util/hash_table: optimize rehash for empty table and no-func clears

if the table is filled with deleted entries, we don't need to rzalloc+free an identical
block of memory for the table, we can just memset the existing one

the same applies to table clears without a function passed in that the table
doesn't need to be iterated and can just be memset

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8450>

src/util/hash_table.c

@@ -234,6 +234,13 @@ _mesa_hash_table_destroy(struct hash_table *ht,
    ralloc_free(ht);
 }
 
+static void
+hash_table_clear_fast(struct hash_table *ht)
+{
+   memset(ht->table, 0, sizeof(struct hash_entry) * hash_sizes[ht->size_index].size);
+   ht->entries = ht->deleted_entries = 0;
+}
+
 /**
  * Deletes all entries of the given hash table without deleting the table
  * itself or changing its structure.
@@ -249,15 +256,17 @@ _mesa_hash_table_clear(struct hash_table *ht,
 
    struct hash_entry *entry;
 
-   for (entry = ht->table; entry != ht->table + ht->size; entry++) {
-      if (entry_is_present(ht, entry) && delete_function != NULL)
-         delete_function(entry);
+   if (delete_function) {
+      for (entry = ht->table; entry != ht->table + ht->size; entry++) {
+         if (entry_is_present(ht, entry))
+            delete_function(entry);
 
-      entry->key = NULL;
-   }
-
-   ht->entries = 0;
-   ht->deleted_entries = 0;
+         entry->key = NULL;
+      }
+      ht->entries = 0;
+      ht->deleted_entries = 0;
+   } else
+      hash_table_clear_fast(ht);
 }
 
 /** Sets the value of the key pointer used for deleted entries in the table.
@@ -362,6 +371,12 @@ _mesa_hash_table_rehash(struct hash_table *ht, unsigned new_size_index)
    struct hash_table old_ht;
    struct hash_entry *table;
 
+   if (ht->size_index == new_size_index && ht->deleted_entries == ht->max_entries) {
+      hash_table_clear_fast(ht);
+      assert(!ht->entries);
+      return;
+   }
+
    if (new_size_index >= ARRAY_SIZE(hash_sizes))
       return;
 

src/util/tests/hash_table/clear.c

@@ -86,6 +86,12 @@ int main()
    hash_table_foreach(ht, entry) {
       assert(key_id(entry->key) < SIZE);
    }
+   _mesa_hash_table_clear(ht, NULL);
+   assert(!ht->entries);
+   assert(!ht->deleted_entries);
+   hash_table_foreach(ht, entry) {
+      assert(0);
+   }
 
    _mesa_hash_table_destroy(ht, NULL);