RFR: 8346567: Make Class.getModifiers() non-native

[Coleen Phillimore]

On Mon, 9 Dec 2024 19:46:43 GMT, Chen Liang <liach at openjdk.org> wrote:

>> The Class.getModifiers() method is implemented as a native method in java.lang.Class to access a field that we've calculated when creating the mirror.  The field is final after that point. The VM doesn't need it anymore, so there's no real need for the jdk code to call into the VM to get it.  This moves the field to Java and removes the intrinsic code.  I promoted the compute_modifiers() functions to return int since that's how java.lang.Class uses the value.  It should really be an unsigned short though.
>> 
>> There's a couple of JMH benchmarks added with this change.  One does show that for array classes for non-bootstrap class loader, this results in one extra load which in a long loop of just that, is observable.  I don't think this is real life code. The other benchmarks added show no regression.
>> 
>> Tested with tier1-8.
>
> src/hotspot/share/classfile/javaClasses.cpp line 1504:
> 
>> 1502:   macro(_reflectionData_offset,      k, "reflectionData",      java_lang_ref_SoftReference_signature, false); \
>> 1503:   macro(_signers_offset,             k, "signers",             object_array_signature, false); \
>> 1504:   macro(_modifiers_offset,           k, vmSymbols::modifiers_name(), int_signature,    false)
> 
> Do we need a trailing semicolon here?

yes. it is needed.

> src/java.base/share/classes/java/lang/Class.java line 1315:
> 
>> 1313: 
>> 1314:     // Set by the JVM when creating the instance of this java.lang.Class
>> 1315:     private transient int modifiers;
> 
> If this is set by the JVM, can this be marked `final` so JIT compiler can trust this field? Also preferable if we can move this together with components/signers/classData fields.

The JVM rearranges these fields so that's why I put it near the caller.  Let me check if final compiles.

Edit: it looks better with the other fields though.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22652#discussion_r1876712191
PR Review Comment: https://git.openjdk.org/jdk/pull/22652#discussion_r1876713323