Re: Usage of the C++ stdlib unordered_map in GCC

[Jason Merrill <jason@redhat.com>]

On Wed, Aug 31, 2022 at 5:38 AM Jonathan Wakely via Gcc <gcc@gcc.gnu.org>
wrote:

> On Tue, 30 Aug 2022 at 21:08, Marek Polacek via Gcc <gcc@gcc.gnu.org>
> wrote:
> >
> > On Tue, Aug 30, 2022 at 09:57:45PM +0200, Tim Lange wrote:
> > > Hello,
> > >
> > > I was preparing a patch for GCC and used the unordered_map from the C++
> > > stdlib in my patch. Later on, I noticed that it is used nowhere else
> inside
> > > GCC except for some files in the go frontend.
> > >
> > > I wondered, now that building GCC requires a C++11 host compiler,
> whether
> > > there is a consensus on which data structure implementation is
> preferred.
> > > Should I rather use a hash_map instead of an unordered_map or is it on
> my
> > > side to decide which one I choose?
> >
> > I think you're probably better off using a hash_map; std::unordered_map
> > has efficiency issues as described in
> > https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2028r0.pdf
>
> I assume you mean the comments on page 6. Does GCC's hash_map actually
> use open addressing and probing to deal with collisions? Do we want to
> be able to change the hash function or use per-compilation salts?
> (Would that break PCH?) If not, I don't see why it would be any better
> when considering the metrics that paper is referring to. It might be
> better based on other properties that benefit GCC, but the case
> against std::unordered_map is often overstated.
>
> If the question was whether to prefer std::unordered_map or
> absl::node_hash_map then I would agree that std::unordered_map is a
> bad choice. But that's not the question.
>

Generally we want to use the GCC hash_map because it works with GCC garbage
collection (and PCH).  Is that not relevant to your patch?

Jason