RFR - Avoid G1 write barriers on newly allocated objects

[Staffan Friberg]

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/
bug: https://bugs.openjdk.java.net/browse/JDK-8057737

I would also need a sponsor for this change if it passes review.

Thanks,
Staffan