Does mtree_lookup_walk need to use ma_data_end?

[Liam R. Howlett]

* Omar Sandoval <osandov at osandov.com> [231026 20:16]:
> Hi,
> 
> I'm reading some of the maple tree code in order to write drgn helpers
> for maple trees, and I was hoping for some clarification on one bit of
> code. Specifically, in mtree_lookup_walk:
> 
> 	end = ma_data_end(node, type, pivots, max);
> 	...
> 	do {
> 		if (pivots[offset] >= mas->index) {
> 			max = pivots[offset];
> 			break;
> 		}
> 	} while (++offset < end);
> 
> So this is looping until the first pivot greater than or equal to the
> desired index, stopping after it has checked the last pivot.
> 
> But we're guaranteed that either the last pivot or the implied maximum
> is greater than or equal to the index, so is it necessary to get the
> actual end with ma_data_end? I.e., would the following be equivalent?
> 
> diff --git a/lib/maple_tree.c b/lib/maple_tree.c
> index bb24d84a4922..18297c6ed06f 100644
> --- a/lib/maple_tree.c
> +++ b/lib/maple_tree.c
> @@ -3739,7 +3739,7 @@ static inline void *mtree_lookup_walk(struct ma_state *mas)
>  		node = mte_to_node(next);
>  		type = mte_node_type(next);
>  		pivots = ma_pivots(node, type);
> -		end = ma_data_end(node, type, pivots, max);
> +		end = mt_pivots[type];
>  		if (unlikely(ma_dead_node(node)))
>  			goto dead_node;
>  		do {
> 
> I have no idea whether this is any better or faster. I guess it does
> avoid a few branches and loads in ma_data_end. I'm mainly wondering
> whether there's something I'm missing in my understanding.

Your analysis looks correct.  This is probably the case elsewhere in
recent code as well (mas_range_walk(), probably).

Thanks,
Liam