code review request for MacOS X compressed oops work around (7118648)

[Daniel D. Daugherty]

David,

Thanks for the quick review!


On 12/6/11 11:08 PM, David Holmes wrote:
> On 7/12/2011 2:51 PM, Vladimir Kozlov wrote:
>> Could you move the comment before #ifndef __APPLE__?
>
> I second that.

Will move the comment.


> If you didn't want the flag turned on at all you could use the 
> UNSUPPORTED_OPTION macro.

Just wanted it false by default. If I disable it entirely, then
it will be (slightly) harder to diagnose via 7118647...

Dan


>
> David
>
>> Vladimir
>>
>> Daniel D. Daugherty wrote:
>>> Greetings,
>>>
>>> The UseCompressedOops flag is enabled ergonomically in a 64-bit VM.
>>> With MacOS X, we're seeing intermittent crashes in our automated
>>> build/test system (JPRT) that look like this:
>>>
>>> Internal Error
>>> (/private/tmp/jprt/P1/050642.jmelvin/source/src/share/vm/runtime/virtualspace.cpp:460), 
>>>
>>> pid=53634, tid=4531154944
>>> # assert((_noaccess_prefix != 0) == (UseCompressedOops && _base !=
>>> NULL && (size_t(_base + _size) > OopEncodingHeapMax) &&
>>> Universe::narrow_oop_use_implicit_null_checks())) failed:
>>> noaccess_prefix should be used only with non zero based compressed oops
>>>
>>> I have filed the following bug to track that failure mode:
>>>
>>> 7118647 3/3 compressed oops crashes on MacOS X with JPRT GCBasher test
>>>
>>> In order to make more reliable progress on getting MacOS X fully
>>> enabled in JPRT, I'm planning to disable compressed oops by default
>>> in the 64-bit MacOS X VM until 7118647 can be fixed.
>>>
>>> The work around is trivial so I'm including it here rather than
>>> generate a webrev:
>>>
>>> diff -r a5a9db0e0203 src/share/vm/runtime/arguments.cpp
>>> --- a/src/share/vm/runtime/arguments.cpp Tue Dec 06 07:23:42 2011 -0800
>>> +++ b/src/share/vm/runtime/arguments.cpp Tue Dec 06 15:29:17 2011 -0800
>>> @@ -1359,9 +1359,12 @@
>>> // by ergonomics.
>>> if (MaxHeapSize <= max_heap_for_compressed_oops()) {
>>> #if !defined(COMPILER1) || defined(TIERED)
>>> +#ifndef __APPLE__
>>> + // disable UseCompressedOops by default on MacOS X until 7118647 is
>>> fixed
>>> if (FLAG_IS_DEFAULT(UseCompressedOops)) {
>>> FLAG_SET_ERGO(bool, UseCompressedOops, true);
>>> }
>>> +#endif // !__APPLE__
>>> #endif
>>> #ifdef _WIN64
>>> if (UseLargePages && UseCompressedOops) {
>>>
>>>
>>> What the above code does is prevent the UseCompressedOops flag
>>> from being set to true ergonomically on MacOS X. It can still be
>>> set to true with the '-XX:+UseCompressedOops' option.
>>>
>>> I'm planning to push this fix to RT_Baseline for the HSX-23-B08
>>> snapshot. Jim Melvin has already reviewed the fix so I just need
>>> one more reviewer.
>>>
>>> Thanks, in advance, for any additional reviews.
>>>
>>> Dan
>>>