RFR (M): 8188224: Generalize Atomic::load/store to use templates

[David Holmes]

Hi Erik,

A lot of jumping through hoops just to do a direct load/store in the 
bulk of cases - but okay, we're embracing templates.

66   // Atomically store to a location
67   // See comment above about using jlong atomics on 32-bit platforms

The comment at #67 and the equivalent one for load can be deleted. The 
"comment above" should only be referring to r-m-w atomic ops not basic 
load and store. All platforms must have a means to do atomic load/store 
of 64-bit due to Java volatile variables (eg by using floating-point 
unit on 32-bit) but may not have cmpxchg<8> capability. (I failed to 
convince the author of this when those comments went in. ;-) )

Cheers,
David

On 3/10/2017 10:29 PM, Erik Österlund wrote:
> Hi,
> 
> The time has come to generalize Atomic::load/store with templates - the 
> last operation to generalize in Atomic.
> The design was inspired by Atomic::xchg and uses a similar mechanism to 
> validate the passed in arguments. It was also designed with coming 
> OrderAccess changes in mind. OrderAccess also contains loads and stores 
> that will reuse the LoadImpl and StoreImpl infrastructure in 
> Atomic::load/store. (the type checking for what is okay to pass in to 
> Atomic::load/store is very much the same for 
> OrderAccess::load_acquire/*store*).
> 
> One thing worth mentioning is that the bsd zero port (but notably not 
> the linux zero port) had a leading fence for atomic stores of jint when 
> #if !defined(ARM) && !defined(M68K) is true without any comment 
> describing why. So I took the liberty of removing it. Atomic should not 
> have any fencing at all - that is what OrderAccess is for. In fact 
> Atomic does not promise any memory ordering semantics for loads and 
> stores. Atomic merely provides relaxed accesses that are atomic. Worth 
> mentioning nevertheless in case anyone wants to keep that jint 
> Atomic::store fence on bsd zero !M68K && !ARM.
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8188224
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8188224/webrev.00/
> 
> Testing: JPRT, mach5 hs-tier3
> 
> Thanks,
> /Erik