Preallocate hash tables when the number of inserts are known in advance

This avoids unnecessary re-allocations and reinsertions. On webkit.git
(i.e. about 182k inserts to the name hash table), this reduces about
100ms out of 3s user time.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-rename.c

@@ -389,6 +389,7 @@ static int find_exact_renames(struct diff_options *options)
 	struct hash_table file_table;
 
 	init_hash(&file_table);
+	preallocate_hash(&file_table, rename_src_nr + rename_dst_nr);
 	for (i = 0; i < rename_src_nr; i++)
 		insert_file_table(&file_table, -1, i, rename_src[i].p->one);
 

hash.h

@@ -40,4 +40,11 @@ static inline void init_hash(struct hash_table *table)
 	table->array = NULL;
 }
 
+static inline void preallocate_hash(struct hash_table *table, unsigned int elts)
+{
+	assert(table->size == 0 && table->nr == 0 && table->array == NULL);
+	table->size = elts * 2;
+	table->array = xcalloc(sizeof(struct hash_table_entry), table->size);
+}
+
 #endif

name-hash.c

@@ -92,6 +92,8 @@ static void lazy_init_name_hash(struct index_state *istate)
 
 	if (istate->name_hash_initialized)
 		return;
+	if (istate->cache_nr)
+		preallocate_hash(&istate->name_hash, istate->cache_nr);
 	for (nr = 0; nr < istate->cache_nr; nr++)
 		hash_index_entry(istate, istate->cache[nr]);
 	istate->name_hash_initialized = 1;