gh-131798: JIT: Further optimize `_CALL_ISINSTANCE` for class tuples by tomasr8 · Pull Request #134543 · python/cpython · GitHub

We can already const eval isinstance(inst, cls) calls when both arguments are known, but only if cls is a single class (e.g. int).
This PR adds support for isinstance(inst, (cls1, cls2, ..., clsN)). This allows us to optimize for example:

isinstance(42, (int, str)) (to True)
isinstance(42, (bool, str)) (to False)

We can narrow to True even when only some of the classes are known, as long as inst is an instance of one of the known classes.

Issue: Better uop coverage in the JIT optimizer #131798

_BUILD_TUPLE is preventing us from optimizing out _POP_CALL_TWO_LOAD_CONST_INLINE_BORROW.
The bytecode is basically:

LOAD_CONST
LOAD_CONST
_BUILD_TUPLE
_POP_CALL_TWO_LOAD_CONST_INLINE_BORROW

To optimize this, we'd need some special handling for _BUILD_TUPLE in remove_unneeded_uops.

Got it, might be worth looking into next if you're up for it! Could be tricky, though.

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

I'll fix the PR hopefully this weekend.

So that was a lie 😆

Anyway, I think I addressed all your comments! :)

I have made the requested changes; please review again

Thanks for making the requested changes!

@brandtbucher: please review the changes made to this pull request.

Looks good, just a few suggestions:

Got it, might be worth looking into next if you're up for it! Could be tricky, though.

I'm confused, why can't we narrow to True? We can't remove the call, but the result is known.

Yep, that was a brainfart 😄 I somehow conflated narrowing to True/False with replacing the op, but we can obviously just narrow without removing the call as you said!

I had to pause and think for a second to figure out what this would even do. Nice edge case.

Could this be continue? We could still narrow to True later to handle cases like the one I called out above, right?

Suggested change

Yup, see #134543 (comment). I just needed to add some extra bookkeeping to know when we can replace the call and when not.

Maybe add a comment explaining that this is to protect against metaclasses definine __instancecheck__.

How would you formulate it? I don't think of it as specifically a guard for __instancecheck__ but basically PyObject_TypeCheck adapted to the JIT optimizer.

We're not only checking that the object is a subclass of type, we're also checking that it is an exact instance of type itself. We care about this second condition because it guarantees that __instancecheck__ doesn't exist (otherwise we would need to look it up to check if it exists or not).

Thanks for the clarification, I added a comment :)

Off-topic, but something I've been thinking about that you may want to check out next: removing bounds checks when indexing into strings and tuples with a constant index.

Thanks for the review! Super helpful as always :) I'll leave this open for a few days so @Fidget-Spinner can also have a look :)

Off-topic, but something I've been thinking about that you may want to check out next: removing bounds checks when indexing into strings and tuples with a constant index.

Sure, I can look into that! I guess this will also require knowing the string/tuple length?

Wait, wouldn't this only work on something that is isinstance(x, type), like the actual type itself, not any type like int? Sorry, I'm confused!

Hmm I don't believe so. If item is e.g. int then sym_matches_type(item, &PyType_Type) should return true

For example something like this seems to work:

JitOptRef ref_type = _Py_uop_sym_new_const(ctx, (PyObject *)&PyLong_Type);
TEST_PREDICATE(_Py_uop_sym_matches_type(ref_type, &PyType_Type), "int is not a type");

That''s kinda strange. The code for _Py_uop_sym_matches_type is:

_Py_uop_sym_get_type(sym) == typ

so it's a pointer comparison, not a subclass check.

The subclass check is done after with PyType_IsSubtype if that's what you mean? The current version of _CALL_ISINSTANCE (which does not handle tuples) works the same way though. @brandtbucher, what do you think?

I'm really sorry Tomas, but the more I look at the code, the more I feel the complexity is not worth it in this case. __instancecheck__ is tricky (and you've handled that well in the code), but I feel for the additional lines we get in the optimizer_bytecodes.c, we don't get much back in terms of optimization :(.

