-load record)
> Cool!
>> With respect to reporting offsets, using code from edebug we have
>> a Lisp-Expression reader that will track source-code locations and
>> store the information in a source-map-expression cl-struct.  The code
>> in progress is here.
> How does the performance of this code compare to that of the "native" `read?

Rough tests indicate it's about three times slower. Running it on all
274 files in the `lisp` directory of the GNU Emacs sources takes ~11-12
seconds (after removing the string from the struct and not
pretty-printing). A similar function which just calls `read` takes ~4
seconds. There are probably ways to further improve the performance of
`source-map-read`, but I don't know much more speed can realistically be

> And to put it into perspective, have you looked at the relative
> proportion of time spent in `read` during a "typical" byte compilation?

I have not yet, but I'll evaluate that and keep it in mind.

> There's no doubt that preserving source code information will slow down
> byte-compilation but depending on how slow it gets we may find it's not
> "worth it".
>> Information currently saved is:
>> * The expression itself
>> * The exact string that was read
>> * Begin and end point​s of the sexp in the buffer
>> * source-map-expression children (for conses and vectors)
> Sounds like a lot of information, which in turn implies a potentially
> high overhead (e.g. the "exact string" sounds like it might cost O(N²)
> in corner cases, yet provides redundant info that can be recovered from
> begin+end points).

Removing the string did improve performance, but not by as much as I
expected. The function that constructs the tree of "children" may be
slower than it needs to be, so I'll look into improving that. It may not
be necessary to create the children for vectors since they're constants
(outside of backquote, at least).

>Note also that while `read` returns a sexp made exclusively of data
>coming from a particular buffer, the code after macro-expansion can
>include chunks coming from other buffers, so if we want to keep the
>same representation of "sexp with extra info" in both cases, we can't
>just assume "the buffer".

Yes, and it won't be easy to maintain the read locations across
macroexpansion, byte-opt and cconv. It's tough to say at this point how
much the final product will slow down compilation, but I suspect it will
be significant.

>         Stefan

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