[foreign-memaccess+abi] RFR: Check for mixed access - remove mem barriers

[Radoslaw Smogura]

On Sat, 24 Jul 2021 16:07:09 GMT, Radoslaw Smogura <github.com+7535718+rsmogura at openjdk.org> wrote:

> That's for previous PR, which I destroyed with bad merge from master.
> 
> I wonder if this can look like this?
> 
> 
> Benchmark                               (size)  Mode  Cnt   Score    Error  Units
> ByteBufferVectorAccess.directBuffers      1024  avgt   10  27.119 ?  0.186  ns/op
> ByteBufferVectorAccess.heapBuffers        1024  avgt   10  28.501 ?  0.118  ns/op
> ByteBufferVectorAccess.pollutedBuffers    1024  avgt   10  64.354 ? 27.778  ns/op
> Finished running test 'micro:ByteBufferVectorAccess'
> 
> 
> https://github.com/openjdk/panama-foreign/pull/566

For polluted case, i think: Scoped memory access have two paths for storing vectors, one producing byte[] result one raw, those are merged together to all mem. The load in loop takes this.

As a consequence loop can't be optimized at all (it effectively works like mem barrier), so even loop limit gets executed with every loop cycle. I think this is more generic JIT case - probably worth investigating as I guess similar case can be produced not only for vectors.

However normal cases works better.

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PR: https://git.openjdk.java.net/panama-foreign/pull/573