[clang] [llvm] [openmp] [Clang][OpenMP] Add `#pragma omp flatten` loop directive + `depth` clause (PR #206977)

[Amit Tiwari via cfe-commits]

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@@ -12226,6 +12226,14 @@ def err_omp_invalid_looprange
 def warn_omp_redundant_fusion : Warning<"looprange clause selects a single "
                                         "loop, resulting in redundant fusion">,
                                 InGroup<OpenMPClauses>;
+def warn_omp_flatten_omitted_depth : Warning<
+  "'flatten' without a 'depth' clause only combines 2 loops, but 3 or more "
+  "loops are perfectly nested">,
+  InGroup<OpenMPClauses>;
+def note_omp_flatten_insert_depth : Note<
+  "insert 'depth(2)' to flatten only the outermost 2 loops">;
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loopacino wrote:

Yep you're right. Reworded it to say the note silences the waning, and makes 2-loop intent explicit.  

https://github.com/llvm/llvm-project/pull/206977