Avoid allocating control bytes in capacity==1 swisstables.

PiperOrigin-RevId: 759275918
Change-Id: Id4e88ae026e3ebee04f5bdb28a7191e54aaea1e9

Author: ezbr

absl/container/internal/raw_hash_set.cc

@@ -443,6 +443,7 @@ void ResetCtrl(CommonFields& common, size_t slot_size) {
   ctrl_t* ctrl = common.control();
   static constexpr size_t kTwoGroupCapacity = 2 * Group::kWidth - 1;
   if (ABSL_PREDICT_TRUE(capacity <= kTwoGroupCapacity)) {
+    if (IsSmallCapacity(capacity)) return;
     std::memset(ctrl, static_cast<int8_t>(ctrl_t::kEmpty), Group::kWidth);
     std::memset(ctrl + capacity, static_cast<int8_t>(ctrl_t::kEmpty),
                 Group::kWidth);
@@ -591,13 +592,14 @@ size_t FindNewPositionsAndTransferSlots(
 
   const auto insert_slot = [&](void* slot) {
     size_t hash = policy.hash_slot(hash_fn, slot);
-    auto target = find_first_non_full(common, hash);
+    FindInfo target =
+        common.is_small() ? FindInfo{0, 0} : find_first_non_full(common, hash);
     SetCtrl(common, target.offset, H2(hash), slot_size);
     policy.transfer_n(&common, SlotAddress(new_slots, target.offset, slot_size),
                       slot, 1);
     return target.probe_length;
   };
-  if (old_capacity == 1) {
+  if (IsSmallCapacity(old_capacity)) {
     if (common.size() == 1) insert_slot(old_slots);
     return 0;
   }
@@ -1343,8 +1345,6 @@ void SmallEmptyNonSooPrepareInsert(CommonFields& common,
   const bool has_infoz = infoz.IsSampled();
   void* alloc = policy.get_char_alloc(common);
 
-  // TODO(b/413062340): don't allocate control bytes for capacity 1 tables. We
-  // don't use the control bytes in this case.
   const auto [new_ctrl, new_slots] =
       AllocBackingArray(common, policy, kNewCapacity, has_infoz, alloc);
   common.set_control</*kGenerateSeed=*/true>(new_ctrl);

absl/container/internal/raw_hash_set.h

@@ -796,7 +796,7 @@ constexpr size_t NumClonedBytes() { return Group::kWidth - 1; }
 
 // Returns the number of control bytes including cloned.
 constexpr size_t NumControlBytes(size_t capacity) {
-  return capacity + 1 + NumClonedBytes();
+  return IsSmallCapacity(capacity) ? 0 : capacity + 1 + NumClonedBytes();
 }
 
 // Computes the offset from the start of the backing allocation of control.

absl/container/internal/raw_hash_set_test.cc

@@ -1184,7 +1184,7 @@ TYPED_TEST(SmallTableResizeTest, InsertIntoSmallTable) {
     t.insert(i);
     ASSERT_EQ(t.size(), i + 1);
     for (int j = 0; j < i + 1; ++j) {
-      EXPECT_TRUE(t.find(j) != t.end());
+      ASSERT_TRUE(t.find(j) != t.end());
       EXPECT_EQ(*t.find(j), j);
     }
   }
@@ -1207,7 +1207,7 @@ TYPED_TEST(SmallTableResizeTest, ResizeGrowSmallTables) {
           t.reserve(target_size);
         }
         for (size_t i = 0; i < source_size; ++i) {
-          EXPECT_TRUE(t.find(static_cast<int>(i)) != t.end());
+          ASSERT_TRUE(t.find(static_cast<int>(i)) != t.end());
           EXPECT_EQ(*t.find(static_cast<int>(i)), static_cast<int>(i));
         }
       }
@@ -1232,7 +1232,7 @@ TYPED_TEST(SmallTableResizeTest, ResizeReduceSmallTables) {
             << "rehash(0) must resize to the minimum capacity";
       }
       for (size_t i = 0; i < inserted_count; ++i) {
-        EXPECT_TRUE(t.find(static_cast<int>(i)) != t.end());
+        ASSERT_TRUE(t.find(static_cast<int>(i)) != t.end());
         EXPECT_EQ(*t.find(static_cast<int>(i)), static_cast<int>(i));
       }
     }