[clang] [OpenMP 6.0 ]Codegen for Reduction over private variables with reduction clause (PR #134709)

[Alexey Bataev via cfe-commits]

================
@@ -4899,6 +4899,237 @@ void CGOpenMPRuntime::emitSingleReductionCombiner(CodeGenFunction &CGF,
   }
 }
 
+void CGOpenMPRuntime::emitPrivateReduction(
+    CodeGenFunction &CGF, SourceLocation Loc, const Expr *Privates,
+    const Expr *LHSExprs, const Expr *RHSExprs, const Expr *ReductionOps) {
+
+  //  Create a shared global variable (__shared_reduction_var) to accumulate the
+  //  final result.
+  //
+  //  Call __kmpc_barrier to synchronize threads before initialization.
+  //
+  //  The master thread (thread_id == 0) initializes __shared_reduction_var
+  //    with the identity value or initializer.
+  //
+  //  Call __kmpc_barrier to synchronize before combining.
+  //  For each i:
+  //    - Thread enters critical section.
+  //    - Reads its private value from LHSExprs[i].
+  //    - Updates __shared_reduction_var[i] = RedOp_i(__shared_reduction_var[i],
+  //    LHSExprs[i]).
+  //    - Exits critical section.
+  //
+  //  Call __kmpc_barrier after combining.
+  //
+  //  Each thread copies __shared_reduction_var[i] back to LHSExprs[i].
+  //
+  //  Final __kmpc_barrier to synchronize after broadcasting
+  QualType PrivateType = Privates->getType();
+  llvm::Type *LLVMType = CGF.ConvertTypeForMem(PrivateType);
+
+  llvm::Constant *InitVal = nullptr;
+  const OMPDeclareReductionDecl *UDR = getReductionInit(ReductionOps);
+  // Determine the initial value for the shared reduction variable
+  if (!UDR) {
+    InitVal = llvm::Constant::getNullValue(LLVMType);
+    if (const auto *DRE = dyn_cast<DeclRefExpr>(Privates)) {
+      if (const auto *VD = dyn_cast<VarDecl>(DRE->getDecl())) {
+        const Expr *InitExpr = VD->getInit();
+        if (InitExpr && !PrivateType->isAggregateType() &&
+            !PrivateType->isAnyComplexType()) {
+          Expr::EvalResult Result;
+          if (InitExpr->EvaluateAsRValue(Result, CGF.getContext())) {
+            APValue &InitValue = Result.Val;
+            if (InitValue.isInt())
+              InitVal = llvm::ConstantInt::get(LLVMType, InitValue.getInt());
+          }
+        }
+      }
+    }
+  } else {
+    InitVal = llvm::Constant::getNullValue(LLVMType);
+  }
+  std::string ReductionVarNameStr;
+  if (const auto *DRE = dyn_cast<DeclRefExpr>(Privates->IgnoreParenCasts())) {
+    ReductionVarNameStr = DRE->getDecl()->getNameAsString();
+  } else {
+    ReductionVarNameStr = "unnamed_priv_var";
+  }
+
+  // Create an internal shared variable
+  std::string SharedName =
+      CGM.getOpenMPRuntime().getName({"internal_pivate_", ReductionVarNameStr});
+  llvm::GlobalVariable *SharedVar = new llvm::GlobalVariable(
+      CGM.getModule(), LLVMType, false, llvm::GlobalValue::InternalLinkage,
+      InitVal, ".omp.reduction." + SharedName, nullptr,
+      llvm::GlobalVariable::NotThreadLocal);
+
+  SharedVar->setAlignment(
+      llvm::MaybeAlign(CGF.getContext().getTypeAlign(PrivateType) / 8));
+
+  Address SharedResult(SharedVar, SharedVar->getValueType(),
+                       CGF.getContext().getTypeAlignInChars(PrivateType));
+
+  llvm::Value *ThreadId = getThreadID(CGF, Loc);
+  llvm::Value *BarrierLoc = emitUpdateLocation(CGF, Loc, OMP_ATOMIC_REDUCE);
+  llvm::Value *BarrierArgs[] = {BarrierLoc, ThreadId};
+
+  llvm::BasicBlock *InitBB = CGF.createBasicBlock("init");
+  llvm::BasicBlock *InitEndBB = CGF.createBasicBlock("init.end");
+
+  llvm::Value *IsWorker = CGF.Builder.CreateICmpEQ(
+      ThreadId, llvm::ConstantInt::get(ThreadId->getType(), 0));
+  CGF.Builder.CreateCondBr(IsWorker, InitBB, InitEndBB);
+
+  CGF.EmitBlock(InitBB);
+
+  auto EmitSharedInit = [&]() {
+    if (UDR) { // Check if it's a User-Defined Reduction
+      if (const Expr *UDRInitExpr = UDR->getInitializer()) {
+        // Use the initializer from the OMPDeclareReductionDecl
+        CGF.EmitAnyExprToMem(UDRInitExpr, SharedResult,
+                             PrivateType.getQualifiers(), true);
----------------
alexey-bataev wrote:

I would suggest to look at emitInitWithReductionInitializer function and try to use it for initialization

https://github.com/llvm/llvm-project/pull/134709