Memory ordering in C2

[Andrew Haley]

Hi,

On 02/25/2014 03:20 PM, Lindenmaier, Goetz wrote:

> We added a a few new node types in this change:
> http://hg.openjdk.java.net/jdk9/hs/hotspot/rev/50fdb38839eb
> This solved the issue that we implement MemBarAcquire empty.

Ah yes.  Good, I can use that too.

> But yes, we would appreciate further modularization in this direction.
> We proposed similar stuff in this mail:
> http://mail.openjdk.java.net/pipermail/hotspot-compiler-dev/2014-February/013384.html

Thanks.  I saw that at the time but didn't quite realize how similar
it was to my own issues.

> Actually, we would like more barrier nodes in some places (around
> CompareAndSwap) because we issue sync instructions in those nodes.
> If they are extra nodes, we can optimize them.  At other places, we
> would like less ... ;)

>From what I've seen, C2 is enthusiastically emitting barrier nodes
around CompareAndSwap, so I'm not quite sure what this means.

> And I guess x86 & friends, that always use empty nodes could profit, too.
> 
> Further we would like to have explicit specification of the required
> barriers in the nodes (Only one node, but listing the barriers
> required (StoreStore | StoreLoad etc)).

That would indeed be nice, but difficult.

Andrew.