Re: [PATCH v3] delta-islands: free island_marks and bitmaps

[Jeff King <peff@peff.net>]

On Fri, Feb 03, 2023 at 11:44:30PM +0000, Eric Wong wrote:

> >  	/* If we aren't using islands, assume everything goes together. */
> > -	if (!island_marks)
> > +	if (!using_island_marks)
> >  		return 1;
> 
> I much prefer to rely on invalid pointers than extra flags since
> having multiple sources of truth confuses me[1].

That's kind of my point, though. It's not multiple sources of truth, but
rather there are two bits "did we ask to use islands" and "is
island_marks still valid". You are shoving both bits into the same
variable by using a special sentinel pointer.

> > And of course that would also be a tiny step in the right direction if
> > the delta islands API learned to use a struct (this would be the same
> > spot where we'd say "we're done with islands; free the struct").
> 
> I do wonder about performance on register-starved systems,
> though, especially if stuff like island_delta_cmp gets called
> frequently.  I already have enough performance problems atm :<

Calling in_same_island() is pretty heavy-weight (it's multiple hash
lookups, and then an arbitrary-length bit-string comparison). I'd be
shocked if replacing a global with a struct pointer is even measurable.

> [1] to go farther, I might even eliminate `int use_delta_islands' as
>     a global from builtin/pack-objects.c and just have that become a
>     `struct delta_islands_foo *' or something.  But I have more
>     pressing performance problems to figure out :<

Right, that's along the same lines I was thinking. But I don't blame
you for not tackling it. The upside is fairly minimal.

> +	/* detect use-after-free with a an address which is never valid: */
> +	island_marks = (void *)-1;

I still hate how magical this line is, but I don't that it's worth
arguing about more.

-Peff