RFR(M): 8028515: PPC64 (part 113.2): opto: Introduce MemBarAcquire/ReleaseWide.

[David Holmes]

On 19/11/2013 2:19 AM, Lindenmaier, Goetz wrote:
> Hi David,
>
> as reply to your comment on the bug:
>
> Well, I sitll would need 2 different nodes, as on PPC we do
>    MemBarAcquireWide --> lwsync
>    MemBarReleaseWide --> lwsync
>    MemBarVolatile      --> sync.
> On Sparc, you even do 3 different operations.
>
> Or should I name them MemBarFenceAcquire and MemBarFenceRelease?
> This all depends a lot on the available instructions of the processors.
> Therefore I think a really clean representation that, at the same time, allows
> to find the cheapest set of instructions to express it on all
> processors, is impossible.

Without a doubt the terminology as presently used throughout the VM is 
somewhat muddled. I was tending to think of acquire in the load.acquire 
sense (loadStore|loadLoad barrier) and similarly of release as a 
store.release (storeSore|storeLoad). But I was overlooking the more 
general acquire/release semantics that we sometimes use when describing, 
for example the memory semantics of entering and leaving a synchronized 
block.

So I prefer your FenceAcquire/FenceRelease suggestion over the "wide" name.

Thanks,
David

> Best regards,
>    Goetz
>
> PS:  Should I respond to comments in the bug right in the bug
> or on the mailing lists?
>
>
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> From: ppc-aix-port-dev-bounces at openjdk.java.net [mailto:ppc-aix-port-dev-bounces at openjdk.java.net] On Behalf Of Lindenmaier, Goetz
> Sent: Montag, 18. November 2013 15:19
> To: 'hotspot-dev at openjdk.java.net'; 'ppc-aix-port-dev at openjdk.java.net'; Vladimir Kozlov
> Subject: RFR(M): 8028515: PPC64 (part 113.2): opto: Introduce MemBarAcquire/ReleaseWide.
>
> Hi,
>
> The c2 compiler inserts MemBarAcquire/Release nodes to enforce memory ordering in various places. Some order a certain load/store with other operations. Inline_unsafe_fence() inserts MemBars that do not correspont to a memory operation. So far, the same nodes were used.
>
> This change introduces MemBarAcquire/ReleaseWide to use where no dedicated load/store is ordered. With this change, these nodes can be matched differently, what is needed on PPC64.
>
> When reviewing 8024921 (part 113) we decided to avoid #defines in inline_unsafe_fence() and to introduce new MemBar operations.
>
> Please review and test this change.
> http://cr.openjdk.java.net/~goetz/webrevs/8028515-0-wide/
>
> Best regards,
>    Goetz.
>