diff --git a/diffcore-rename.c b/diffcore-rename.c
index 512d0ac5fd2bc0..6c7a72fbe74e8d 100644
--- a/diffcore-rename.c
+++ b/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);
 
diff --git a/hash.h b/hash.h
index b875ce67c49eb3..1d43ac0ba0120e 100644
--- a/hash.h
+++ b/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
diff --git a/name-hash.c b/name-hash.c
index d8d25c23e99ddd..2a1f1082470a28 100644
--- a/name-hash.c
+++ b/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;