[string] ARC hack around hashValue #14118
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Avoid a source of ARC for hashValue, which is perf-sensitive especially for hashed collection growth. This improves the Dictionary benchmark by around 30%.
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Build comment file:Optimized (O)Regression (13)
Improvement (19)
No Changes (326)
Unoptimized (Onone)Regression (12)
Improvement (9)
No Changes (337)
Hardware Overview |
lorentey
reviewed
Jan 24, 2018
| @@ -177,7 +191,9 @@ extension String : Hashable { | |||
| /// your program. Do not save hash values to use during a future execution. | |||
| @_inlineable // FIXME(sil-serialize-all) | |||
| public var hashValue: Int { | |||
| return _guts._computeHashValue() | |||
| defer { _fixLifetime(self) } | |||
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Is the _fixLifetime necessary here? If it is, we should audit for missing cases elsewhere; I started eliding it under the assumption that self lifetime is guaranteed.
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Self lifetime is not guaranteed, and technically these are always necessary. _LegacyStringCore violated this rule all over the place, and never blew up, but doing this is a good idea. @atrick was working on a dependent-lifetime improvement wherein we could say that some values/expressions/properties have lifetime dependent on self, and that would alleviate this.
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I think there's some confusion because self has a guaranteed calling convention, which is completely different from a fixed lifetime--it doesn't give you any lifetime guarantee. Currently, the best thing you can do is defer { _fixLifetime }. That's terrible for multiple reasons, but I haven't had any time to introduce a reasonable alternative.
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Avoid a source of ARC for hashValue, which is perf-sensitive
especially for hashed collection growth.
This improves the Dictionary benchmark by around 30%.