[Openmp-commits] [PATCH] D75546: [libomptarget] Implement locks for amdgcn

[Jon Chesterfield via Phabricator via Openmp-commits]

JonChesterfield added a comment.

If we say thread == lane of simd, and every lane executes the same instruction with some masked off, then we can 'lock' a thread with respect to the rest of the device if and only if it is the only active one in the warp.

It's a symptom of defining thread to have finer granularity than instruction pointer, which I believe is a serious design mistake passed down from cuda. I'd like openmp to map SIMD onto the warp instead, at which point we can lock the newly defined 'thread' using an implementation like this.

If we continue doing thread==lane, and want to support this API, I think it can be done by rewriting the cfg. The end result will perform horrendously, but that's fairly likely of any code using spin locks.

The above is also a spin lock, which is not traditionally a great idea, so I'd like to add a futex syscall equivalent to the kernel driver. That's a longer term goal.

In the meantime, this is the most useful functionality I have been able to work out under the openmp lock API.


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https://reviews.llvm.org/D75546