RFR (XS): 8023472: C2 optimization breaks with G1

[Azeem Jiva]

Adding the GC dev alias to get their opinion.

--
Azeem Jiva
@javawithjiva

On Aug 22, 2013, at 6:34 AM, "Doerr, Martin" <martin.doerr at sap.com> wrote:

> Hi,
> 
> we have an addon to your G1 pre-barrier change which reduces
> its cost.
> It is possible to get rid of the do_load in inline_unsafe_load_store
> because we always know the value which gets overwritten.
> 
> In case of cmpxchg, the only value which might get overwritten
> is known in advance.
> In case of xchg, the old value is returned by the load_store.
> It is allowed to do the barrier after the xchg because there's
> no safepoint between them.
> 
> Please review
> http://cr.openjdk.java.net/~goetz/webrevs/opto_g1_barr/
> 
> Best regards,
> Martin and Goetz
> 
> 
> -----Original Message-----
> From: hotspot-compiler-dev-bounces at openjdk.java.net [mailto:hotspot-compiler-dev-bounces at openjdk.java.net] On Behalf Of Vladimir Kozlov
> Sent: Donnerstag, 22. August 2013 02:55
> To: hotspot compiler
> Subject: Re: RFR (XS): 8023472: C2 optimization breaks with G1
> 
> Thanks, Christian
> 
> On 8/21/13 4:04 PM, Christian Thalinger wrote:
>> 
>> On Aug 21, 2013, at 3:18 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:
>> 
>>> http://cr.openjdk.java.net/~kvn/8023472/webrev/
>>> 
>>> The load of previous value in G1 pre-barrier code (method GraphKit::g1_write_barrier_pre()) does not have control. It is from the time when G1 code was merged into Hotspot sources. I can only speculate that it was done to allow commoning with a preceding normal load if it exists and to avoid generation of 2 loads.
>>> This problem, I think, only affects loads from an array because a load from a field should be guarded by NULL check of object.
>>> 
>>> The fix is to put control on this load to always generate it after if (marking) check. I doubt it affect performance much. If there was load before the value should be in cache.
>> 
>> The fix looks good.  Can you at least do a refworkload run to do some due diligence?
> 
> Will do it tonight.
> 
>>> 
>>> Added regression test.
>> 
>> Could you change the name of the test?  Since we moved away from using bug ids the test name should more or less explain what it tests.  I suppose there will be more G1CrashTest's in the future :-)
> 
> It is external test. I don't want to change it much or its name.
> 
> Vladimir
> 
>> 
>> -- Chris
>> 
>>> 
>>> Thanks,
>>> Vladimir
>>> 

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