Remove class alloc check

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. 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).

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.

[Feature #21966]

1 files changed, 1 insertions, 5 deletions

diff --git a/object.c b/object.c
index c3241a198d..cb21741f31 100644
--- a/object.c
+++ b/object.c
@@ -2337,11 +2337,7 @@ class_call_alloc_func(rb_alloc_func_t allocator, VALUE klass)
 
     obj = (*allocator)(klass);
 
-    if (UNLIKELY(RBASIC_CLASS(obj) != klass)) {
-        if (rb_obj_class(obj) != rb_class_real(klass)) {
-            rb_raise(rb_eTypeError, "wrong instance allocation");
-        }
-    }
+    RUBY_ASSERT(rb_obj_class(obj) == rb_class_real(klass));
     return obj;
 }