[10] RFR(S): 8182036: Load from initializing arraycopy uses wrong memory state

[Roland Westrelin]

Thanks for reviewing this, Vladimir,

> What if you have a second arraycopy after 0x42 store which also modify 
> src[1]?

Can you post the code snippet you think could cause a problem? I'm not
sure I'm following.
I should have made it clearer: this covers a corner case where the
arraycopy is responsible for the initialization of a just allocated
array. The arraycopy is right after the allocation and nothing is
allowed between the arraycopy and the allocation.

> You did not explain changes in memnode.cpp

During arraycopy node expansion, a ClearArrayNode is created to
initialize the head section followed by the LoadI/StoreI pair for the
first element to copy. The ClearArrayNode is transformed into a StoreL
so we have a StoreI followed by a StoreL. The LoadI used to have the
StoreL as memory input but I moved it to another slice which is why the
assert now fails: the StoreL used to have 2 uses so we wouldn't go into
the loop where the assert is.

> I would like to fix it first in JDK 10 and let it be tested before we 
> backport into previous releases.

Sure. That sounds fine to me.

Roland.