RFR: 8138659: Speed up InstanceKlass subclass discrimination (original) (raw)

Kim Barrett kim.barrett at oracle.com
Sat Oct 3 18:26:06 UTC 2015

Please review this change to speed up discrimination among the classes rooted at InstanceKlass.

We use 2 bits from InstanceKlass::_misc_flags (the _misc_kind field) for discrimination among the four kinds of InstanceKlass. We use the low 2 bits of _misc_flags to avoid a shift when accessing the field. This means that all the existing flags are being shifted up 2 bits. Added an accessor and a suite of predicates to InstanceKlass. Added kind argument to InstanceKlass constructor, which is used to set the _misc_kind field. Updated all callers to provide the appropriate value.

With these changes we can use the layout_helper-based Klass::oop_is_instance to get to the InstanceKlass case, and use the _misc_kind value to further discriminate from there. In particular, the concrete InstanceKlass case is (after inlining)

(klass->_layout_helper > 0) && ((static_cast<InstanceKlass*>(klass)->_misc_flags & 3) == 0)

We could do better with a single discriminator, but there doesn't appear to be a good place to add such in Klass, and we don't want to make Klass bigger.

Changed calls to Klass::oop_is_instanceXXX to instead call Klass::oop_is_instance then cast to InstanceKlass and use the corresponding kind_is_xxx predicate. Removed the no longer used Klass::oop_is_instanceXXX functions.

Removed unused InstanceRefKlass::cast. InstanceMirrorKlass::cast is retained because it is used. InstanceClassLoaderKlass::cast does not exist.

CR: https://bugs.openjdk.java.net/browse/JDK-8138659

Webrev: http://cr.openjdk.java.net/~kbarrett/8138659/webrev.00/

Testing: JPRT Aurora ad-hoc defaults + GC Nightly + Runtime Nightly

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