[CIR][Lowering] Fix lowering for multi dimensional array (#851)

This PR fixes the lowering for multi dimensional arrays.

Consider the following code snippet `test.c`: 
```
void foo() {
  char arr[4][1] = {"a", "b", "c", "d"};
}
```

When ran with `bin/clang test.c -Xclang -fclangir -Xclang -emit-llvm -S
-o -`, It produces the following error:
```
~/clangir/llvm/include/llvm/Support/Casting.h:566: decltype(auto) llvm::cast(const From&) [with To = mlir::ArrayAttr; From = mlir::Attribute]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
```

The bug can be traced back to `LoweringHelpers.cpp`. It considers the
values in the array as integer types, and this causes an error in this
case.

This PR updates `convertToDenseElementsAttrImpl` when the array contains
string attributes. I have also added one more similar test. Note that in
the tests I used a **literal match** to avoid matching as regex, so
`!dbg` is useful.

Author: bruteforceboy

clang/lib/CIR/Lowering/LoweringHelpers.cpp

@@ -61,16 +61,43 @@ mlir::Type getNestedTypeAndElemQuantity(mlir::Type Ty, unsigned &elemQuantity) {
   return nestTy;
 }
 
+template <typename StorageTy>
+void fillTrailingZeros(mlir::cir::ConstArrayAttr attr,
+                       llvm::SmallVectorImpl<StorageTy> &values) {
+  auto numTrailingZeros = attr.getTrailingZerosNum();
+  if (numTrailingZeros) {
+    auto localArrayTy = mlir::dyn_cast<mlir::cir::ArrayType>(attr.getType());
+    assert(localArrayTy && "expected !cir.array");
+
+    auto nestTy = localArrayTy.getEltType();
+    if (!mlir::isa<mlir::cir::ArrayType>(nestTy))
+      values.insert(values.end(), numTrailingZeros,
+                    getZeroInitFromType<StorageTy>(nestTy));
+  }
+}
+
 template <typename AttrTy, typename StorageTy>
 void convertToDenseElementsAttrImpl(mlir::cir::ConstArrayAttr attr,
                                     llvm::SmallVectorImpl<StorageTy> &values) {
+  if (auto stringAttr = mlir::dyn_cast<mlir::StringAttr>(attr.getElts())) {
+    if (auto arrayType = mlir::dyn_cast<mlir::cir::ArrayType>(attr.getType())) {
+      for (auto element : stringAttr) {
+        auto intAttr = mlir::cir::IntAttr::get(arrayType.getEltType(), element);
+        values.push_back(mlir::dyn_cast<AttrTy>(intAttr).getValue());
+      }
+      return;
+    }
+  }
+
   auto arrayAttr = mlir::cast<mlir::ArrayAttr>(attr.getElts());
   for (auto eltAttr : arrayAttr) {
     if (auto valueAttr = mlir::dyn_cast<AttrTy>(eltAttr)) {
       values.push_back(valueAttr.getValue());
     } else if (auto subArrayAttr =
                    mlir::dyn_cast<mlir::cir::ConstArrayAttr>(eltAttr)) {
       convertToDenseElementsAttrImpl<AttrTy>(subArrayAttr, values);
+      if (mlir::dyn_cast<mlir::StringAttr>(subArrayAttr.getElts()))
+        fillTrailingZeros(subArrayAttr, values);
     } else if (auto zeroAttr = mlir::dyn_cast<mlir::cir::ZeroAttr>(eltAttr)) {
       unsigned numStoredZeros = 0;
       auto nestTy =
@@ -84,16 +111,7 @@ void convertToDenseElementsAttrImpl(mlir::cir::ConstArrayAttr attr,
 
   // Only fill in trailing zeros at the local cir.array level where the element
   // type isn't another array (for the mult-dim case).
-  auto numTrailingZeros = attr.getTrailingZerosNum();
-  if (numTrailingZeros) {
-    auto localArrayTy = mlir::dyn_cast<mlir::cir::ArrayType>(attr.getType());
-    assert(localArrayTy && "expected !cir.array");
-
-    auto nestTy = localArrayTy.getEltType();
-    if (!mlir::isa<mlir::cir::ArrayType>(nestTy))
-      values.insert(values.end(), numTrailingZeros,
-                    getZeroInitFromType<StorageTy>(nestTy));
-  }
+  fillTrailingZeros(attr, values);
 }
 
 template <typename AttrTy, typename StorageTy>

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

@@ -1,10 +1,27 @@
 // RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm %s -o %t.ll
 // RUN: FileCheck --input-file=%t.ll %s -check-prefix=LLVM
 
+// LLVM: charInit1.ar = internal global [4 x [4 x i8]] {{.*}}4 x i8] c"aa\00\00", [4 x i8] c"aa\00\00", [4 x i8] c"aa\00\00", [4 x i8] c"aa\00\00"], align 16
+char charInit1() {
+  static char ar[][4] = {"aa", "aa", "aa", "aa"};
+  return ar[0][0];
+}
+
 // LLVM: define dso_local void @zeroInit
 // LLVM: [[RES:%.*]] = alloca [3 x i32], i64 1
 // LLVM: store [3 x i32] zeroinitializer, ptr [[RES]]
 void zeroInit() {
   int a[3] = {0, 0, 0};
 }
 
+// LLVM: %1 = alloca [4 x [1 x i8]], i64 1, align 1
+// LLVM: store [4 x [1 x i8]] {{.*}}1 x i8] c"a", [1 x i8] c"b", [1 x i8] c"c", [1 x i8] c"d"], ptr %1, align 1
+void charInit2() {
+  char arr[4][1] = {"a", "b", "c", "d"};
+}
+
+// LLVM: %1 = alloca [4 x [2 x i8]], i64 1, align 1
+// LLVM: store [4 x [2 x i8]] {{.*}}2 x i8] c"ab", [2 x i8] c"cd", [2 x i8] c"ef", [2 x i8] c"gh"], ptr %1, align 1
+void charInit3() {
+  char arr[4][2] = {"ab", "cd", "ef", "gh"};
+}