RFR: JDK-8201483 Make it possible to disable JVM features

[David Holmes]

On 12/04/2018 11:33 PM, Magnus Ihse Bursie wrote:
> On 2018-04-12 14:15, David Holmes wrote:
>> Hi Magnus,
>>
>> On 12/04/2018 9:39 PM, Magnus Ihse Bursie wrote:
>>> It is currently easy to add new JVM features to the JVM build, but it 
>>> is not possible to remove features.
>>>
>>> With this change, features can be both added or removed from the 
>>> default set. They are added using --with-jvm-features=f1,f2 and 
>>> removed using --with-jvm-features=-f1,-f2. The syntax can be 
>>> combined, so --with-jvm-features=dtrace,-nmt will enable dtrace but 
>>> disable native memory tracking.
>>
>> I need to point out that we have never tested disabling individual VM 
>> features likes this. They are either all on, or all off for the 
>> minimal VM! There may be implicit dependencies between features.
> 
> Well, I have. :-) However, I don't do that regularly, and changes might 
> very well have crept in. As always, if you build something non-standard 
> that is not regularly tested, you're on your own.

Feels to me like you've taken away the safety-fence and are encouraging 
people to attempt these unsupported configurations. Whether that was 
your intent or not.

> In any case, the purpose of this is not so much to disable existing JVM 
> features (after all, no one has really been missing this functionality), 
> as to pave the way for the upcoming patch for including/excluding 
> individual GCs.

Surely a GC selection flag would have sufficed.

David

> /Magnus
> 
>>
>> David
>>
>>> I also included some additional code cleanup and fixes, such as 
>>> printing the JVM feature set at the summary.
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8201483
>>> WebRev: 
>>> http://cr.openjdk.java.net/~ihse/JDK-8201483-disable-JVM-features/webrev.01 
>>>
>>>
>