gh-121459: Deferred LOAD_ATTR (methods) #124101

Fidget-Spinner · 2024-09-15T10:30:04Z

Issue: PEP 703 Deferred reference counts #121459

Objects/object.c

Objects/dictobject.c

Include/internal/pycore_stackref.h

Objects/typeobject.c

Python/bytecodes.c

Objects/typeobject.c

Include/internal/pycore_dict.h

Objects/dictobject.c

colesbury · 2024-09-16T19:18:04Z

Objects/object.c

+        _PyObject_TryGetInstanceAttributeStackRef(obj, name, method)) {
+        if (!PyStackRef_IsNull(*method)) {
+            PyStackRef_XCLOSE(descr_st);


This overwrites *method, but not descr_st, so descr_st may store a stack reference that's not visible to the GC. That seems fragile and potentially a problem.

We want to restore *method to descr_st after this in some cases (see below). I guess the safe thing would be to convert it to a strong ref?

I don't think you need both *method and descr_st if you refactor these function.

Alternatively take in a second _PyStackRef * to store the temporary values in a place that is visible to the GC.

I think the second approach needs to wait for @mpage 's PR on adding an extra stack slot?

Oh we don't, we can use self_or_null.

Ok nevermind, this does indeed need Matt's fix. Because we unconditionally write to self_or_null now.

Objects/object.c

Fidget-Spinner · 2024-10-29T00:45:23Z

@colesbury gentle ping to respond to the review comments please

Co-Authored-By: Sam Gross <[email protected]>

markshannon

This seems to add a lot of complexity.
LOAD_ATTR is already the slow path, do you have performance numbers?
Without a significant speedup, this doesn't seem worth it.

For future reference, for large PRs like this, could you make sure there is specific issue where the change can be discussed and justified, rather adding them to a more general (and vague) issue like #121459

markshannon · 2024-11-14T11:52:11Z

Include/internal/pycore_object.h

+// This belongs here rather than pycore_stackref.h because including pycore_object.h
+// there causes a circular include.
+static inline _PyStackRef
+_Py_TryXGetStackRef(PyObject **ptr)


Please add clear comments as to what this does.
What is it "trying" to do? What happens if it fails? etc.

markshannon · 2024-11-14T11:53:00Z

Include/internal/pycore_object.h

@@ -816,7 +834,8 @@ extern int _PyObject_StoreInstanceAttribute(PyObject *obj,
                                            PyObject *name, PyObject *value);
 extern bool _PyObject_TryGetInstanceAttribute(PyObject *obj, PyObject *name,
                                              PyObject **attr);
-
+extern bool _PyObject_TryGetInstanceAttributeStackRef(PyObject *obj, PyObject *name,


Likewise. Anything with "try" in the name needs to be carefully documented.

markshannon · 2024-11-14T11:53:37Z

Include/internal/pycore_object.h

@@ -863,6 +882,7 @@ PyAPI_FUNC(PyObject*) _PyObject_LookupSpecialMethod(PyObject *self, PyObject *at
 extern int _PyObject_IsAbstract(PyObject *);

 PyAPI_FUNC(int) _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method);
+PyAPI_FUNC(int) _PyObject_GetMethodStackRef(PyObject *obj, PyObject *name, _PyStackRef *method, _PyStackRef *spare);


Why does this have a "spare" parameter?

markshannon · 2024-11-14T11:54:41Z

Include/internal/pycore_object.h

@@ -900,6 +920,8 @@ PyAPI_DATA(int) _Py_SwappedOp[];

 extern void _Py_GetConstant_Init(void);

+extern void _PyType_LookupStackRef(PyTypeObject *type, PyObject *name, _PyStackRef *result);


Can't this fail? It should probably return an int indicating success/failure/error.

markshannon · 2024-11-14T12:01:38Z

Python/bytecodes.c

@@ -2054,20 +2054,19 @@ dummy_func(
            #endif  /* ENABLE_SPECIALIZATION */
        }

-        op(_LOAD_ATTR, (owner -- attr, self_or_null if (oparg & 1))) {
+        op(_LOAD_ATTR, (owner -- attr[1], self_or_null[1] if (oparg & 1))) {


Suggested change

op(_LOAD_ATTR, (owner -- attr[1], self_or_null[1] if (oparg & 1))) {

op(_LOAD_ATTR, (owner -- attr, self_or_null if (oparg & 1))) {

You don't need the arrays. The code generator should spill the values to the in-memory stack if necessary.

markshannon · 2024-11-14T12:02:21Z

Python/compile.c

@@ -1335,6 +1335,11 @@ optimize_and_assemble_code_unit(struct compiler_unit *u, PyObject *const_cache,
        goto error;
    }

+    /* Reserve an extra word on the stack to ensure there is space for uops to


Why is this necessary?

Fidget-Spinner · 2024-11-14T12:20:09Z

@markshannon I'm tempted to wait for Matt to land his LOAD_ATTR specialization PR then take a look again. It may be possible that specialization makes this PR less useful.

Fidget-Spinner added 3 commits September 15, 2024 18:20

Make LOAD_ATTR deferred

be5fa22

_PyObject_TryGetInstanceAttributeStackRef

f93588a

cleanup

1050673

Fidget-Spinner added the skip news label Sep 15, 2024

Fidget-Spinner requested a review from colesbury September 15, 2024 10:30

Fidget-Spinner requested review from markshannon and methane as code owners September 15, 2024 10:30

bedevere-app bot added the awaiting core review label Sep 15, 2024

bedevere-app bot mentioned this pull request Sep 15, 2024

PEP 703 Deferred reference counts #121459

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fix default build

f0afe98

Fidget-Spinner commented Sep 15, 2024

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Objects/object.c Outdated Show resolved Hide resolved

colesbury reviewed Sep 16, 2024

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partially address review

c3fe777

Fidget-Spinner and others added 3 commits November 14, 2024 18:49

Merge remote-tracking branch 'upstream/main' into deferred_method_final

c3448d9

Partially address review

ba19770

Co-Authored-By: Sam Gross <[email protected]>

Write to second slot on stack

d383437

Fidget-Spinner requested a review from iritkatriel as a code owner November 14, 2024 11:32

markshannon reviewed Nov 14, 2024

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gh-121459: Deferred LOAD_ATTR (methods) #124101

gh-121459: Deferred LOAD_ATTR (methods) #124101

Fidget-Spinner commented Sep 15, 2024 •

edited by bedevere-app bot

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colesbury Sep 16, 2024

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colesbury Oct 29, 2024 •

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Fidget-Spinner commented Nov 14, 2024

		@@ -900,6 +920,8 @@ PyAPI_DATA(int) _Py_SwappedOp[];

		extern void _Py_GetConstant_Init(void);

		extern void _PyType_LookupStackRef(PyTypeObject type, PyObject name, _PyStackRef *result);

	op(_LOAD_ATTR, (owner -- attr[1], self_or_null[1] if (oparg & 1))) {
	op(_LOAD_ATTR, (owner -- attr, self_or_null if (oparg & 1))) {

gh-121459: Deferred LOAD_ATTR (methods) #124101

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gh-121459: Deferred LOAD_ATTR (methods) #124101

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Fidget-Spinner commented Sep 15, 2024 • edited by bedevere-app bot Loading

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Fidget-Spinner commented Oct 29, 2024

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Fidget-Spinner commented Nov 14, 2024

Fidget-Spinner commented Sep 15, 2024 •

edited by bedevere-app bot

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colesbury Oct 29, 2024 •

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