Fix unintentionally-quadratic COW copies on insertion#1626
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harlanhaskins merged 6 commits intomainfrom Mar 16, 2026
Merged
Fix unintentionally-quadratic COW copies on insertion#1626harlanhaskins merged 6 commits intomainfrom
harlanhaskins merged 6 commits intomainfrom
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Nice. Please give me a chance to review on Monday before merging! |
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| @testable import Testing | ||
| @_spi(Experimental) @_spi(ForToolsIntegrationOnly) @testable import Testing |
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Nitpick: anything experimental actually being used here?
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Just Test.Clock, but also that doesn't need ForToolsIntegrationOnly
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I think you have that backwards?
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Ah, turns out you only need one of the SPI tags to get the declaration. So I could pick either or leave both
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This fixes an accidentally-quadratic COW copy of a subgraph whenever an item is inserted.
Motivation:
For large Swift Testing test suites with many parameterized test cases, we accidentally tripped this specific pathological failure mode of the graph insertion code; when we insert a new child, we mutate each graph entry along the ancestor chain. If we mutate our local copy while leaving the existing copy in the tree, it causes a COW copy of the whole tree on every insertion.
For the attached stress test, without the
.take()it takes 2 seconds on my MacBook Pro, and with the.take()it takes 40 milliseconds.Modifications:
The bulk of this fix is to
.take()the child from the dictionary (Optional.take()sets the receiver to nil and returns the previous wrapped value).This is a very similar problem and fix to this PR in
swift-driver: swiftlang/swift-driver#654Checklist: