Re: [C++ PATCH] Reimplement ADL

[Jason Merrill <jason@redhat.com>]

On Thu, May 25, 2017 at 9:03 AM, Nathan Sidwell <nathan@acm.org> wrote:
> Anyway, this implementation introduces LOOKUP_SEEN_P and LOOKUP_FOUND_P to
> directly mark the DECL node.  Hence determination is now O(1) rather than
> O(N^2).  We still have a vector to recall which decls we need to unmark at
> the end of the lookup.  We need two markers on a class, because depending on
> how we found it we may need to search additional things about it. (These two
> maker bits will be used in later changes too.)
>
> One quirk is that ADL can be recursive.  ADL can cause template
> instantiation, which can in turn cause a different ADL to happen.  The new
> testcase is an example of this.  So, we need to detect this and undo/redo
> the outer DECL marking during the inner ADL.  Thus implementing a simple
> chain of ADLs and using their record of which decls got marked to undo/redo.
> The fiddly bit there is recording whether LOOKUP_FOUND_P was set or not
> (LOOKUP_SEEN_P will be).  To record that I simply push those DECLS with
> lookup_found_p set onto the stack.  They'll thus appear twice, and we can
> infer from the second sighting that it had FOUND_P set (and pop the stack).
> The recursion is a rare event, so we optimize the non-recursive case.

Sounds like it would make sense to use a hash_set rather than flags on
the decls.

Jason