Implement small object optimization in swisstable - disabled for now.

Details:
- We use the space for control/slots pointers as the inline buffer.
- We use a max inline capacity of 1 to make the implementation much simpler and to avoid having to randomize the iteration order for inline tables.
- For iteration of inline tables, we introduce the kSooControl buffer which just has 1 full control byte followed by 1 sentinel control byte so that incrementing yields an end() iterator. We don't access kSooControl during lookups - only iteration.
PiperOrigin-RevId: 613253492
Change-Id: Id98ff11842f8bef27ac7ed88138dc03b46ce4fa6

1 files changed, 15 insertions, 0 deletions

diff --git a/absl/container/internal/hash_policy_traits.h b/absl/container/internal/hash_policy_traits.h
index 86ffd1be..ec08794a 100644
--- a/absl/container/internal/hash_policy_traits.h
+++ b/absl/container/internal/hash_policy_traits.h
@@ -168,6 +168,9 @@ struct hash_policy_traits : common_policy_traits<Policy> {
 #endif
   }
 
+  // Whether small object optimization is enabled. False by default.
+  static constexpr bool soo_enabled() { return soo_enabled_impl(Rank1{}); }
+
  private:
   template <class Hash>
   struct HashElement {
@@ -183,6 +186,18 @@ struct hash_policy_traits : common_policy_traits<Policy> {
     return Policy::apply(HashElement<Hash>{*static_cast<const Hash*>(hash_fn)},
                          Policy::element(static_cast<slot_type*>(slot)));
   }
+
+  // Use go/ranked-overloads for dispatching. Rank1 is preferred.
+  struct Rank0 {};
+  struct Rank1 : Rank0 {};
+
+  // Use auto -> decltype as an enabler.
+  template <class P = Policy>
+  static constexpr auto soo_enabled_impl(Rank1) -> decltype(P::soo_enabled()) {
+    return P::soo_enabled();
+  }
+
+  static constexpr bool soo_enabled_impl(Rank0) { return false; }
 };
 
 }  // namespace container_internal