RFR - Avoid G1 write barriers on newly allocated objects

[Staffan Friberg]

Thanks Vladimir,

Updated with your suggestions. Added a small comment in 
g1_can_remove_pre_barrier method since the alloc == NULL check is quite 
a bit earlier.

New webrev, http://cr.openjdk.java.net/~sfriberg/8057737/webrev.03

Cheers,
Staffan


On 09/15/2014 05:55 PM, Vladimir Kozlov wrote:
> 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
>>>>
>>