RE: Getting involved in Bison

Hello!


> I recommend that you first build by hand a push parser before trying to get this done by m4.  For instance take examples/c++/calc++, and change the generated parser so that it works as a push parser.  Keep this preciously somewhere to make sure "make" doesn't kill it.  When it works, you can show us a diff between the regular pull parser and the push parser, and when we agree, we can proceed to make it work in m4.

Ok, I will first create a draft parser, review it with everyone and then make it work in m4. I will use Java as reference, should be closer to the final solution than C.
I have been fiddling this weekend with the push parsers and some examples, but I didn't look at pushcalc. Although I have used a lot Bison I am still unfamiliar with jargon and some notions. Thanks for all the suggestions and examples.

> Maybe you should start with that: find a means to benchmark two pull parsers: one off-the-shelf, generated by today's lalr1.cc, and another one where the local variables that need to be member variables in push-mode are made member variables.

What is the expected difference in performance between local and member variables? Besides constructor creation/destruction and variable lifetime, I would say there should not be other issues. Anyway let's test both 😊



Related to this topic:
#################
Do you remember Akim that I told you that in my Mac tests did not pass completely but in yours they did? Specifically all in these categories:
	LALR(1) Calculator
	LALR(1) C++ Calculator

Since this time will I need them, I decided to checked them. Seems that in Mac `wc -l` indents the result with the number of lines, mismatching with the expected pattern:
	tests/testsuite.log:
	./calc.at:1111: grep -v 'Return for a new token:' stderr | wc -l 
	--- -   2020-02-22 13:15:01.000000000 +0800 
	+++ /Users/Victor/Projects/bison/tests/testsuite.dir/at-groups/456/stdout       2020-02-22 13:15:01.000000000 +0800 
	@@ -1,2 +1,2 @@
	-0
	+       0
Has someone previously reported this? Seems I am predestined to deal with indentations XD



Separate topic about C++:
#####################
I believe we could improve a bit the C++ generated code and standardize it with modern syntax.

For example, C++ does no longer recommend constructing objects with parenthesis, but with braces. I saw yesterday this statement:
	symbol_type yylookahead (yylex ());
Which should be rewritten as:
	symbol_type yylookahead {yylex ()};

There are also some keywords that help the compiler optimizing. For example, this structure has a default constructor:
	struct by_type	{ /* Default constructor */ by_type (); ... }
We might add "= default" (or even "= delete" if it is not used and we prefer to avoid it being created without parameters):
	struct by_type	{ /* Default constructor */ by_type () = default; ... }

We can discuss all this after the push parser.


Regards,
Victor

-----Original Message-----
From: Akim Demaille <[email protected]> 
Sent: 2020年2月23日 15:08
To: Morales Cayuela, Victor (NSB - CN/Hangzhou) <[email protected]>
Cc: Bison Bugs <[email protected]>; Bison Patches <[email protected]>
Subject: Re: Getting involved in Bison

Hi Victor!

> Le 20 févr. 2020 à 04:07, Morales Cayuela, Victor (NSB - CN/Hangzhou) <[email protected]> a écrit :
> 
> Hi!
> 
>> It really depends what you'd like to do.  You reported you're fluent in C++.  Something quite interesting would be to enable push-parser in lalr1.cc.  It's more ambitious than the subcomplain task, but it will probably take you less time, because it's more compact, it will impact much less code.
> 
> Ok, seems very interesting! I use C++ (currently C++17) in my daily job as a developer and I always try to keep up with the latest of the standard.
> I will familiarize myself first with push/pull parsers and then I will start with it 😊 I will also take a look at TODO to check if there is already any specification about how it should look like.

I think all the information you need is in the manual, in the push example in C (see examples/c/pushcalc), and in yacc.c's implementation of push.  Actually, Java also has push-parser support, and is probably a better source of inspiration than C.

I recommend that you first build by hand a push parser before trying to get this done by m4.  For instance take examples/c++/calc++, and change the generated parser so that it works as a push parser.  Keep this preciously somewhere to make sure "make" doesn't kill it.  When it works, you can show us a diff between the regular pull parser and the push parser, and when we agree, we can proceed to make it work in m4.

Actually, maybe examples/c++/calc++ is too bloated: first create a very simple example in C++, similar to examples/c/calc.  Start from this one, it is much simpler, and yet sufficient.

Most of the effort, I guess, is finding what local variables must be turned into member variables, to survive across calls to yyparse (amusingly enough coroutines would make this easy, but we can't afford them yet ;-).  But lalr1.java should help.

Note that C++ allows to consider other options than the C parsers.  Since the C++ parser is an object, maybe we can have push and pull use the same object.  I mean, *maybe*, we can *always* move most local variables into member variables, even for regular pull-parsing.  Of course it's technically possible, the "maybe" refers to the runtime cost: *if* benchmarks show that it is not a regression to move to a model with more member variables and less local variables, then the m4 part will be much easier.

Maybe you should start with that: find a means to benchmark two pull parsers: one off-the-shelf, generated by today's lalr1.cc, and another one where the local variables that need to be member variables in push-mode are made member variables.

How does that sound?