[WIP] Introduce disjoint agents fn attribute/intrinsic property

[jchlanda]

This patch attempts to generalise the workaround: #1334
The problem is as follows:

if (T0)
  create/store variable for the use by all the threads
barrier()
use variable/load

LLVM doesn't know that barrier (__syncthreads) interacts with the store/load sequence. The offending spot is here: https://git.office.codeplay.com/oneapi-core/intel-llvm-mirror/-/blob/sycl/llvm/lib/Analysis/GlobalsModRef.cpp#L536 it is assumed that all intrinsics are save to process (even though they have hasSideEffects set by default). The pass in question works on llvm::Function declaration and the only available information is:

; Function Attrs: convergent nounwind
declare void @llvm.nvvm.barrier0() #3
...
attributes #3 = { convergent nounwind }

This patch adds intrinsic property that is propagated all the way up to function attribute, modifying the declaration of the barrier0 to:

...
attributes #3 = { convergent disjoint_agents nounwind }

something that the pass can then check against.

The original bug report: #1258

[againull]

Thank you for working on this issue! Should we consider opening a PR in llvm-project repo?

[jchlanda]

Thank you for working on this issue! Should we consider opening a PR in llvm-project repo?

We discussed it and came to a conclusion that first we should consult the wider community and gather their opinion on the problem/solution. Codeplay will send request to the mailing list to address that.

[bader]

@jchlanda, can we close this PR?

[jchlanda]

Yeap, closing now.