Don't reuse backing array in clear(), except for capacity 1 tables.

The motivation is to save memory.

PiperOrigin-RevId: 1004850661

absl/container/internal/raw_hash_set.cc

diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index 82ec819..baed443 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc
@@ -803,9 +803,8 @@
   // new_ctrl after 2nd store  =      EHNSEHNEEEE
 }
 
-// ClearBackingArrayNoReuse clears the backing array and sets the common
-// fields to the default values for empty non-allocated tables.
-// REQUIRES: c.capacity > policy.soo_capacity.
+// Clears the backing array and sets the common fields to the default values for
+// empty non-allocated tables. REQUIRES: c.capacity > policy.soo_capacity.
 void ClearBackingArrayNoReuse(CommonFields& c,
                               const PolicyFunctions& __restrict policy,
                               void* alloc) {
@@ -947,6 +946,8 @@
     ABSL_ASSUME(cap > 0);
   }
   if (c.is_small()) {
+    // In the small case, we keep the backing array allocation because the
+    // memory overhead is small compared to the potential benefits of reuse.
     if (!c.empty()) {
       if (destroy_slot != nullptr) {
         destroy_slot(&c, SingleSlotAddress<kSooEnabled>(c));
@@ -960,12 +961,7 @@
     }
     // Iterating over this container is O(bucket_count()). When bucket_count()
     // is much greater than size(), iteration becomes prohibitively expensive.
-    // For clear() it is more important to reuse the allocated array when the
-    // container is small because allocation takes comparatively long time
-    // compared to destruction of the elements of the container. So we pick the
-    // largest bucket_count() threshold for which iteration is still fast and
-    // past that we simply deallocate the array.
-    ClearBackingArray(c, policy, alloc, /*reuse=*/cap < 128);
+    ClearBackingArrayNoReuse(c, policy, alloc);
   }
   c.set_reserved_growth(0);
   c.set_reservation_size(0);

absl/container/internal/raw_hash_set.h

diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 49e5b2e..9781b5c 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -2265,9 +2265,8 @@
                                         const PolicyFunctions& policy,
                                         size_t new_capacity);
 
-// ClearBackingArray clears the backing array, either modifying it in place,
-// or creating a new one based on the value of "reuse".
-// REQUIRES: c.capacity > MaxSmallCapacity().
+// Clears the backing array, either modifying it in place, or creating a new one
+// based on the value of "reuse". REQUIRES: c.capacity > MaxSmallCapacity().
 void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,
                        void* alloc, bool reuse);
 

absl/container/internal/raw_hash_set_test.cc

diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
index 1268851..65c8886 100644
--- a/absl/container/internal/raw_hash_set_test.cc
+++ b/absl/container/internal/raw_hash_set_test.cc
@@ -2074,7 +2074,7 @@
   }
   ASSERT_EQ(capacity, t.capacity());
   intptr_t original = reinterpret_cast<intptr_t>(&*t.find(2));
-  t.clear();
+  t.erase(t.begin(), t.end());
   ASSERT_EQ(capacity, t.capacity());
   for (size_t i = 0; i < max_size; ++i) {
     t.insert(i);