Cast-free type tags #1306
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Cast-free type tags #1306
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By replacing the structural type with an abstract [existential] type, the subst/unsubst code sees that a tagged type is equal to the untagged type, so it doesn't need to cast. * Now, subst cannot be used to derive unsafeCoerce in concert with singleton type patterns. * Tags/untags no longer form an optimization barrier for inlining optimizers, because there are neither internal nor external inconsistencies when tagging.
xuwei-k
approved these changes
Dec 16, 2016
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@S11001001 You mention that tags with casts form an inlining barrier. Did you come across a concrete case where this version performs better? |
puffnfresh
approved these changes
Dec 16, 2016
| "k.@@" should { | ||
| "be abstract" in { | ||
| val r = (List[Int](42) ++ List[Int @@ Mult]()).toSet | ||
| r: Set[Any] |
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@edmundnoble No, it's just been bugging me since either @sjrd or @DarkDimius (or maybe both? 😕) mentioned casting as a problem at Scala Days NYC. |
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Not me. So it must have been @DarkDimius. |
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I remember @DarkDimius mentioning this during a dotty linker talk, saying casts can mess up the call graph analysis and end up disabling a lot of the specialization optimizations in linker. Perhaps that's it. |
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By replacing the structural type with an abstract [existential] type, the
subst/unsubstcode sees that a tagged type is equal to the untagged type, so it doesn't need to cast.substcannot be used to deriveunsafeCoercein concert with singleton type patterns.