RFC: add "cmpxchg weak" to LLVM IR

[Chandler Carruth]

On Fri, Jun 13, 2014 at 11:00 AM, Tim Northover <t.p.northover at gmail.com>
wrote:

> Hi Chandler,
>
> > So, I see where you're going here, but I'm curious -- why not just switch
> > ATOMIC_CMP_SWAP to have a second i1 value, and still be strong? Is this
> > *just* to support expanding? I wonder if that's really useful rather than
> > just lowering it directly on the various targets....
>
> I tried that originally, but quickly got into murky waters with all
> the targets I've got basically no clue about.
>
> It still could have been the best option (I started poking at Mips),
> but for one particularly nasty facet: you can select ATOMIC_CMP_SWAP
> in TableGen, but not ATOMIC_CMP_SWAP_WITH_SUCCESS (multiple results
> and all that).
>
> That means even getting selection back to the status quo would have
> been a lot of duplicated C++ code in multiple targets, all for what's
> basically a dead-end in most cases. It's far easier to make use of the
> i1 before the DAG level if possible.
>
> Anyway, that was my reasoning. Possibly more pragmatism than principle.
>

No, it makes a certain amount of sense.

I'd personally much rather we had *just* the multiple result variant in the
DAG, but I can see it being really annoying to get there directly. Perhaps
eventually targets will have adapted far enough that we can kill off the
expanded form, or invert them.

Certainly, I agree with only exposing the strong variant once you hit the
DAG.

So, I say carry on unless others on the list see serious problems here.
-Chandler
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