RFR: 8221477: Inject os/cpu-specific constants into Unsafe from JVM

[David Holmes]

Hi Peter,

On 5/04/2019 5:37 pm, Peter Levart wrote:
> Hi Andrew,
> 
> For a casual reader (like me) the comments in the UnsafeConstants for 
> each field:
> 
>    69      * @implNote
>    70      * The actual value for this field is injected by the JVM.
> 
> ...make one wonder what is actually going on regarding the static 
> initializer at the end of the class. Is it actually executed? Is it 
> there just to silence the javac and prevent fields from becoming 
> compile-time constants?

That's what the earlier class comment is about:

  * @implNote
  *
  * The JVM injects hardware-specific values into all the static fields
  * of this class during JVM initialization. The static initialization
  * block exists to prevent the fields from being considered constant
  * variables, so the field values will be not be compiled directly into
  * any class that uses them.
  */

Cheers,
David

> Reading the patch further uncovers the secret:
> 
> 3645   // initialize the hardware-specific constants needed by Unsafe
> 3646 initialize_class(vmSymbols::jdk_internal_misc_UnsafeConstants(), 
> CHECK);
> 3647   jdk_internal_misc_UnsafeConstants::set_unsafe_constants();
> 
> If my understanding is correct, then the static initializer *is* 
> executed and then the fields' values are overwritten with injected values.
> 
> So perhaps it might be nice to write that down in the javadoc like:
> 
>    69      * @implNote
>    70      * The actual value for this field is injected by the JVM 
> immediately after the class initialization.
> 
> Or something similar in the class-level javadoc.
> 
> Just for the peace of mind of casual readers or perhaps someone that 
> might later add a field to this class and try to initialize it in the 
> static initializer, although I think this class is reserved for injected 
> fields only...
> 
> Regards, Peter
> 
> On 4/4/19 5:19 PM, Andrew Dinn wrote:
>> New webrev is now available. Re-reviews welcome.
>>
>> JIRA:   https://bugs.openjdk.java.net/browse/JDK-8221477
>> Webrev: http://cr.openjdk.java.net/~adinn/8221477/webrev.03
>>
>> This version should address all comments from Thomas, David and Coleen.
>>
>> Testing
>> Tier1 test passed.
>> Submit test passed.
>>
>> regards,
>>
>>
>> Andrew Dinn
>> -----------
>> Senior Principal Software Engineer
>> Red Hat UK Ltd
>> Registered in England and Wales under Company Registration No. 03798903
>> Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander
>