[Foundation] Performance improvements for IndexPath bridging and comparison (#9339)#9653
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tkremenek merged 2 commits intoswiftlang:swift-4.0-branchfrom May 17, 2017
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…arison (swiftlang#9339) * [Foundation] Refactor the backing of IndexPath to favor stack allocations The previous implementation of IndexPath would cause a malloc of the underlying array buffer upon bridging from ObjectiveC. This impacts graphical APIs (such as UICollectionView or AppKit equivalents) when calling delegation patterns. Since IndexPath itself can be a tagged pointer and most often just a pair of elements it can be represented as an enum of common cases. Those common cases of empty, single, or pair can be represented respectively as no associated value, a single Int, and a tuple of Ints. These cases will be exclusively stack allocations, which is markably faster than the allocating code-path. IndexPaths that have a count greater than 2 will still fall into the array storage case. As an added performance benefit, accessing count and subscripting is now faster by aproximately 30% due to more tightly coupled inlining potential under whole module optimizations. Accessing count is also faster since it has better cache-line effeciency (lesson learned: the branch predictor is more optimized than pointer indirection chasing). Benchmarks performed on x86_64, arm and arm64 still pending results but should be applicable across the board. Resolves the following issues: https://bugs.swift.org/browse/SR-3655 https://bugs.swift.org/browse/SR-2769 Resolves the following radars: rdar://problem/28207534 rdar://problem/28209456 * [Foundation] remove temp IndexPath hashing that required bridging to ref types * [Foundation] IndexPath does not guarentee hashing to be the same as objc # Conflicts: # stdlib/public/SDK/Foundation/IndexPath.swift
…to calculate hash values (swiftlang#9476)
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@swift-ci please test |
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cc @parkera |
itaiferber
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May 17, 2017
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itaiferber
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Big diff, but a good performance improvement for IndexPath in the most common cases (small number of indexes in the path). LGTM
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Explanation: IndexPath was refactored to avoid un-nessicary allocations by utilizing an enum backing store that switches upon the payload size of the path. The previous implementation of IndexPath would cause a malloc of the underlying array buffer upon bridging from ObjectiveC. This impacts graphical APIs (such as UICollectionView or AppKit equivalents) when calling delegation patterns. Since IndexPath itself can be a tagged pointer and most often just a pair of elements it can be represented as an enum of common cases. Those common cases of empty, single, or pair can be represented respectively as no associated value, a single Int, and a tuple of Ints. These cases will be exclusively stack allocations, which is markably faster than the allocating code-path. IndexPaths that have a count greater than 2 will still fall into the array storage case. As an added performance benefit, accessing count and subscripting is now faster by aproximately 30% due to more tightly coupled inlining potential under whole module optimizations. Accessing count is also faster since it has better cache-line effeciency (lesson learned: the branch predictor is more optimized than pointer indirection chasing).
Scope: This is limited to the specific functionality of IndexPath. No API changes were made in this.
Radar (and possibly SR Issue):
https://bugs.swift.org/browse/SR-3655
https://bugs.swift.org/browse/SR-2769
rdar://problem/28207534
rdar://problem/28209456
Tracked by rdar://problem/32110579
Risk: The hashing algorithm was changed for IndexPaths so they no longer match their objc counterparts. This could result in runtime assumptions failing but any heterogenous collections of objc and non objc paths will be converted into a uniform collection either via bridges or AnyHashable boxes.
Testing: This was tested against the Foundation overlay unit tests