[clang] [CIR] Upstream initial support for union type (PR #137501)

[Iris Shi via cfe-commits]

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@@ -306,3 +320,71 @@ CIRGenTypes::computeRecordLayout(const RecordDecl *rd, cir::RecordType *ty) {
   // TODO: implement verification
   return rl;
 }
+
+void CIRRecordLowering::lowerUnion() {
+  CharUnits layoutSize = astRecordLayout.getSize();
+  mlir::Type storageType = nullptr;
+  bool seenNamedMember = false;
+
+  // Iterate through the fields setting bitFieldInfo and the Fields array. Also
+  // locate the "most appropriate" storage type.  The heuristic for finding the
+  // storage type isn't necessary, the first (non-0-length-bitfield) field's
+  // type would work fine and be simpler but would be different than what we've
+  // been doing and cause lit tests to change.
+  for (const FieldDecl *field : recordDecl->fields()) {
+    mlir::Type fieldType;
+    if (field->isBitField())
+      cirGenTypes.getCGModule().errorNYI(recordDecl->getSourceRange(),
+                                         "bitfields in lowerUnion");
+    else
+      fieldType = getStorageType(field);
+
+    fields[field->getCanonicalDecl()] = 0;
+
+    // Compute zero-initializable status.
+    // This union might not be zero initialized: it may contain a pointer to
+    // data member which might have some exotic initialization sequence.
+    // If this is the case, then we aught not to try and come up with a "better"
+    // type, it might not be very easy to come up with a Constant which
+    // correctly initializes it.
+    if (!seenNamedMember) {
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el-ev wrote:

If the first named member is a `RecordDecl`, it will trigger `ClangIR code gen Not Yet Implemented: isZeroInitializable for RecordType: 'S'`

https://github.com/llvm/llvm-project/pull/137501