[Mlir-commits] [mlir] [flang][mlir] Add support for translating task_reduction to LLVMIR (PR #120957)

[Tom Eccles]

================
@@ -1787,16 +1779,264 @@ convertOmpTaskOp(omp::TaskOp taskOp, llvm::IRBuilderBase &builder,
   return success();
 }
 
+template <typename OP>
+llvm::Value *createTaskReductionFunction(
+    llvm::IRBuilderBase &builder, const std::string &name, llvm::Type *redTy,
+    LLVM::ModuleTranslation &moduleTranslation,
+    SmallVectorImpl<omp::DeclareReductionOp> &reductionDecls, Region &region,
+    OP &op, unsigned Cnt, llvm::ArrayRef<bool> &isByRef,
+    SmallVectorImpl<llvm::Value *> &privateReductionVariables,
+    DenseMap<Value, llvm::Value *> &reductionVariableMap) {
+  llvm::LLVMContext &Context = builder.getContext();
+  llvm::Type *OpaquePtrTy = llvm::PointerType::get(Context, 0);
+  if (region.empty()) {
+    return llvm::Constant::getNullValue(OpaquePtrTy);
----------------
tblah wrote:

Nice spot Kareem!

It looks like these pointers should not be null because init and combiner are called unconditionally by the runtime. E.g. https://github.com/llvm/llvm-project/blob/7e01a322f850e86be9eefde8ae5a30e532d22cfa/openmp/runtime/src/kmp_tasking.cpp#L2511.

For normal reductions there will always be combiner and initializer regions. For `declare reduction` the initializer expression is optional so I think one could write legal code without an init region. The combiner expression is required.

The finalization does seem to be optional: https://github.com/llvm/llvm-project/blob/7e01a322f850e86be9eefde8ae5a30e532d22cfa/openmp/runtime/src/kmp_tasking.cpp#L2753C9-L2753C19

https://github.com/llvm/llvm-project/pull/120957