On Tue, 4 Jul 2023 22:03:58 GMT, John R Rose <jr...@openjdk.org> wrote:
>>> Hmm, I think that issue refers to code that have explicit non-Object >>> parameter types (like `X::equals(Object)boolean` in the issue's sample). >>> This method already have both arguments as `Object`, so I don't think >>> there's any type-specific inlining opportunities. >> >> If that's true, then perhaps those (and some other) locations got that idea >> wrong: >> * >> https://github.com/openjdk/jdk/blob/faf1b822d03b726413d77a2b247dfbbf4db7d57e/src/java.base/share/classes/java/util/Collections.java#L5712-L5719 >> * >> https://github.com/openjdk/jdk/blob/faf1b822d03b726413d77a2b247dfbbf4db7d57e/src/java.base/share/classes/java/util/AbstractMap.java#L577-L585 >> >> Maybe @rose00 could clarify that? >> >> FWIW, I also note that `HashMap` does not use similar private static >> methods; it uses `Objects.equals(Object, Object)` and `Objects.hashCode` >> overloads that take parameters. > > I wrote a little case study on `Objects::equals` that talks about how it > should optimize, when it does, why it doesn’t, and how (maybe) to fix that. > > https://cr.openjdk.org/~jrose/jvm/equals-profile.html > https://cr.openjdk.org/~jrose/jvm/equals-profile.md > > This is also attached to the JBS bug. > > The work on [JDK-8026251](https://bugs.openjdk.org/browse/JDK-8026251) with > the `TypeProfileLevel` switch bring us closer to correctly optimizing > `Objects::equals` in more cases. Sadly, JDK refactoring by itself will not > get all the way to where we want to go. The JVM’s profiling logic needs > tweaking. Thanks @rose00 for the writeup and @pavelrappo for asking pertinent followup questions. For me the issue here is that there is a bunch of lore about avoiding `Objects::equals` and it's embodied in comments like this: > NB: Do not replace with Object.equals until JDK-8015417 is resolved. These comments are almost exactly ten years old, and we can't seem to find any evidence showing that a slowdown occurred if `Objects::equals` were used. The comments are a "dead hand" at this point. Is there a way to demonstrate whether there is or is not any difference when using `Objects::equals`? As a side note, I observe that the `eq` method in Collections.java and AbstractMap.java does this: return o1 == null ? o2 == null : o1.equals(o2); whereas `Objects::equals` will test `o1 == o2` and skip the `equals()` call if the references are non-null and equals. This might confound the comparison a bit. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/14752#discussion_r1255024667
