Release store in C2 putfield
aph at redhat.com
Wed Sep 3 17:00:50 UTC 2014
On 09/03/2014 05:49 PM, Aleksey Shipilev wrote:
> Hi Andrew,
> On 09/03/2014 06:16 PM, Andrew Haley wrote:
>> In Parse::do_put_xxx, I see
>> const MemNode::MemOrd mo = is_vol ? // Volatile fields need releasing stores. MemNode::release : // Non-volatile fields also need releasing stores if they hold an // object reference, because the object reference might point to // a freshly created object. StoreNode::release_if_reference(bt);
>> AArch64 doesn't need a release store here: its memory guarantees are strong enough that a simple store is sufficient. But my question is not about that, but how to handle it properly.
> I can't answer the question you posed, but let me challenge your premise.
> Why is a simple store is sufficient here for AArch64? Do the stores ordered on AArch64 (I thought not)? I thought the "RC" part in "RCsc" only applies to explicit synchronization instructions.
I discussed this with Peter Sewell, and it's explained in his
(co-authored) paper "A Tutorial Introduction to the ARM and POWER
Relaxed Memory Models" at
Section 4.1, "Enforcing Order with Dependencies"
In the AArch64 spec, we have:
B2.7.2 Ordering requirements
If an address dependency exists between two reads or between a read
and a write, then those memory accesses are observed in program
order by all observers within the shareability domain of the memory
So, an address dependency and a DMB when an object is created is all
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