RFR: 8186838: Generalize Atomic::inc/dec with templates

[Andrew Haley]

On 04/09/17 09:14, Erik Österlund wrote:
> On PPC Atomic::inc/dec and Atomic::add have the following differences:
> 
> Atomic::inc/dec uses addic between the LL and SC instructions with an 
> immediate value for adding, whereas Atomic::add uses the add instruction 
> with an extra register.
> Atomic::add has a leading lwsync fence and Atomic::inc/dec has no 
> leading fence.
> Atomic::add has a trailing isync fence and Atomic::inc/dec has no 
> trailing fence.

One of those must be a bug.  Either one of them is unnecessary or
both are necessary.

> So the current implementation of Atomic::add uses heavier fencing than 
> Atomic::inc/dec. I can imagine that does matter for performance today.
> However, the documented semantics of Atomic::inc/dec requires a leading 
> sync fence - so they are both arguably too weak and should have stronger 
> fencing than they do today. And I would argue that if both conformed to 
> the fencing required by our public API, then the difference would 
> probably be small.

Right.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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