Skip __class__ lookup in object_isinstance for standard getattro (#7303)

In object_isinstance(), when is_subtype() returns false, the __class__
attribute lookup via get_attribute_opt is redundant for objects using
standard __getattribute__, since __class__ is a data descriptor on
object that always returns obj.class().

crates/vm/src/protocol/object.rs

@@ -4,8 +4,8 @@
 use crate::{
     AsObject, Py, PyObject, PyObjectRef, PyRef, PyResult, TryFromObject, VirtualMachine,
     builtins::{
-        PyBytes, PyDict, PyDictRef, PyGenericAlias, PyInt, PyList, PyStr, PyTuple, PyTupleRef,
-        PyType, PyTypeRef, PyUtf8Str, pystr::AsPyStr,
+        PyBaseObject, PyBytes, PyDict, PyDictRef, PyGenericAlias, PyInt, PyList, PyStr, PyTuple,
+        PyTupleRef, PyType, PyTypeRef, PyUtf8Str, pystr::AsPyStr,
     },
     common::{hash::PyHash, str::to_ascii},
     convert::{ToPyObject, ToPyResult},
@@ -565,9 +565,12 @@ impl PyObject {
             // PyType_Check(cls) - cls is a type object
             let mut retval = self.class().is_subtype(cls);
             if !retval {
-                // Check __class__ attribute, only masking AttributeError
-                if let Some(i_cls) =
-                    vm.get_attribute_opt(self.to_owned(), identifier!(vm, __class__))?
+                // __class__ is a data descriptor on object that always returns
+                // obj.class() under standard __getattribute__. Only do the
+                // expensive attribute lookup when getattro is overridden.
+                if !self.has_standard_getattro()
+                    && let Some(i_cls) =
+                        vm.get_attribute_opt(self.to_owned(), identifier!(vm, __class__))?
                     && let Ok(i_cls_type) = PyTypeRef::try_from_object(vm, i_cls)
                     && !i_cls_type.is(self.class())
                 {
@@ -584,14 +587,15 @@ impl PyObject {
                 )
             })?;
 
-            // Get __class__ attribute and check, only masking AttributeError
-            if let Some(i_cls) =
-                vm.get_attribute_opt(self.to_owned(), identifier!(vm, __class__))?
-            {
-                i_cls.abstract_issubclass(cls, vm)
+            let i_cls: PyObjectRef = if self.has_standard_getattro() {
+                self.class().to_owned().into()
             } else {
-                Ok(false)
-            }
+                match vm.get_attribute_opt(self.to_owned(), identifier!(vm, __class__))? {
+                    Some(cls) => cls,
+                    None => return Ok(false),
+                }
+            };
+            i_cls.abstract_issubclass(cls, vm)
         }
     }
 
@@ -769,6 +773,15 @@ impl PyObject {
         Err(vm.new_type_error(format!("'{}' does not support item deletion", self.class())))
     }
 
+    /// Returns true if the object uses the standard `__getattribute__`
+    /// (i.e. `object.__getattribute__`), meaning attribute access follows
+    /// the normal descriptor protocol without custom interception.
+    #[inline]
+    fn has_standard_getattro(&self) -> bool {
+        let getattro = self.class().slots.getattro.load().unwrap();
+        getattro as usize == PyBaseObject::getattro as *const () as usize
+    }
+
     /// Equivalent to CPython's _PyObject_LookupSpecial
     /// Looks up a special method in the type's MRO without checking instance dict.
     /// Returns None if not found (masking AttributeError like CPython).