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Re: [ARM] Add atomic_loaddi pattern


On 04/30/2012 11:50 AM, Richard Earnshaw wrote:
> On 30/04/12 09:51, Andrew Haley wrote:
>> On 04/27/2012 10:30 PM, Richard Henderson wrote:
>>> We can perform a single-copy atomic load with an ldrexd insn.
>>> If the load is all we care about, we need not pair this with
>>> a strexd.
>>
>> Can we?  It's good to know.  I have had a long email exchange with
>> engineers at ARM, and they would not say that this was safe.  If they
>> have changed their mind, I'd like to see chapter and verse.
> 
> The ARM ARM lists a number of single-copy atomic operations.  For v7,
> the list includes:
> 
> - memory accesses caused by LDREXD and STREXD instructions to
> doubleword-aligned locations.
>
> Of course, there is a potential performance issue from heavy use of the
> global monitor to ensure coherency.

Aha!  That is excellent news. that none of us managed to discover.
I shall immediately change the code we use in OpenJDK.

Thanks,
Andrew.


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