[10] RFR: 8182487: Add Unsafe.objectFieldOffset(Class, String)

Claes Redestad claes.redestad at oracle.com
Wed Jun 21 12:32:39 UTC 2017

Hi Mikael,

On 06/21/2017 09:49 AM, Mikael Gerdin wrote:
> Hi Claes,
> This looks like a good change but I do have some comments on exception 
> management details.
> On 2017-06-20 15:14, Claes Redestad wrote:
>> Hi,
>> as a startup optimization, we can avoid a number of reflective 
>> operations on
>> variouscore classes by adding a specialized objectFieldOffset method 
>> taking
>> Class and String rather than Field:
>> Webrev: https://bugs.openjdk.java.net/browse/JDK-8182487
>> Hotspot: http://cr.openjdk.java.net/~redestad/8182487/hotspot.00
> The UNSAFE_ENTRY macro expands to among other things a transition into 
> _thread_in_vm (normal JNI code runs as _thread_in_native).
> When running "in VM" we are (fairly) free to do things such as 
> resolving JNI handles and looking at the fields of classes so that is 
> clearly correct for find_field_offset.
> For code running in VM we have to avoid invoking functions in the 
> public JNI api since those entry points perform transitions from 
> native to VM, causing assertions to fire and other potentially nasty 
> stuff happening.
> Instead of reusing the throw_new helper function you should do 
> something similar to the other find_field_offset function and do
> if (offset < 0) {
>   THROW_0(vmSymbols::java_lang_InternalError());
> }
> return field_offset_from_byte_offset(offset);

makes sense.

I've also cleaned up and removed static blocks as suggested by
Brent Christian, and even though I reverted the throw_new method
back to its original place I think it's reasonable to clean it up as
suggested by Christian Thalinger:

JDK: http://cr.openjdk.java.net/~redestad/8182487/jdk.01/
Hotspot: http://cr.openjdk.java.net/~redestad/8182487/hotspot.01/



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