util/hash_table: don't allocate the smallest table, declare it in the struct

When compiling Heaven shaders with radeonsi+ACO, this reduces the total
number of ralloc calls by 10.8% (same number as the sets, which also reduced
it by 10.8%). Apparently we like to create a lot of hash tables.

Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36728>

src/util/hash_table.c

@@ -167,6 +167,7 @@ _mesa_hash_table_init(struct hash_table *ht,
                       bool (*key_equals_function)(const void *a,
                                                   const void *b))
 {
+   ht->mem_ctx = mem_ctx;
    ht->size_index = 0;
    ht->size = hash_sizes[ht->size_index].size;
    ht->rehash = hash_sizes[ht->size_index].rehash;
@@ -175,12 +176,14 @@ _mesa_hash_table_init(struct hash_table *ht,
    ht->max_entries = hash_sizes[ht->size_index].max_entries;
    ht->key_hash_function = key_hash_function;
    ht->key_equals_function = key_equals_function;
-   ht->table = rzalloc_array(mem_ctx, struct hash_entry, ht->size);
+   assert(ht->size == ARRAY_SIZE(ht->_initial_storage));
+   ht->table = ht->_initial_storage;
+   memset(ht->table, 0, sizeof(ht->_initial_storage));
    ht->entries = 0;
    ht->deleted_entries = 0;
    ht->deleted_key = &deleted_key_value;
 
-   return ht->table != NULL;
+   return true;
 }
 
 /* It's preferred to use _mesa_hash_table_init instead of this to skip ralloc. */
@@ -243,16 +246,23 @@ _mesa_hash_table_clone(struct hash_table *src, void *dst_mem_ctx)
    if (ht == NULL)
       return NULL;
 
-   memcpy(ht, src, sizeof(struct hash_table));
+   /* Copy the whole structure except the initial storage. */
+   memcpy(ht, src, offsetof(struct hash_table, _initial_storage));
+   ht->mem_ctx = dst_mem_ctx;
 
-   ht->table = ralloc_array(ht, struct hash_entry, ht->size);
-   if (ht->table == NULL) {
-      ralloc_free(ht);
-      return NULL;
+   if (src->table != src->_initial_storage) {
+      ht->table = ralloc_array(dst_mem_ctx, struct hash_entry, ht->size);
+      if (ht->table == NULL) {
+         ralloc_free(ht);
+         return NULL;
+      }
+
+      memcpy(ht->table, src->table, ht->size * sizeof(struct hash_entry));
+   } else {
+      ht->table = ht->_initial_storage;
+      memcpy(ht->table, src->_initial_storage, sizeof(src->_initial_storage));
    }
 
-   memcpy(ht->table, src->table, ht->size * sizeof(struct hash_entry));
-
    return ht;
 }
 
@@ -272,7 +282,9 @@ _mesa_hash_table_fini(struct hash_table *ht,
          delete_function(entry);
       }
    }
-   ralloc_free(ht->table);
+   if (ht->table != ht->_initial_storage)
+      ralloc_free(ht->table);
+
    ht->table = NULL;
 }
 
@@ -439,12 +451,19 @@ _mesa_hash_table_rehash(struct hash_table *ht, unsigned new_size_index)
    if (new_size_index >= ARRAY_SIZE(hash_sizes))
       return;
 
-   table = rzalloc_array(ralloc_parent(ht->table), struct hash_entry,
+   table = rzalloc_array(ht->mem_ctx, struct hash_entry,
                          hash_sizes[new_size_index].size);
    if (table == NULL)
       return;
 
-   old_ht = *ht;
+   if (ht->table == ht->_initial_storage) {
+      /* Copy the whole structure including the initial storage. */
+      old_ht = *ht;
+      old_ht.table = old_ht._initial_storage;
+   } else {
+      /* Copy everything except the initial storage. */
+      memcpy(&old_ht, ht, offsetof(struct hash_table, _initial_storage));
+   }
 
    ht->table = table;
    ht->size_index = new_size_index;
@@ -462,7 +481,8 @@ _mesa_hash_table_rehash(struct hash_table *ht, unsigned new_size_index)
 
    ht->entries = old_ht.entries;
 
-   ralloc_free(old_ht.table);
+   if (old_ht.table != old_ht._initial_storage)
+      ralloc_free(old_ht.table);
 }
 
 static struct hash_entry *

src/util/hash_table.h

@@ -44,6 +44,7 @@ struct hash_entry {
 };
 
 struct hash_table {
+   void *mem_ctx;
    struct hash_entry *table;
    uint32_t (*key_hash_function)(const void *key);
    bool (*key_equals_function)(const void *a, const void *b);
@@ -56,6 +57,15 @@ struct hash_table {
    uint32_t size_index;
    uint32_t entries;
    uint32_t deleted_entries;
+
+   /* "table" points to here at first. A bigger storage is allocated separately
+    * when a bigger size is needed.
+    */
+   struct hash_entry _initial_storage[19]; /* hash_sizes[0].size */
+
+   /* Don't insert any new fields here. All other fields must be before
+    * _initial_storage.
+    */
 };
 
 struct hash_table *