[CIR][CIRGen] Improve emission for array of unions

clang/lib/CIR/CodeGen/CIRGenExprConst.cpp

@@ -1015,6 +1015,20 @@ class ConstExprEmitter
         return {};
     }
 
+    auto desiredType = CGM.getTypes().ConvertType(T);
+    bool isDesiredArrayOfUnionDirectly = [&]() {
+      auto desiredArrayType = dyn_cast<cir::ArrayType>(desiredType);
+      if (!desiredArrayType)
+        return false;
+
+      auto elementStructType =
+          dyn_cast<cir::StructType>(desiredArrayType.getEltType());
+      if (!elementStructType)
+        return false;
+
+      return elementStructType.isUnion();
+    }();
+
     // Emit initializer elements as MLIR attributes and check for common type.
     mlir::Type CommonElementType;
     for (unsigned i = 0; i != NumInitableElts; ++i) {
@@ -1024,10 +1038,12 @@ class ConstExprEmitter
         return {};
       if (i == 0)
         CommonElementType = C.getType();
+      else if (isDesiredArrayOfUnionDirectly &&
+               C.getType() != CommonElementType)
+        CommonElementType = nullptr;
       Elts.push_back(std::move(C));
     }
 
-    auto desiredType = CGM.getTypes().ConvertType(T);
     auto typedFiller = llvm::dyn_cast_or_null<mlir::TypedAttr>(Filler);
     if (Filler && !typedFiller)
       llvm_unreachable("We shouldn't be receiving untyped attrs here");

clang/test/CIR/Lowering/union-array.c

@@ -0,0 +1,19 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm -fno-clangir-call-conv-lowering %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s --check-prefix=LLVM
+
+typedef struct {
+  char a;
+} S_1;
+
+typedef struct {
+  long a, b;
+} S_2;
+
+typedef union {
+  S_1 a;
+  S_2 b;
+} U;
+
+void foo() { U arr[2] = {{.b = {1, 2}}, {.a = {1}}}; }
+
+// LLVM: store { { %struct.S_2 }, { %struct.S_1, [15 x i8] } } { { %struct.S_2 } { %struct.S_2 { i64 1, i64 2 } }, { %struct.S_1, [15 x i8] } { %struct.S_1 { i8 1 }, [15 x i8] zeroinitializer } }