[clang] [CIR][OpenMP] Implement lowering for the 'if' clause for 'parallel' d… (PR #204999)

[Pedro da Rosa Pinheiro via cfe-commits]

https://github.com/pedropiin created https://github.com/llvm/llvm-project/pull/204999

Implementing the lowering of the `if` clause for the `#pragma omp parallel` directive. The clause accepts an `Expr` and adapts the `mlir::omp::ParallelOp.getIfExprMutable()` accordingly. If the `Expr` is later evaluated to a non-zero value, the region is executed in parallel.

Followed the same approach as the one in my other PR #202466 and based myself off of PR #172501 that implements the lowering for the `proc_bind` clause. Also following the adaptations done in PR #181841 to support the use of `UnrealizedConversionCast` when enabling the CIR pipeline. PR #196363 implemented a separate approach to deal with this, but got blocked. 

### Evidence:
Output of: `python3 build/bin/llvm-lit -sv clang/test/CIR/CodeGenOpenMP/parallel.c`

```
/CodeGenOpenMP/parallel.c
llvm-lit: /home/pedro/dev/projects/llvm-project-fork/llvm/utils/lit/lit/llvm/config.py:569: note: using clang: /home/pedro/dev/projects/llvm-project-fork/build/bin/clang

Testing Time: 0.05s

Total Discovered Tests: 1
  Passed: 1 (100.00%)
```

>From 6a6ee355c2b8e979578799fd827871d93f887b58 Mon Sep 17 00:00:00 2001
From: pedropiin <pedarosa04 at gmail.com>
Date: Sun, 21 Jun 2026 15:12:26 -0300
Subject: [PATCH] [CIR][OpenMP] Implement lowering for the 'if' clause for
 'parallel' directive

---
 clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp  | 20 +++++++++++++++++
 .../CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp |  3 +++
 clang/test/CIR/CodeGenOpenMP/parallel.c       | 22 +++++++++++++++++++
 3 files changed, 45 insertions(+)

diff --git a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
index a0f0ea9299c8d..5ed80c7a01b5f 100644
--- a/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
+++ b/clang/lib/CIR/CodeGen/CIRGenOpenMPClause.cpp
@@ -66,6 +66,26 @@ class OpenMPClauseCIREmitter final
     }
   }
 
+  void VisitOMPIfClause(const OMPIfClause *clause) {
+    if constexpr (std::is_same_v<OpTy, mlir::omp::ParallelOp>) {
+      Expr *ifExpr = clause->getCondition();
+      mlir::Value ifValue = cgf.emitScalarExpr(ifExpr);
+      mlir::Location ifLoc =
+          cgf.cgm.getLoc(ifExpr->getBeginLoc());
+
+      mlir::Type stdBoolType = builder.getI1Type();
+
+      mlir::Value unrealizedCastIf =
+          mlir::UnrealizedConversionCastOp::create(builder, ifLoc, stdBoolType, ifValue).getResult(0);
+
+      operation.getIfExprMutable().append(unrealizedCastIf);
+    } else {          
+      cgf.cgm.errorNYI(
+          clause->getBeginLoc(),
+          "OMPProcBindClause unimplemented on this directive kind");
+    }
+  }
+
   void emitClauses(ArrayRef<const OMPClause *> clauses) {
     for (const auto *c : clauses)
       this->Visit(c);
diff --git a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
index c844375a000e0..2dc8dc45258dc 100644
--- a/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
+++ b/clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp
@@ -17,6 +17,7 @@
 
 #include "mlir/Conversion/LLVMCommon/TypeConverter.h"
 #include "mlir/Conversion/OpenMPToLLVM/ConvertOpenMPToLLVM.h"
+#include "mlir/Conversion/ReconcileUnrealizedCasts/ReconcileUnrealizedCasts.h"
 #include "mlir/Dialect/DLTI/DLTI.h"
 #include "mlir/Dialect/Func/IR/FuncOps.h"
 #include "mlir/Dialect/LLVMIR/LLVMDialect.h"
@@ -3695,6 +3696,7 @@ void ConvertCIRToLLVMPass::runOnOperation() {
   mlir::populateOpenMPToLLVMConversionPatterns(converter, patterns);
   target.addIllegalDialect<mlir::BuiltinDialect, cir::CIRDialect,
                            mlir::func::FuncDialect>();
+  target.addLegalOp<mlir::UnrealizedConversionCastOp>();
 
   llvm::SmallVector<mlir::Operation *> ops;
   ops.push_back(module);
@@ -4935,6 +4937,7 @@ void populateCIRToLLVMPasses(mlir::OpPassManager &pm) {
   mlir::populateCIRPreLoweringPasses(pm);
   pm.addPass(mlir::omp::createMarkDeclareTargetPass());
   pm.addPass(createConvertCIRToLLVMPass());
+  pm.addPass(mlir::createReconcileUnrealizedCastsPass());
 }
 
 std::unique_ptr<llvm::Module>
diff --git a/clang/test/CIR/CodeGenOpenMP/parallel.c b/clang/test/CIR/CodeGenOpenMP/parallel.c
index 36a48b38ee789..b3b82fbd3ea23 100644
--- a/clang/test/CIR/CodeGenOpenMP/parallel.c
+++ b/clang/test/CIR/CodeGenOpenMP/parallel.c
@@ -84,3 +84,25 @@ void proc_bind_parallel() {
   // CHECK-NEXT: omp.terminator
   // CHECK-NEXT: }
 }
+
+void if_parallel() {
+  // CHECK: omp.parallel if(%{{.*}}) {
+#pragma omp parallel if (1)
+  {}
+  // CHECK-NEXT: omp.terminator
+  // CHECK-NEXT: }
+
+int validCondition = 10;
+  // CHECK: omp.parallel if(%{{.*}}) {
+#pragma omp parallel if (validCondition)
+  {}
+  // CHECK-NEXT: omp.terminator
+  // CHECK-NEXT: }
+
+void *nullPtr = ((void *)0);
+  // CHECK: omp.parallel if(%{{.*}}) {
+#pragma omp parallel if (nullPtr)
+  {}
+  // CHECK-NEXT: omp.terminator
+  // CHECK-NEXT: }
+}