Refactor `EraseMetaOnly` to speed up single group tables.

PiperOrigin-RevId: 592272653
Change-Id: I895c5786555227bdc88ab0a4cce8cf5ba65222a1

absl/container/internal/raw_hash_set.cc

@@ -221,26 +221,35 @@ void DropDeletesWithoutResize(CommonFields& common,
   common.infoz().RecordRehash(total_probe_length);
 }
 
-void EraseMetaOnly(CommonFields& c, ctrl_t* it, size_t slot_size) {
-  assert(IsFull(*it) && "erasing a dangling iterator");
-  c.decrement_size();
-  const auto index = static_cast<size_t>(it - c.control());
+static bool WasNeverFull(CommonFields& c, size_t index) {
+  if (is_single_group(c.capacity())) {
+    return true;
+  }
   const size_t index_before = (index - Group::kWidth) & c.capacity();
-  const auto empty_after = Group(it).MaskEmpty();
+  const auto empty_after = Group(c.control() + index).MaskEmpty();
   const auto empty_before = Group(c.control() + index_before).MaskEmpty();
 
   // We count how many consecutive non empties we have to the right and to the
   // left of `it`. If the sum is >= kWidth then there is at least one probe
   // window that might have seen a full group.
-  bool was_never_full = empty_before && empty_after &&
+  return empty_before && empty_after &&
          static_cast<size_t>(empty_after.TrailingZeros()) +
                  empty_before.LeadingZeros() <
              Group::kWidth;
+}
 
-  SetCtrl(c, index, was_never_full ? ctrl_t::kEmpty : ctrl_t::kDeleted,
-          slot_size);
-  c.set_growth_left(c.growth_left() + (was_never_full ? 1 : 0));
+void EraseMetaOnly(CommonFields& c, size_t index, size_t slot_size) {
+  assert(IsFull(c.control()[index]) && "erasing a dangling iterator");
+  c.decrement_size();
   c.infoz().RecordErase();
+
+  if (WasNeverFull(c, index)) {
+    SetCtrl(c, index, ctrl_t::kEmpty, slot_size);
+    c.set_growth_left(c.growth_left() + 1);
+    return;
+  }
+
+  SetCtrl(c, index, ctrl_t::kDeleted, slot_size);
 }
 
 void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,

absl/container/internal/raw_hash_set.h

@@ -1788,7 +1788,7 @@ void ClearBackingArray(CommonFields& c, const PolicyFunctions& policy,
                        bool reuse);
 
 // Type-erased version of raw_hash_set::erase_meta_only.
-void EraseMetaOnly(CommonFields& c, ctrl_t* it, size_t slot_size);
+void EraseMetaOnly(CommonFields& c, size_t index, size_t slot_size);
 
 // Function to place in PolicyFunctions::dealloc for raw_hash_sets
 // that are using std::allocator. This allows us to share the same
@@ -2871,7 +2871,8 @@ class raw_hash_set {
   // This merely updates the pertinent control byte. This can be used in
   // conjunction with Policy::transfer to move the object to another place.
   void erase_meta_only(const_iterator it) {
-    EraseMetaOnly(common(), it.control(), sizeof(slot_type));
+    EraseMetaOnly(common(), static_cast<size_t>(it.control() - control()),
+                  sizeof(slot_type));
   }
 
   // Resizes table to the new capacity and move all elements to the new

absl/container/internal/raw_hash_set_test.cc

@@ -653,6 +653,23 @@ TEST(Table, InsertCollisionAndFindAfterDelete) {
   EXPECT_TRUE(t.empty());
 }
 
+TEST(Table, EraseInSmallTables) {
+  for (int64_t size = 0; size < 64; ++size) {
+    IntTable t;
+    for (int64_t i = 0; i < size; ++i) {
+      t.insert(i);
+    }
+    for (int64_t i = 0; i < size; ++i) {
+      t.erase(i);
+      EXPECT_EQ(t.size(), size - i - 1);
+      for (int64_t j = i + 1; j < size; ++j) {
+        EXPECT_THAT(*t.find(j), j);
+      }
+    }
+    EXPECT_TRUE(t.empty());
+  }
+}
+
 TEST(Table, InsertWithinCapacity) {
   IntTable t;
   t.reserve(10);