Re: [RFC] resource: Avoid unnecessary resource tree walking in __region_intersects()

[David Hildenbrand]

On 11.10.24 13:19, Andy Shevchenko wrote:
> On Fri, Oct 11, 2024 at 02:15:55PM +0300, Andy Shevchenko wrote:
> > On Fri, Oct 11, 2024 at 12:51:09PM +0200, David Hildenbrand wrote:
> > > On 11.10.24 12:49, Andy Shevchenko wrote:
> > > > On Fri, Oct 11, 2024 at 09:06:37AM +0800, Huang, Ying wrote:
> > > > > David Hildenbrand <david@xxxxxxxxxx> writes:
> > > > > > On 10.10.24 08:55, Huang Ying wrote:
>
> ...
>
> > > > > > 	for ((_p) = (_root)->child; (_p); (_p) = next_resource_XXX(_root, _p))
> > > > >
> > > > > Yes.  This can improve code readability.
> > > > >
> > > > > A possible issue is that "_root" will be evaluated twice in above macro
> > > > > definition.  IMO, this should be avoided.
> > > >
> > > > Ideally, yes. But how many for_each type of macros you see that really try hard
> > > > to achieve that? I believe we shouldn't worry right now about this and rely on
> > > > the fact that root is the given variable. Or do you have an example of what you
> > > > suggested in the other reply, i.e. where it's an evaluation of the heavy call?
> > > >
> > > > > Do you have some idea about
> > > > > how to do that?  Something like below?
> > > > >
> > > > > #define for_each_resource_XXX(_root, _p)                                \
> > > > > 	for (typeof(_root) __root = (_root), __p = (_p) = (__root)->child; \
> > > > > 	     __p && (_p); (_p) = next_resource_XXX(__root, _p))
> > > >
> > > > This is a bit ugly :-( I would avoid ugliness as long as we have no problem to
> > > > solve (see above).
> > >
> > > Fully agreed, I didn't quite understand the concern about "evaluation" at
> > > first.
> >
> > It's a basic concept for macros and a good mine field even for the simple
> > cases.
> >
> > > If it's just reading a variable twice, it doesn't matter at all right
> > > now.
> >
> > The problem (even if it's a variable) is that the content of variable can be
> > changed when run in non-atomic context, i.e. two evaluations will give two
> > different results. Most "simple" for_each macros leave this exercise to the
> > caller. That's what I also suggest for now.
>
> For any context as Ying provided an example with calls, they have to be
> idempotent, or you definitely get two different pointers for these, which is
> bigger issue that what I described above.

Ah, now I understood what Ying meant: if the root pointer is modified 
within the loop body we'd be in trouble.

--
Cheers,

David / dhildenb