[5.4] In DI, cache whether a memory object is a box. #37484
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Boxes tend to have a small number of uses, so frequently finding the unique projection isn't too bad. Non-boxes, however, can have a large number of uses: for example, a class instance has expected uses proportionate to the number of stored properties. So if we do a linear scan of the uses on a non-box instruction, we'll scale quadratically. Fixes SR-14532.
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DougGregor
approved these changes
May 18, 2021
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Boxes tend to have a small number of uses, so frequently finding the unique projection isn't too bad. Non-boxes, however, can have a large number of uses: for example, a class instance has expected uses proportionate to the number of stored properties. So if we do a linear scan of the uses on a non-box instruction, we'll scale quadratically.
Fixes SR-14532. 5.4 version of #37328.
Explanation: Avoid a quadratic slowdown in DI in class initializers when a class reference has many uses. This is typical when a class has a large number of properties.
Scope: The change is to Swift’s definitive initialization pass, which is run on all functions
Risk: Low
Testing: Manual; no straightforward way to test future regressions here
Issue: rdar://77217125
Reviewer: Erik Eckstein (on original PR)