Issue #21966 has been reported by jhawthorn (John Hawthorn). ---------------------------------------- Feature #21966: Remove rb_obj_class runtime check on rb_define_alloc_func returned objects https://bugs.ruby-lang.org/issues/21966 * Author: jhawthorn (John Hawthorn) * Status: Open ---------------------------------------- I want to remove the following runtime check in `class_call_alloc_func`, replacing it with a `RUBY_ASSERT` (so we would still have the check, but only on a debug build). ``` if (rb_obj_class(obj) != rb_class_real(klass)) { rb_raise(rb_eTypeError, "wrong instance allocation"); } ``` This checks that the value returned from the function registered with `rb_define_alloc_func` is of the correct class. When this was first introduced in 1fe40b7cc5 (by Matz on 2001-10-03), allocation was done via user-defined `Object#allocate`, so it made sense to have a runtime check in release builds. Now that it's defined via `rb_define_alloc_func` in the C extension API, I don't think it's necessary. The check is surprisingly expensive. Removing it makes `Object.new` [about 10% faster](https://gist.github.com/jhawthorn/318ca96f471574080e4f08b6dce759f2). It allows the C compiler to optimize the call to the function pointer as a tail-call. Removing this also allows ZJIT/YJIT to call the function directly (ZJIT already does this by having a list of known safe allocation functions). The diff in the assembly from skipping this is substantial https://gist.github.com/jhawthorn/20834b654bec4ba0bafee729592d5156 (though much of that is `rb_class_real` which I also have plans to improve). There's no way for users to ever have seen this check, other than by writing a misbehaving C extension, which returns objects with the wrong class. https://github.com/ruby/ruby/pull/16571 I don't think this should be controversial, but I'm filing a feature to make sure it has visibility. If anyone has concerns I'll revert the change and discuss here. -- https://bugs.ruby-lang.org/