RFR - Avoid G1 write barriers on newly allocated objects

[Vladimir Kozlov]

You need only check (alloc == st_alloc) because alloc != NULL at this point:

+    if (st_alloc != NULL && alloc == st_alloc) {


You can clean up g1_can_remove_pre_barrier() too (remove st_alloc == 
NULL check).

I would also reverse the last check there because you bailout from loop 
anyway:

+        if (captured_store == NULL || captured_store == 
st_init->zero_memory()) {
+          return true;
+        }
+      }
+    }
+
+    // Unless there is an explicit 'continue', we must bail out here,


Vladimir

On 9/15/14 3:31 PM, Staffan Friberg wrote:
> Hi Vladimir,
>
> Uploaded a new webrev with fixes to the below issues,
> http://cr.openjdk.java.net/~sfriberg/8057737/webrev.02
>
> Still removes expected barriers.
>
> Thanks,
> Staffan
>
> On 09/12/2014 04:15 PM, Vladimir Kozlov wrote:
>> 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.
>>
>> Thanks,
>> Vladimir
>>
>> 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
>>>
>