RFC (S) JDK-8050149: Experimental option to select the instruction sequence for x86 StoreLoad barrier

[Vladimir Kozlov]

On 7/14/14 3:13 PM, Aleksey Shipilev wrote:
> On 07/15/2014 02:00 AM, Vitaly Davidovich wrote:
>> In case you're interested (and haven't checked yourself), both
>> clang(3.4.1) and gcc(4.9) use mfence (under O2+) for
>> std::atomic_thread_fence(std::memory_order_seq_cst), which I think is
>> comparable to StoreLoad that's emitted on x86 backend in hotspot.
>
> See "case 0" in my patch. I think C++11 folks use the docs from Sewell
> et al.: http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html. (It is
> interesting that since Azeem's Jiva / Dave Dice's research we switched
> to "lock addl". Linux uses "lock addl" as well).
>
> Any non-trivial ideas? In fact, I would be grateful to see the x86
> instruction sequences which provide the semantics (e.g. locked
> instructions referencing memory), and (try to) do not waste registers,
> and/or (try to) avoid stores.

May be instructions stream:

   address rip = pc();
   lock(); addl(InternalAddress(rip), 0);

Vladimir

>
> -Aleksey.
>