Review Request CR#7118743 : Alternative Hashing for String with Hash-based Maps (original) (raw)

Rémi Forax forax at univ-mlv.fr
Fri May 25 16:25:49 UTC 2012

On 05/25/2012 04:41 AM, Krystal Mok wrote:

Hi Mike,

On Fri, May 25, 2012 at 10:12 AM, Mike Duigou<mike.duigou at oracle.com>wrote:

On May 24 2012, at 16:32 , Vitaly Davidovich wrote:

That's a bit odd as I thought the Klass object in the VM stored something like 7 supers, which includes interfaces (if I'm not mistaken). I know that instanceof checks against final classes are optimized into a simple cmp against an address, but I'm surprised that a check against an interface for classes in a very shallow type hierarchy is up to x25 slower. Do you know why that is Mike? Did you ask the compiler guys by chance? I didn't actually look much further into it other than to write a small microbenchmark to make sure it was the instanceof check. I tested a couple of different configs of classes and interfaces and supers. I was able to validate that 'x instanceof String' was as fast as 'x.getClass() == String.class' and that other cases were slower. Inheritance checks seemed faster than checks on interfaces. That was enough to tell me that I was on the wrong track with 'x instanceof Hashable' as a way to determine which hash algorithm to use. At least for C2 server compiler. FYI, "x.getClass() == String.class" will be just as fast when 7170463 [1] is in. @Vitaly The fast subtype checking algorithm in HotSpot treats interfaces as "secondary supers", which goes through a slow path. The super type display in instanceKlass is only for "primary supers", which does not include interfaces for instance classes. You may want to check out the details in this paper [2].

Also the code of HashMap is used from too many callsites so the JIT will not be able to use any profiling information.

Definition 5: A class T is a secondary type iff T is an interface or an array of a secondary type. Every type is either a primary type or a secondary type but not both. Regards, Kris [1]: http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2012-May/007730.html [2]: http://dl.acm.org/citation.cfm?id=583821

cheers, Rémi

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