heterogeneous lookup for F14 map fallback

Summary:
This diff implements efficient heterogeneous lookup, insert,
and erase for the fallback (non-SIMD) mode of F14.  It relies on internal
implementation details of the underlying std::unordered_map implementation
that happen to be true for libstdc++, libc++, and MSVC.

Extending this technique to F14 sets would be straightforward, but has
not been done here.

(Note: this ignores all push blocking failures!)

Differential Revision: D21408628

fbshipit-source-id: 72e9ddd82a50c02ec762d4b64dc3303848bd0218

folly/container/detail/F14IntrinsicsAvailability.h

@@ -44,6 +44,14 @@
 #define FOLLY_F14_CRC_INTRINSIC_AVAILABLE 0
 #endif
 
+// The F14 extension eraseInto is only available in fallback mode for
+// c++17 or later, because it relies on unordered_map::extract.
+#if FOLLY_F14_VECTOR_INTRINSICS_AVAILABLE || __cplusplus >= 201703L
+#define FOLLY_F14_ERASE_INTO_AVAILABLE 1
+#else
+#define FOLLY_F14_ERASE_INTO_AVAILABLE 0
+#endif
+
 namespace folly {
 namespace f14 {
 namespace detail {

folly/container/detail/F14SetFallback.h

@@ -43,6 +43,10 @@ class F14BasicSet : public std::unordered_set<K, H, E, A> {
   //// PUBLIC - F14 Extensions
 
   bool containsEqualValue(value_type const& value) const {
+    // bucket is only valid if bucket_count is non-zero
+    if (this->empty()) {
+      return false;
+    }
     auto slot = this->bucket(value);
     auto e = this->end(slot);
     for (auto b = this->begin(slot); b != e; ++b) {

folly/container/detail/F14Table.h

@@ -198,6 +198,27 @@ void erase_if_impl(Container& c, Predicate& predicate) {
   }
 }
 
+template <
+    typename TableKey,
+    typename Hasher,
+    typename KeyEqual,
+    typename ArgKey>
+struct EligibleForHeterogeneousFind
+    : Conjunction<
+          is_transparent<Hasher>,
+          is_transparent<KeyEqual>,
+          is_invocable<Hasher, ArgKey const&>,
+          is_invocable<KeyEqual, ArgKey const&, TableKey const&>> {};
+
+template <
+    typename TableKey,
+    typename Hasher,
+    typename KeyEqual,
+    typename ArgKey>
+using EligibleForHeterogeneousInsert = Conjunction<
+    EligibleForHeterogeneousFind<TableKey, Hasher, KeyEqual, ArgKey>,
+    std::is_constructible<TableKey, ArgKey>>;
+
 } // namespace detail
 } // namespace f14
 
@@ -239,27 +260,6 @@ template <typename Arg, typename Default>
 using Defaulted =
     std::conditional_t<std::is_same<Arg, void>::value, Default, Arg>;
 
-template <
-    typename TableKey,
-    typename Hasher,
-    typename KeyEqual,
-    typename ArgKey>
-struct EligibleForHeterogeneousFind
-    : Conjunction<
-          is_transparent<Hasher>,
-          is_transparent<KeyEqual>,
-          is_invocable<Hasher, ArgKey const&>,
-          is_invocable<KeyEqual, ArgKey const&, TableKey const&>> {};
-
-template <
-    typename TableKey,
-    typename Hasher,
-    typename KeyEqual,
-    typename ArgKey>
-using EligibleForHeterogeneousInsert = Conjunction<
-    EligibleForHeterogeneousFind<TableKey, Hasher, KeyEqual, ArgKey>,
-    std::is_constructible<TableKey, ArgKey>>;
-
 ////////////////
 
 template <typename T>