Fix #2563: Unconditionally emit mixin forwarders #6081
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test performance please |
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performance test scheduled: 1 job(s) in queue, 1 running. |
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Performance test finished successfully: Visit http://dotty-bench.epfl.ch/6081/ to see the changes. Benchmarks is based on merging with master (3c9935f) |
odersky
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Mar 17, 2019
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This controls when mixin forwarders are generated like the homonymous flag in scalac, for now we default to "junit" to keep our existing behavior, next commit will change it to "true" to match scalac.
This is what Scala 2.12+ does for cold performance reasons (see scala#5928 for more details) and we should align ourselves with them when possible. About two years ago in scala#2563, Dmitry objected that we may not need to do that if we found another way to get performance back, or if newer JDKs improved the performance of default method resolution. It doesn't look like these things have happened so far (but there's some recent glimmer of hope here: https://bugs.openjdk.java.net/browse/JDK-8036580). Dmitry also said "I don't recall triggering bugs by emitting more forwarders rather then less.", but in fact since then I've found one case where the standard library failed to compile with extra forwarders causing name clashes, requiring a non-binary-compatible change: scala/scala@e3ef657. As of scala#6079 this is no longer a problem since we now emit mixin forwarders after erasure like scalac, but it still seems prudent to emit as many forwarders as scalac to catch potential name clash issues.
Now that -Xmixin-force-forwarders defaults to true, tests that showcase some behavior of mixin forwarders can be simplified since we need less indirections to generate them.
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This PR is a draft because it depends on #6079, only the last three commits are new.