add rb_hash_new_with_size()

Sometimes, size of a hash can be calcluated a priori.  By providing
such info to the constructor we can avoid unnecessary internal re-
allocations.  This can boost for instance creation of hash literals.
[Bug #13861]

Signed-off-by: Urabe, Shyouhei <shyouhei@ruby-lang.org>


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59744 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -432,6 +432,15 @@ rb_hash_new(void)
     return hash_alloc(rb_cHash);
 }
 
+VALUE
+rb_hash_new_with_size(st_index_t size)
+{
+    VALUE ret = rb_hash_new();
+    if (size)
+        RHASH(ret)->ntbl = st_init_table_with_size(&objhash, size);
+    return ret;
+}
+
 static VALUE
 hash_dup(VALUE hash, VALUE klass, VALUE flags)
 {
@@ -1927,7 +1936,7 @@ rb_hash_transform_values(VALUE hash)
     VALUE result;
 
     RETURN_SIZED_ENUMERATOR(hash, 0, 0, hash_enum_size);
-    result = rb_hash_new();
+    result = rb_hash_new_with_size(RHASH_SIZE(hash));
     if (!RHASH_EMPTY_P(hash)) {
         rb_hash_foreach(hash, transform_values_i, result);
     }

insns.def

@@ -496,7 +496,7 @@ newhash
 {
     RUBY_DTRACE_CREATE_HOOK(HASH, num);
 
-    val = rb_hash_new();
+    val = rb_hash_new_with_size(num / 2);
 
     if (num) {
         rb_hash_bulk_insert(num, STACK_ADDR_FROM_TOP(num), val);

internal.h

@@ -1242,6 +1242,7 @@ void ruby_sized_xfree(void *x, size_t size);
 
 /* hash.c */
 struct st_table *rb_hash_tbl_raw(VALUE hash);
+VALUE rb_hash_new_with_size(st_index_t size);
 VALUE rb_hash_has_key(VALUE hash, VALUE key);
 VALUE rb_hash_default_value(VALUE hash, VALUE key);
 VALUE rb_hash_set_default_proc(VALUE hash, VALUE proc);

vm.c

@@ -2694,7 +2694,7 @@ m_core_hash_from_ary(VALUE self, VALUE ary)
 static VALUE
 core_hash_from_ary(VALUE ary)
 {
-    VALUE hash = rb_hash_new();
+    VALUE hash = rb_hash_new_with_size(RARRAY_LEN(ary) / 2);
 
     RUBY_DTRACE_CREATE_HOOK(HASH, (Check_Type(ary, T_ARRAY), RARRAY_LEN(ary)));
     return core_hash_merge_ary(hash, ary);