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

[Nick Gasson (Arm Technology China)]

> 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