On 05/25/2012 04:41 AM, Krystal Mok wrote:
Hi Mike,
On Fri, May 25, 2012 at 10:12 AM, Mike Duigou<mike.dui...@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
