Heapster Rust layout optimization for option-like types #1627
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added 5 commits
April 14, 2022 12:17
…n of Option-like types
…r is going to work for now...
…the recognizers for niches simplify the payload shape first
eddywestbrook
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subsystem: heapster
ChrisEPhifer
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Apr 14, 2022
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PR: ready to merge
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This PR changes Heapster's Rust layout engine to handle the option-like type optimization of Rust.
An "option-like" type in Rust is a non-recursive enum with two variants, one of which is empty and one of which is not. The canonical example is
Option<T>. The field type of the non-empty variant is called the "payload". If the payload of an option-like type has illegal values, called "niches", then Rust optimizes the layout of the option-like type so that the empty variant is represented as a value in one of these niches. So, for example,Option<&T>is laid out like a&Tbut where the null pointer (which is an invalid reference) represents theNoneconstructor, whileOption<T>whenTis itself an enum withNconstructors is laid out asTbut with an extraN+1th constructor for `None.This PR also adds a test case, the
list64_tailfunction inrust_data.rs, to test out this option-like type optimization.