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

[Amit Tiwari via llvm-commits]

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@@ -16528,6 +16535,305 @@ StmtResult SemaOpenMP::ActOnOpenMPInterchangeDirective(
                                          buildPreInits(Context, PreInits));
 }
 
+StmtResult
+SemaOpenMP::ActOnOpenMPFlattenDirective(ArrayRef<OMPClause *> Clauses,
+                                        Stmt *AStmt, SourceLocation StartLoc,
+                                        SourceLocation EndLoc) {
+  ASTContext &Context = getASTContext();
+  DeclContext *CurContext = SemaRef.CurContext;
+  Scope *CurScope = SemaRef.getCurScope();
+
+  // Empty statement should only be possible if there already was an error.
+  if (!AStmt)
+    return StmtError();
+
+  // flatten without 'depth' clause combines two loops; 'depth(k)' selects k.
+  unsigned NumLoops = 2;
+  bool DepthIsDependent = false;
+  const auto *DepthClause =
+      OMPExecutableDirective::getSingleClause<OMPDepthClause>(Clauses);
+  if (DepthClause) {
+    Expr *DepthExpr = DepthClause->getDepth();
+    if (DepthExpr && DepthExpr->isInstantiationDependent()) {
+      DepthIsDependent = true;
+    } else if (DepthExpr) {
+      Expr::EvalResult EvalResult;
+      if (DepthExpr->EvaluateAsInt(EvalResult, Context))
+        NumLoops = EvalResult.Val.getInt().getLimitedValue(
+            std::numeric_limits<unsigned>::max());
+    }
+  }
+
+  // Count perfectly nested loops with doForAllLoops. When 'depth' is present,
+  // walk NumLoops iterations to diagnose an insufficient nest. When it is
+  // omitted, walk one extra loop (3 total) so we can warn that default
+  // flatten only combines 2 of a deeper nest.
+  if (!DepthIsDependent) {
+    unsigned WalkLimit = DepthClause ? NumLoops : 3;
+    unsigned Found = 0;
+    bool Enough = OMPLoopBasedDirective::doForAllLoops(
+        AStmt->IgnoreContainers(), /*TryImperfectlyNestedLoops=*/false,
----------------
loopacino wrote:

The walk counts syntactic ForStmt, not canonical loops. So, the triangular `k` does still warn despite being non-flattenable. `k` is not flatten-affected unless `depth(3)` is used, where it correctly errors. Didn't change Sema here. Added For actionable part: aded a note + fix-it that inserts `depth(2)`

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