[clang] [CIR][OpenMP] Implement lowering for the 'num_threads' clause for 'parallel' directive (PR #202466)

[Pedro da Rosa Pinheiro via cfe-commits]

https://github.com/pedropiin updated https://github.com/llvm/llvm-project/pull/202466

>From 76acfb1baa6ef80a3fba3410d2b7c88cb79698da Mon Sep 17 00:00:00 2001
From: pedropiin <pedarosa04 at gmail.com>
Date: Mon, 8 Jun 2026 20:53:51 -0300
Subject: [PATCH] [CIR][OpenMP] Implement lowering for the 'num_threads' clause
 for 'parallel' directive

---
 clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp | 21 +++++++++++++++++++-
 clang/lib/CIR/CodeGen/CIRGenOpenMPClause.h   |  2 ++
 clang/lib/CIR/CodeGen/CIRGenStmtOpenMP.cpp   | 11 +++++-----
 clang/test/CIR/CodeGenOpenMP/parallel.c      | 15 ++++++++++++++
 4 files changed, 43 insertions(+), 6 deletions(-)

diff --git a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
index 16ac4440660b5..fab66241de112 100644
--- a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
@@ -93,6 +93,25 @@ bool OpenMPClauseEmitter::emitProcBind(
   return false;
 }
 
+bool OpenMPClauseEmitter::emitNumThreads(
+    mlir::omp::NumThreadsClauseOps &result) const {
+  for (const OMPClause *clause : clauses) {
+    const auto *ntc = dyn_cast<OMPNumThreadsClause>(clause);
+    if (!ntc)
+      continue;
+
+    const Expr *numThreadsExpr = ntc->getNumThreads();
+    mlir::Value numThreadsValue = cgf.emitScalarExpr(numThreadsExpr);
+    auto intType = builder.getIntegerType(32); // Assuming 32-bit integer type.
+    numThreadsValue = builder.createBuiltinIntCast(numThreadsValue, intType);
+
+    result.numThreadsVars.assign({numThreadsValue});
+
+    return true;
+  }
+  return false;
+}
+
 bool OpenMPClauseEmitter::emitMap(
     mlir::omp::MapClauseOps &result,
     llvm::SmallVectorImpl<const VarDecl *> *mapSyms) const {
@@ -142,4 +161,4 @@ bool OpenMPClauseEmitter::emitMap(
     }
   }
   return found;
-}
+}
\ No newline at end of file
diff --git a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.h b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.h
index 54c7366b1d769..12fa6d9459a19 100644
--- a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.h
+++ b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.h
@@ -42,6 +42,8 @@ class OpenMPClauseEmitter {
 
   bool emitProcBind(mlir::omp::ProcBindClauseOps &result) const;
 
+  bool emitNumThreads(mlir::omp::NumThreadsClauseOps &result) const;
+
   /// Emit map clauses. The optional \p mapSyms parameter collects the
   /// VarDecls corresponding to each map operand.
   bool emitMap(mlir::omp::MapClauseOps &result,
diff --git a/clang/lib/CIR/CodeGen/CIRGenStmtOpenMP.cpp b/clang/lib/CIR/CodeGen/CIRGenStmtOpenMP.cpp
index 17a1fb8090f5c..cc9fa6cfe7268 100644
--- a/clang/lib/CIR/CodeGen/CIRGenStmtOpenMP.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenStmtOpenMP.cpp
@@ -39,11 +39,12 @@ CIRGenFunction::emitOMPParallelDirective(const OMPParallelDirective &s) {
   mlir::omp::ParallelOperands clauseOps;
   OpenMPClauseEmitter ce(*this, getCIRGenModule(), builder, begin, s.clauses());
   ce.emitProcBind(clauseOps);
-  ce.emitNYI</*supported=*/OMPProcBindClause>(
-      /*nyi=*/OpenMPNYIClauseList<
-          OMPAllocateClause, OMPCopyinClause, OMPDefaultClause,
-          OMPFirstprivateClause, OMPIfClause, OMPNumThreadsClause,
-          OMPPrivateClause, OMPReductionClause, OMPSharedClause>{},
+  ce.emitNumThreads(clauseOps);
+  ce.emitNYI</*supported=*/OMPProcBindClause, OMPNumThreadsClause>(
+      /*nyi=*/OpenMPNYIClauseList<OMPAllocateClause, OMPCopyinClause,
+                                  OMPDefaultClause, OMPFirstprivateClause,
+                                  OMPIfClause, OMPPrivateClause,
+                                  OMPReductionClause, OMPSharedClause>{},
       llvm::omp::Directive::OMPD_parallel);
 
   auto parallelOp = mlir::omp::ParallelOp::create(builder, begin, clauseOps);
diff --git a/clang/test/CIR/CodeGenOpenMP/parallel.c b/clang/test/CIR/CodeGenOpenMP/parallel.c
index 36a48b38ee789..c256e9ff25e71 100644
--- a/clang/test/CIR/CodeGenOpenMP/parallel.c
+++ b/clang/test/CIR/CodeGenOpenMP/parallel.c
@@ -84,3 +84,18 @@ void proc_bind_parallel() {
   // CHECK-NEXT: omp.terminator
   // CHECK-NEXT: }
 }
+
+void num_threads_parallel() {
+  // CHECK: omp.parallel num_threads(%{{.*}}: i32) {
+  #pragma omp parallel num_threads(16)
+  {}
+  // CHECK-NEXT: omp.terminator
+  // CHECK-NEXT: }
+
+int numThreads = 4;
+  // CHECK: omp.parallel num_threads(%{{.*}}: i32) {
+#pragma omp parallel num_threads(numThreads) 
+  {}
+  // CHECK-NEXT: omp.terminator
+  // CHECK-NEXT: }
+}