RFR - Avoid G1 write barriers on newly allocated objects

Vladimir Kozlov vladimir.kozlov at oracle.com
Fri Sep 12 23:15:30 UTC 2014

Fix indention in 2 checks:

+    if (use_ReduceInitialCardMarks() &&
+            g1_can_remove_*_barrier(&_gvn, adr, bt, alias_idx)) {

Typo 'iff' in both comments:

+ * Returns true iff the barrier can be removed

In g1_can_remove_post_barrier() use:

+  // Start search from Store node
+  Node* ctrl = store->in(MemNode::Control);

Control edge can't point to an other Store so you should not check it inside loop.
As result you don't need loop.


On 9/12/14 2:26 PM, Staffan Friberg wrote:
> Hi Mikael,
> I updated the documentation, and uploaded a new webrev, http://cr.openjdk.java.net/~sfriberg/8057737/webrev.01
> Thanks for agreeing to sponsor, and the good idea about pushing to hs-gc for extra coverage.
> Thanks,
> Staffan
> On 09/09/2014 07:31 AM, Mikael Gerdin wrote:
>> Hi Staffan,
>> On Friday 05 September 2014 16.31.44 Staffan Friberg wrote:
>>> Hi,
>>> Copying both the compiler and GC alias as the optimization involves both
>>> groups.
>>> This optimization aims to avoid generating G1 barriers for newly
>>> allocated objects where the compiler can prove that the object has not
>>> been written to earlier and there is no safepoint between the allocation
>>> and the write. The bug has some further details and microbenchmark
>>> result. The new code has fairly extensive comments about the optimization.
>>> It would be great if the GC team can help validate that the premise of
>>> the optimization is correct as well.
>>> webrev: http://cr.openjdk.java.net/~sfriberg/8057737/webrev/
>> I mostly read through the comments since I can't decode what the IR transforms
>> intend to do :)
>> We usually stick to the terms "old generation" and "young generation" instead
>> of "Old Space" and "Young Space".
>> "G1 also requires to keep track of objects between different
>> + * regions to enable evacuation of old regions"
>> should probably be.
>> .."to keep track of references between different regions"
>> I leave it to the compiler team to review the actual code changes.
>>> bug: https://bugs.openjdk.java.net/browse/JDK-8057737
>>> I would also need a sponsor for this change if it passes review.
>> I'll push this to hs-gc after review since we have more G1 test coverage on
>> hs-gc.
>> /Mikael
>>> Thanks,
>>> Staffan

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