RFR: 8214512: Jtreg test compiler/c2/Test8062950.java fails on ARM32

[dean.long at oracle.com]

Nick, can you please confirm that you are covered under the

# ARM Ltd. - OpenJDK, MySQL

OCA agreement?  I'm guessing Arm Technology China is under ARM Ltd., judging
by your email domain, but please confirm.

dl

On 12/11/18 5:30 PM, dean.long at oracle.com wrote:
> I can help you commit it, but first I think another review would be good.
>
> dl
>
> On 12/11/18 5:15 PM, Nick Gasson (Arm Technology China) wrote:
>>> Hi Nick.  This change looks good to me.
>> Thanks Dean. Could you help me to commit this patch if it doesn't 
>> need other reviews?
>>
>> Nick
>>
>>
>>> -----Original Message-----
>>> From: dean.long at oracle.com <dean.long at oracle.com>
>>> Sent: 12 December 2018 06:05
>>> To: Nick Gasson (Arm Technology China) <Nick.Gasson at arm.com>; hotspot-
>>> dev at openjdk.java.net; Boris Ulasevich <boris.ulasevich at bell-sw.com>
>>> Cc: nd <nd at arm.com>; hotspot-compiler-dev at openjdk.java.net compiler
>>> <hotspot-compiler-dev at openjdk.java.net>; aarch32-port-
>>> dev at openjdk.java.net
>>> Subject: Re: RFR: 8214512: Jtreg test compiler/c2/Test8062950.java 
>>> fails on
>>> ARM32
>>>
>>> Hi Nick.  This change looks good to me.
>>>
>>> dl
>>>
>>> On 12/2/18 10:06 PM, Nick Gasson wrote:
>>>> Hi,
>>>>
>>>> Could someone please help me review this fix for a test failure on 
>>>> ARM32:
>>>>
>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8214512
>>>> Webrev: http://cr.openjdk.java.net/~njian/8214512/webrev.0/
>>>>
>>>> This fixes two assertion failures related to biased locking when C2's
>>>> bias inlining feature is disabled (-XX:-OptoBiasInlining):
>>>>
>>>> MacroAssembler::atomic_cas_bool takes three register arguments plus a
>>>> temporary register for use in the CAS loop. If the caller passes
>>>> `noreg' as this temporary register (which happens in the
>>>> !OptoBiasInlining call path from MacroAssembler::fast_lock) then
>>>> MacroAssembler::atomic_cas_bool will default to using LR as a
>>>> temporary. But it's possible with C2 that LR is one of the other three
>>>> arguments which then triggers an assert_different_registers assertion
>>>> failure. Fix this by supplying an additional scratch register to
>>>> MacroAssembler::fast_lock if !OptoBiasInlining.
>>>>
>>>> In the !OptoBiasInlining case MacroAssembler::fast_lock calls
>>>> MacroAssembler::biased_locking_enter to handle the case where the mark
>>>> word contains the biased lock pattern which fast_lock wouldn't
>>>> otherwise see if OptoBiasInlining was true. However in the case that
>>>> we fail to acquire the biased lock this falls through to label
>>>> `failed' and runs the normal fast_lock code that implicitly assumes
>>>> the mark word does not have the bias pattern. This means we can end up
>>>> with a BasicLock where _displaced_header contains the biased lock
>>>> pattern which is an illegal state and subsequently triggers an
>>>> assertion failure in ObjectSynchronizer::fast_exit.
>>>>
>>>> The right thing to do here is branch to `done' whenever the bias lock
>>>> pattern is present and let the runtime handle the failure case.  Also
>>>> edited the comment describing MacroAssembler::biased_locking_enter to
>>>> more accurately describe what it does (the comment about `slow_case'
>>>> being optional is wrong).
>>>>
>>>> Jtreg test compiler/c2/Test8062950.java now passes on ARM32 with this
>>>> patch.
>>>>
>>>> Thanks,
>>>> Nick
>