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Will be needed by a subsequent fix for the atomics.
GPUAtomic() expanded to metal::atomic, but the counters it guards are plain integers in structures that the host allocates and transfers, and the code reads them directly outside the atomic operations. That is exactly the CUDA and HIP situation, where GPUAtomic() is a no-op and atomicAdd() takes a plain pointer, so Metal now does the same. MSL's atomic_*_explicit do want metal::atomic, which has the same size and alignment, so the operand is cast at the one point where the operation is issued. That cast has to be made in the address space the pointer is really in, since casting a threadgroup pointer to a device atomic is a hard error, hence the two overloads: threadgroup stays threadgroup for the *Shared entry points, anything else resolves to device.
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GPUAtomic() expanded to metal::atomic, but the counters it guards are plain
integers in structures that the host allocates and transfers, and the code reads
them directly outside the atomic operations. That is exactly the CUDA and HIP
situation, where GPUAtomic() is a no-op and atomicAdd() takes a plain pointer,
so Metal now does the same.
MSL's atomic_*_explicit do want metal::atomic, which has the same size and
alignment, so the operand is cast at the one point where the operation is
issued. That cast has to be made in the address space the pointer is really in,
since casting a threadgroup pointer to a device atomic is a hard error, hence
the two overloads: threadgroup stays threadgroup for the *Shared entry points,
anything else resolves to device.