Speed up raw_hash_map::[] with ABSL hardening enabled by unchecking dereference of iterator returned by try_emplace.

PiperOrigin-RevId: 597920257
Change-Id: I1b2e8f10a2f1efa763a6f0760294beafdb6fd9c0

Author: Abseil Team

absl/container/internal/raw_hash_map.h

@@ -177,13 +177,20 @@ class raw_hash_map : public raw_hash_set<Policy, Hash, Eq, Alloc> {
   template <class K = key_type, class P = Policy, K* = nullptr>
   MappedReference<P> operator[](key_arg<K>&& key)
       ABSL_ATTRIBUTE_LIFETIME_BOUND {
-    return Policy::value(&*try_emplace(std::forward<K>(key)).first);
+    // It is safe to use unchecked_deref here because try_emplace
+    // will always return an iterator pointing to a valid item in the table,
+    // since it inserts if nothing is found for the given key.
+    return Policy::value(
+        &this->unchecked_deref(try_emplace(std::forward<K>(key)).first));
   }
 
   template <class K = key_type, class P = Policy>
   MappedReference<P> operator[](const key_arg<K>& key)
       ABSL_ATTRIBUTE_LIFETIME_BOUND {
-    return Policy::value(&*try_emplace(key).first);
+    // It is safe to use unchecked_deref here because try_emplace
+    // will always return an iterator pointing to a valid item in the table,
+    // since it inserts if nothing is found for the given key.
+    return Policy::value(&this->unchecked_deref(try_emplace(key).first));
   }
 
  private:

absl/container/internal/raw_hash_set.h

@@ -1943,7 +1943,7 @@ class raw_hash_set {
     // PRECONDITION: not an end() iterator.
     reference operator*() const {
       AssertIsFull(ctrl_, generation(), generation_ptr(), "operator*()");
-      return PolicyTraits::element(slot_);
+      return unchecked_deref();
     }
 
     // PRECONDITION: not an end() iterator.
@@ -2023,6 +2023,10 @@ class raw_hash_set {
     // Should be used when the lifetimes of the iterators are well-enough
     // understood to prove that they cannot be invalid.
     bool unchecked_equals(const iterator& b) { return ctrl_ == b.control(); }
+
+    // Dereferences the iterator without ABSL Hardening iterator invalidation
+    // checks.
+    reference unchecked_deref() const { return PolicyTraits::element(slot_); }
   };
 
   class const_iterator {
@@ -3159,6 +3163,8 @@ class raw_hash_set {
     return {control() + i, slot_array() + i, common().generation_ptr()};
   }
 
+  reference unchecked_deref(iterator it) { return it.unchecked_deref(); }
+
  private:
   friend struct RawHashSetTestOnlyAccess;