Re: [Tinycc-devel] make tcc reentrant

[Michael Matz]

Hello,

On Tue, 3 Dec 2019, Ulrich Schmidt wrote:

> i try to write a lua binding for tcc. To work out propperly, the tcc lib
> needs to be reentrant.

As demonstrated down-thread, that isn't correct.  It doesn't _need_ to be, 
it would be an feature.  As usual with features it needs to be measured 
against the downsides.  The downsides for your proposed changes are the 
following at least:
1) more complicated/boiler-platy source code of TCC (a TCCState
   argument almost everywhere)
2) slower code: most of the time the indirection through a pointer 
   variable (the state) in comparison to a direct access to a static 
   variable doesn't matter.  But it does matter for the symbol/token 
   table (and potentially for the register/evaluation stack).  I have 
   measured this years ago for the token table, so this might or might not 
   still be the case.

So, while I can see the wish for this feature, I don't necessarily see 
that tcc should be changed to accomodate.

If anything I would expect a _complete_ transition to exist, in order to 
measure the impact.  The worst thing that could happen is if someone added 
TCCState arguments everywhere, moved some static variables to that state, 
and then leaves: none of the features of this whole excercise would be 
had, but all the downsides would be there.

And yes, this is a big project.  I really think it would be better
if you simply write a wrapper for libtcc that ensures single-threadedness 
and that regards TCCState as a singleton.  I think such thing would be 
well-suited in the TCC sources itself.

(In a way it seems prudent for a tiny C compiler to only be usable as a 
singleton)


Ciao,
Michael.

> 
> I took a look into the sources and found some comments (XXX:...) and
> started with removing
> 
> the static var tcc_state. As a result allmost all lib functions needs a
> 1st parameter of
> 
> type TCCState*. I did this in my own local branch and tcc is still
> running :).
> 
> But this is a really HUGE change. in addition most of the local vars in
> tccpp, tccgen, ... needs
> 
> to be moved to TCCState. I can do that but at some points i will have
> some questions and i
> 
> can only test on windows and probably on linux.
> 
> My 1st question is: Are you interested in these changes or should i do
> this locally?
> 
> I would like to this together with you.
> 
> 
> Greetings.
> 
> Ulrich.
> 
> 
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