RFR: JDK-8207169: X86: Modularize cmpxchg-oop assembler for C1 and C2

[Erik Osterlund]

Hi Roman,

> On 31 Aug 2018, at 16:26, Roland Westrelin <rwestrel at redhat.com> wrote:
> 
> 
> Hi Erik,
> 
>> So if you have some kind of loop for your CAS, your
>> ShenandoahBarrierSetC1 can generate the LIR for that, and allocate
>> registers accordingly, letting the register allocator do its thing.
> 
> I doubt it's as simple as you make it sound. A loop or any control flow
> construct would require us to hack the CFG. In c1, the CFG is
> constructed at parse time from the bytecodes of the method being
> compiled. There's no code that I know of that changes the CFG once
> parsing is over. The c1 way of doing something like that is to implement
> the loop (or any control flow) in assembly code in the backend.

Well... C1 uses CAS in the heap only for the Unsafe CAS intrinsic, which is indeed inserted at parse time. And all other GCs alter the CFG for the GC barriers in their CAS barriers, using LIR. Except Epsilon I suppose.

Thanks,
/Erik

> Roland.