[llvm] [openmp] [OpenMP][offload] Fix single-thread SPMD cross-team reductions (PR #228216)

[Joseph Huber via llvm-commits]

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@@ -235,14 +235,19 @@ int32_t __kmpc_gpu_xteam_reduce_nowait(IdentTy *Loc, void *reduce_data,
   uint32_t ThreadId;
   uint32_t NumThreads;
 
-  if (mapping::isSPMDMode()) {
+  // In SPMD mode, we're dealing with full warps unless the kernel was SPMD-ized
+  // without parallel regions: then, only the main thread is alive (see
+  // OpenMPOpt).
+  bool AllThreads =
+      mapping::isSPMDMode() && utils::popc(mapping::activemask()) > 1;
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jhuber6 wrote:

This is a little confusing, I feel like if we're handling a very specific single threaded case in SPMD mode then maybe we could just check if the number of threads is 1?

https://github.com/llvm/llvm-project/pull/228216