RFR (S) 8031818: Experimental VM flag for enforcing safe object construction

[Aleksey Shipilev]

Hi Remi,

On 01/22/2014 12:44 PM, Remi Forax wrote:
> My mail is a little OT but anyway, Aleksey, I don't understand why the
> performance are not mostly identical.

...modulo quality of implementation issues. Rigorously quantify the
real-world impact is the rationale of introducing this experimental
feature in the mainline VM. This is what we learned so far (in progress;
so unless you are really anxious and bored, don't read it yet):
  http://shipilev.net/blog/2014/all-fields-are-final/

> Correct me if I'm wrong, for TSO architecture, you basically do nothing
> so no impact. 

Right, but also prohibit the compiler reorderings, which can matter.

> For ARM or PPC, you need a barrier but anyway you need to
> emit a barrier when you store the class pointer in the header of the
> object, so it should not perf at all the JIT is able to see that the
> class pointer is not read in the constructor

There is also the tidbit: securing the field stores with StoreStore is
arguably not enough, which is why C2 emits MemBarRelease
(LoadStore+StoreStore) at the end of constructor. It is heavier than
ordinary StoreStore, at least for ARMs.

Anyhow, fixing this requires StoreStore/(LoadStore+StoreStore) barrier
coalescing logic, which is still missing. The performance model in the
absence of smart optimizing compiler is still of interest in spec work.
More on that later.

-Aleksey.