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Re: scratch/accurate-warning-pos: next steps.

From: Alan Mackenzie
Subject: Re: scratch/accurate-warning-pos: next steps.
Date: Tue, 11 Dec 2018 20:51:28 +0000
User-agent: Mutt/1.10.1 (2018-07-13)

Hello, Paul.

On Tue, Dec 11, 2018 at 11:59:27 -0800, Paul Eggert wrote:
> On 12/11/18 11:20 AM, Alan Mackenzie wrote:

> If we required objdump or similar utilities, that would be yet another 
> porting hassle. And we might run into platforms where there is no 
> objdump-like utility and we have to write one ourselves. This doesn't 
> sound good at all.

Let's just assume we can get a list of functions from somewhere.
Exactly how is a minor implementation detail.  I only suggested objdump
to demonstrate it was possible and easy.  I think the C compiler, any C
compiler, can generate cross references.  That would be another source
of the info.

[ .... ]

> >> It should be easy enough to move shared file-static data into another
> >> file, that would be compiled only once.
> > Possibly not.  The same would have to be done with file global data,
> > too.  But doing it that way would involve a great deal of change to the
> > source code (testing for the -D option) which would not be popular.

> It'd be less change than having to decorate every function that might 
> call EQ.

True, but an irrelevant diversion.  Nobody but you is suggesting
decorating every function.

> > It would likely slow down the compilation by a very great deal.

> That's OK, if the cost is borne only by people who want accurate 
> diagnostics. People who want compilation speed can simply turn off the 
> accurate-diagnostics flag.

WHAT????  There is no such flag, will be no such flag, MUST be no such
flag.  We give accurate diagnostics to EVERYBODY, and we do this FAST.

> > You're conflating "tedious" with "tedious in the extreme".

> We're estimating how much work would be needed. Even if there would be 
> more work in changing the byte compiler, it shouldn't be so much more 
> work that we need to contort all the rest of Emacs.

Nobody but you is talking about "contorting all the rest of Emacs".  The
byte compiler is well over 8000 lines of code, much, possibly most, of
which would need to be rewritten.

Writing the aforementioned preprocessor is MUCH less work.  It is
something I can do and intend to do.

As for amending the reader and byte compiler to work with "Stefan's
format", I know that that is beyond my hacking capacity, even if it
could work.  I suggest you take on the task yourself, or organise a team
to do it.  If at the end you have a working solution to the bug, we can
compare approaches and merge the better one into the master branch.

[ .... ]

> > This would place onerous restrictions on what macros were allowed to do,
> > and likely be incompatible with a vast proportion of existing macros.

> But under both proposals I mentioned, existing macros would work just 
> fine with no new restrictions. So what I think you're saying is that if 
> people want to write macros that allow for more-accurate diagnostics, 
> they'll find that they can't easily do it for some reason.

No, I'm saying that people writing macros don't have to and mustn't have
to care about diagnostic mechanisms.  Lisp hackers deserve to get the
best diagnostics without any such ugly compromises being made.

Alan Mackenzie (Nuremberg, Germany).

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