fix(es): avoid incorrect `instanceof` optimizations by teamleaderleo · Pull Request #12110 · swc-project/swc

Leo (teamleaderleo) · 2026-08-10T15:32:52Z

Description:

Avoid folding instanceof from operand shape alone. The operator may invoke Symbol.hasInstance, and an invalid right-hand operand may throw, so folds such as object-like values to true or primitive-like values to false are not generally valid.

For example:

({ __proto__: null }) instanceof Object

evaluates to false, but could previously fold to true.

This keeps SWC's existing Terser-compatible treatment of discarded instanceof expressions unchanged. Tests under tests/terser are unchanged.

The change removes the operand-shape-based instanceof folds in the minifier and expression simplifier and adds SWC-owned regressions for cases where the result is used.

in / pure_getters are unchanged.

BREAKING CHANGE:

None.

Validation:

cargo fmt --all -- --check
cargo clippy -p swc_ecma_transforms_optimization --all-targets -- -D warnings
cargo clippy -p swc_ecma_minifier --all-targets -- -D warnings
focused instanceof used-result minifier fixture

changeset-bot · 2026-08-10T15:32:57Z

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Latest commit: 9f838a5

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CLAassistant · 2026-08-10T15:33:01Z

All committers have signed the CLA.

Leo (teamleaderleo) · 2026-08-10T15:35:54Z

Updated the draft.

codspeed-hq · 2026-08-10T15:47:14Z

Merging this PR will not alter performance

✅ 200 untouched benchmarks
⏩ 61 skipped benchmarks¹

_{Comparing teamleaderleo:fix/instanceof-observable-effects (9f838a5) with main (394c7c9)²}

61 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩
No successful run was found on main (d7d7434) during the generation of this report, so 394c7c9 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

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Preserve observable instanceof evaluation across effect extraction, simplification, minifier ignored-result handling, and dead-branch cleanup. Remove stale instanceof fold eligibility, add fixture-based regression coverage, and update the affected expectations.

Austaras

As I stated in related issue

Leo (teamleaderleo) · 2026-08-11T09:30:51Z

Ok, so we keep the existing Terser assumptions for the unused instanceof case, but the other changes are acceptable?

Leo (teamleaderleo) · 2026-08-15T07:15:25Z

I updated this PR. I kept the existing discarded-result behavior and reduced the PR to the incorrect instanceof constant folding.

Austaras · 2026-08-15T10:16:46Z

@@ -1,4 +1,4 @@
-foo() instanceof bar();


This seems wrong

Okay, my interpretation: keep the existing discarded instanceof behavior generally,

but leave the full expression intact when one of its operands comes from an expression SWC already marked pure.

I updated the PR that way.

Sorry, I haven't seen foo has been marked as pure. foo() should be removed in this case.

The general idea is not to change any test case under tests/terser.

Thank you for clarifying. I restored the tests/terser expectations and removed the pure-marked instanceof exception. The PR is back to only fixing the incorrect constant folding.

Despite being useless, could you restore some harmless correct constant folding like 1 instanceof Object?

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Leo (teamleaderleo) requested review from a team as code owners August 10, 2026 15:32

Leo (teamleaderleo) marked this pull request as draft August 10, 2026 15:34

Leo (teamleaderleo) mentioned this pull request Aug 10, 2026

Optimizer can miscompile or discard instanceof evaluation #12111

Open

Leo (teamleaderleo) changed the title ~~fix(es): preserve observable instanceof semantics during optimization~~ fix(es): preserve observable instanceof evaluation Aug 10, 2026

Leo (teamleaderleo) force-pushed the fix/instanceof-observable-effects branch from 3724575 to d9cc58e Compare August 10, 2026 16:34

Leo (teamleaderleo) marked this pull request as ready for review August 10, 2026 16:44

Leo (teamleaderleo) marked this pull request as draft August 10, 2026 16:45

Leo (teamleaderleo) force-pushed the fix/instanceof-observable-effects branch from d9cc58e to 0f2ad51 Compare August 10, 2026 16:47

chatgpt-codex-connector Bot reviewed Aug 10, 2026

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Comment thread crates/swc_ecma_transforms_optimization/tests/instanceof.rs Outdated

Leo (teamleaderleo) force-pushed the fix/instanceof-observable-effects branch 2 times, most recently from 9a7abca to 551c0d2 Compare August 10, 2026 17:03

Leo (teamleaderleo) force-pushed the fix/instanceof-observable-effects branch from 551c0d2 to a8ee1a6 Compare August 10, 2026 17:47

Leo (teamleaderleo) marked this pull request as ready for review August 10, 2026 18:28

Austaras requested changes Aug 11, 2026

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fix(es): narrow instanceof fix to constant folding

a39678b

Leo (teamleaderleo) changed the title ~~fix(es): preserve observable instanceof evaluation~~ fix(es): avoid incorrect instanceof constant folding Aug 15, 2026

Austaras reviewed Aug 15, 2026

View reviewed changes

Leo (teamleaderleo) added 2 commits August 15, 2026 19:45

fix(es): preserve instanceof with pure_funcs

22e8892

fix(es): preserve pure call values in instanceof

c1ffb99

chatgpt-codex-connector Bot reviewed Aug 15, 2026

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Comment thread crates/swc_ecma_minifier/src/compress/optimize/mod.rs Outdated

fix(es): preserve pure-marked instanceof values

5628a67

Leo (teamleaderleo) changed the title ~~fix(es): avoid incorrect instanceof constant folding~~ fix(es): avoid incorrect instanceof optimizations Aug 15, 2026

fix(es): restore fold-only instanceof scope

9f838a5

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