[PATCH] D71726: Let clang atomic builtins fetch add/sub support floating point types

[James Y Knight via Phabricator via cfe-commits]

jyknight added a comment.

In D71726#2165445 <https://reviews.llvm.org/D71726#2165445>, @yaxunl wrote:

> In D71726#2165424 <https://reviews.llvm.org/D71726#2165424>, @jyknight wrote:
>
> > Why not have clang always emit atomicrmw for floats, and let AtomicExpandPass handle legalizing that into integer atomics if necessary, rather than adding a target hook in clang?
>
>
> Not all targets can legalize fp atomics by AtomicExpandPass. Some targets need library support.


That isn't true, because you can do so generically with a cmpxchg loop, assuming that size of atomic is supported by the target. This might not be the most efficient lowering choice, but it's always possible as a fallback. (And if the size is too large, then AtomicExpandPass will lower the cmpxchg to the libatomic call.)

If a target wants to tell AtomicExpandPass that fp add/sub are supported, and then lower the resulting ATOMIC_LOAD_FSUB sdag node into a libcall of its choice, that's also ok (as long as the libcall is lock-free).


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