We need a test for a type with __instancecheck__, otherwise it LGTM, and you can merge after the test is added

This PR is stale because it has been open for 30 days with no activity.

tomasr8 added 2 commits May 22, 2025 22:20

Add news entry

1d582e8

Optimize _CALL_ISINSTANCE for class tuples

97aca36

tomasr8 changed the title gh-134369: JIT: Further optimize _CALL_ISINSTANCE for class tuples gh-131798: JIT: Further optimize _CALL_ISINSTANCE for class tuples May 22, 2025

bedevere-app Bot mentioned this pull request May 22, 2025

Better uop coverage in the JIT optimizer #131798

Open

Add more tests

467bcb9

tomasr8 commented May 22, 2025

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tomasr8 marked this pull request as ready for review May 22, 2025 22:34

tomasr8 requested a review from Fidget-Spinner as a code owner May 22, 2025 22:34

bedevere-app Bot added the awaiting review label May 22, 2025

tomasr8 requested a review from brandtbucher May 22, 2025 22:34

brandtbucher requested changes Jul 1, 2025

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Comment thread Python/optimizer_bytecodes.c

Comment thread Python/optimizer_bytecodes.c Outdated

bedevere-app Bot added awaiting changes and removed awaiting review labels Jul 1, 2025

tomasr8 added 5 commits July 5, 2025 23:17

Bail when encountering an unknown class

d2e339f

Remove an extra test case

0465c9a

Simplify code

088ccd8

Merge branch 'main' into jit-tuple-isinstance

fb28e06

Fix incorrect type

1e9b95f

bedevere-app Bot added awaiting change review and removed awaiting changes labels Jul 19, 2025

bedevere-app Bot requested a review from brandtbucher July 19, 2025 09:38

brandtbucher approved these changes Jul 29, 2025

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bedevere-app Bot added awaiting merge and removed awaiting change review labels Jul 29, 2025

Narrow to true but keep the isinstance call

a09b860

tomasr8 requested a review from brandtbucher July 29, 2025 18:59

brandtbucher approved these changes Jul 29, 2025

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Add a comment about __instancecheck__

17701d3

tomasr8 mentioned this pull request Aug 10, 2025

gh-131798: Remove bounds check when indexing into tuples with a constant index #137607

Merged

Fidget-Spinner reviewed Aug 20, 2025

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tomasr8 added 2 commits December 27, 2025 22:33

Merge remote-tracking branch 'upstream/main' into jit-tuple-isinstance

cf71d8b

Regenerate files

25ae15f

tomasr8 requested a review from markshannon as a code owner December 27, 2025 21:34

tomasr8 closed this Dec 28, 2025

tomasr8 reopened this Jan 8, 2026

Fidget-Spinner reviewed Jan 9, 2026

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github-actions Bot added the stale Stale PR or inactive for long period of time. label Apr 25, 2026

+                          for _ in range(n):
+                              # One of the classes is unknown, so we can't narrow
+                              # to True or False, only bool
+                              y = isinstance(42, (eval('str'), int))

+                      def testfunc(n):
+                          x = 0
+                          for _ in range(n):
+                              y = isinstance(42, ())

+                                      // It could potentially define its own __instancecheck__
+                                      // method so we can only deduce bool.
+                                      out = NULL;
+                                      break;

+                                  if (cls_o &&
+                                      // Ensure that item is an exact instance of `type` ensuring that
+                                      // there is no __instancecheck__ defined.
+                                      sym_matches_type(item, &PyType_Type) &&

		self.assertIn("_BUILD_TUPLE", uops)
		self.assertIn("_POP_CALL_TWO_LOAD_CONST_INLINE_BORROW", uops)

gh-131798: JIT: Further optimize `_CALL_ISINSTANCE` for class tuples by tomasr8 · Pull Request #134543 · python/cpython · GitHub

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gh-131798: JIT: Further optimize _CALL_ISINSTANCE for class tuples - #134543

gh-131798: JIT: Further optimize _CALL_ISINSTANCE for class tuples#134543tomasr8 wants to merge 12 commits intopython:mainpython/cpython:mainfrom tomasr8:jit-tuple-isinstancetomasr8/cpython:jit-tuple-isinstanceCopy head branch name to clipboard

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tomasr8 commented May 22, 2025 • edited by bedevere-app Bot Loading Uh oh! There was an error while loading. Please reload this page.

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gh-131798: JIT: Further optimize _CALL_ISINSTANCE for class tuples#134543
tomasr8 wants to merge 12 commits into
python:mainfrom
tomasr8:jit-tuple-isinstance

tomasr8 commented May 22, 2025 •
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