JDK-8166840 : Synthetic bridge constructor in ArrayList$Itr blocks inlining
- Type: Sub-task
- Component: core-libs
- Sub-Component: java.util:collections
- Priority: P4
- Status: Resolved
- Resolution: Fixed
- OS: generic
- CPU: generic
- Submitted: 2016-09-28
- Updated: 2021-03-03
- Resolved: 2016-09-28
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| JDK 8 | JDK 9 |
|---|---|
8u152Fixed  |
9 b139Fixed |
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Description
As discussed recently on hotspot-compiler-dev[1], having a private class with no default constructor can lead to C2 failing to inline, due to the synthetic bridge constructor using a dummy argument of an uninitialized class. This is really a problem in C2, as well as something which could ultimately be resolved by nestmates... However, there is an easy workaround in adding an empty package-private constructor. In the most recently found case - a microbenchmark stressing MethodHandles.iteratedLoop - adding this to ArrayList$Itr lead to a 2.5-3x speedup. It thus makes sense to do this simple change in this particular case as a quick fix and solve the larger C2 issue separately. [1] http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2016-September/024407.html
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