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
