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

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D71726#2207700 <https://reviews.llvm.org/D71726#2207700>, @yaxunl wrote:

> clang does not always emit atomic instructions for atomic builtins. Clang may emit lib calls for atomic builtins. Basically clang checks target info about max atomic inline width and if the desired atomic operation exceeds the supported atomic inline width, clang will emit lib calls for atomic builtins. The rationale is that the lib calls may be faster than the IR generated by the LLVM pass. This behavior has long existed and it also applies to fp atomics. I don't think emitting lib calls for atomic builtins is a bug. However, this does introduce the issue about whether the library functions for atomics are available for a specific target. As I said, only the target owners have the answer and therefore I introduced the target hook.

If we want the frontend to emit an error when the target doesn't support library-based atomics, that seems fine, but there's no reason to only do so for floating-point types.  That is, we should have a TargetInfo method that asks whether atomics at a given size and alignment are supported at all, similar to what we have for "builtin" (lock-free) atomics, and we should check it for all the atomic types and operations.

Actually, maybe we should take the existing hook and have it return one of { LockFree, Library, Unsupported }.


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  https://reviews.llvm.org/D71726/new/

https://reviews.llvm.org/D71726