[CIR][Lowering] fix lowering for the structs inited with zeros (llvm#315

)

Basically that is, the  next code should work now
```
typedef struct {
    int a;
    int b;
} A;
...
A a = {0, 0};
```

clang/lib/CIR/Lowering/DirectToLLVM/LowerToLLVM.cpp

@@ -9,7 +9,6 @@
 // This file implements lowering of CIR operations to LLVMIR.
 //
 //===----------------------------------------------------------------------===//
-
 #include "mlir/Conversion/AffineToStandard/AffineToStandard.h"
 #include "mlir/Conversion/ControlFlowToLLVM/ControlFlowToLLVM.h"
 #include "mlir/Conversion/FuncToLLVM/ConvertFuncToLLVM.h"
@@ -1076,6 +1075,17 @@ class CIRConstantLowering
       rewriter.replaceAllUsesWith(op, initVal);
       rewriter.eraseOp(op);
       return mlir::success();
+    } else if (auto strTy = op.getType().dyn_cast<mlir::cir::StructType>()) {
+      if (auto zero = op.getValue().dyn_cast<mlir::cir::ZeroAttr>()) {
+        auto initVal =
+          lowerCirAttrAsValue(op, zero, rewriter, typeConverter);
+        rewriter.replaceAllUsesWith(op, initVal);
+        rewriter.eraseOp(op);
+        return mlir::success();
+      }
+
+      return op.emitError()
+        << "unsupported lowering for struct constant type " << op.getType();
     } else
       return op.emitError() << "unsupported constant type " << op.getType();
 

clang/test/CIR/Lowering/struct-init.c

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir-enable -emit-llvm %s -o %t.ll
+// RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM
+struct S {
+    int x;    
+};
+
+// LLVM: define void @zeroInit
+// LLVM: [[TMP0:%.*]] = alloca %struct.S, i64 1
+// LLVM: store %struct.S zeroinitializer, ptr [[TMP0]]
+void zeroInit() {
+  struct S s = {0};
+}