Re: Question regarding back-porting Lua 5.4 GC to 5.3

[Dibyendu Majumdar <mobile@...>]

On Mon, 22 Jun 2020 at 14:39, Roberto Ierusalimschy
<roberto@inf.puc-rio.br> wrote:
>
> > I am trying to understand whether to backport the 5.4 GC to 5.3 one
> > would need some of the other changes that were made or are these
> > orthogonal to the GC. In particular will the 5.4 GC work correctly if
> > the UpValue model was left as is in 5.3?
>
> The changes themselves are certainly not orthogonal, as the
> implementation of the GC is intimately connected to the implementation
> of the data graph.

Of course the GC must be aware of the changes in the data graph, but
the opposite  is not true? I mean that the user of the GC (in this
case Lua implementation) simply needs to invoke write barriers
appropriately - i.e. there is no difference in how write barriers are
invoked between say the incremental GC and the generational GC I
assume.

> In a more abstract level, I don't know.  In
> particular, the management of upvalues is somewhat complex, due to the
> fact that stacks do not have write barriers, so Lua can change the
> values of open upvalues behind their backs.
>
> However, the upvalue model was changed after the new GC, so there
> was a time when Lua (internally) had generational GC with the
> old upvalue model. I don't remember exactly why we changed. The
> comment says only "simpler and better for generational mode"
> (see commit a3d36fe2).
>

I see. I was wondering if the upvalue change was somehow necessitated
by the generational GC changes, or it was simply for conceptual
clarity. I have previously looked at the commit you mention and my
previous attempt included the upvalue changes, but I wanted to keep
the ref counted upvalues for my current attempt to backport the new
GC.

Thanks and Regards
Dibyendu