RFR (S): 8007898: Incorrect optimization of Memory Barriers in Matcher::post_store_load_barrier()

[Aleksey Shipilev]

On 07/11/2013 07:38 AM, Vladimir Kozlov wrote:
> http://cr.openjdk.java.net/~kvn/8007898/webrev/

This looks good, but the regression test needs cleanup, see below.

> I also cleaned/fixed corresponding code in Matcher which tries to
> optimize out those multiply barriers. The problem with it was it did not
> take into account biased locking when cas could be not executed.

That's an interesting corner case, thanks for fixing it! I can't readily
reproduce it though, here are two potential jcstress tests [1].

On x86 maintaining the total store order, the effect can only be due to
the compiler reordering, yet it does not happen with -XX:+StressLCM/GCM.
The disassembly also shows the release barrier is emitted.

Volkner, Martin, Goetz, can you try those on your PPC C2 port?

> Added regression test which shows the problem with -XX:+StressLCM[GCM]
> flags added by Aleksey some time ago.

That regression test is the rewrite of the existing jcstress test. I
think it needs cleanup and the rename, see the update here:
  http://cr.openjdk.java.net/~shade/8007898/DekkerTest.java

Note that because StressLCM/GCM are only working once during the code
generation, this test is inherently intermittently passing. I don't see
how to tell jtreg to do a few runs in the row, is it possible? Or, can
we use whitebox API to deoptimize actor1/2?

For the record, the complete jcstress [2] run with -XX:+StressLCM/GCM on
Solaris 10/x86_64 yield no failures on the patched build. That means
lots of previous failures are accounted for with this patch.

> Aleksey is testing performance of these changes and no regression is
> observed so far.

Yes, the targeted microbenchmarks [3] show no red flags [4]. The larger
workloads are not regressing either, at least SPECjvm98 and SPECjbb2005
are unaffected. SPECjvm2008 and SPECjbb2013 runs are still pending, I'll
update in 24 hours.

-Aleksey.

[1] http://hg.openjdk.java.net/code-tools/jcstress/rev/6c339a5aa00d
[2] openjdk.java.net/projects/code-tools/jcstress/
[3] http://cr.openjdk.java.net/~shade/8007898/volatile-reorder.tar.gz
[4] http://cr.openjdk.java.net/~shade/8007898/micros-scores.txt