[llvm-dev] RFC: Extending atomic loads and stores to floating point and vector types

[James Y Knight via llvm-dev]

On Fri, Dec 11, 2015 at 3:05 PM, Philip Reames via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> One open question I don't know the answer to: Are there any special
>> semantics required from floating point stores which aren't met by simply
>> bitcasting their result to i32 (float) or i64 (double) and storing the
>> result?  In particular, I'm unsure of the semantics around canonicalization
>> here.  Are there any? Same for loads?
>>
>
> I'd go a bit further: should you also support basic FP operations
> atomically? The above C++ paper adds add/sub, and we've discussed adding
> FMA as well.
>
> Just to be clear, I assume you mean extending the atomicrmw, and cmpxchg
> instructions right?  I agree this is worth doing, but I'm purposely
> separating that into a possible later proposal.  I don't need this right
> now and keeping the work scoped reasonably is key to making progress.
>

It seems a unfortunate, and potentially problematic if llvm doesn't
support, at least, "atomicrmw xchg" and "cmpxchg". If you support just
those two additionally to load/store, it'll cover everything the C frontend
actually needs to be able to do *now* with floating point atomics.

Atomic floating point math isn't actually a supported operation in C now,
so it seems reasonable to leave the rest of the atomicrmw ops for a future
design.
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