RFR - Avoid G1 write barriers on newly allocated objects

[Staffan Friberg]

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
>>