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

[David Holmes]

On 13/04/2018 5:25 PM, Volker Simonis wrote:
> On Thu, Apr 12, 2018 at 11:30 PM, David Holmes <david.holmes at oracle.com> wrote:
>> 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.
> 
> I think that would be great. If people would try out different
> configurations AND fix them that would be a great code clean-up for
> HotSpot. I've recently tried various "unsupported" configurations
> (i.e. minimal plus some selected features) and found that the
> resulting build failures are mostly artificial because the
> corresponding features aren't correctly ifdefed.

These features were never expected to be individually selectable. The 
minimal VM was a special case. There is likely more work needed than 
just build time ifdefs to allow this to actually work.

For fun try building on 64-bit and ask for no compiler2 to see what 
happens. It makes no sense. But there are no constraints implemented so 
you can ask for nonsensical things. That's not a good thing in my book.

David
-----


> 
>>
>>> 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
>>>>>
>>>
>>