Unnamed bit-fields in a union should be laid out with a type that doesn't affect alignment.

llvm-svn: 101673

clang/lib/CodeGen/CGRecordLayoutBuilder.cpp

@@ -332,10 +332,26 @@ CGRecordLayoutBuilder::LayoutUnionField(const FieldDecl *Field,
     if (FieldSize == 0)
       return 0;
 
+    const llvm::Type *FieldTy;
+
+    if (!Field->getDeclName()) {
+      // This is an unnamed bit-field, which shouldn't affect alignment on the
+      // struct so we use an array of bytes for it.
+
+      FieldTy = llvm::Type::getInt8Ty(Types.getLLVMContext());
+
+      unsigned NumBytesToAppend =
+        llvm::RoundUpToAlignment(FieldSize, 8) / 8;
+
+      if (NumBytesToAppend > 1)
+        FieldTy = llvm::ArrayType::get(FieldTy, NumBytesToAppend);
+    } else 
+      FieldTy = Types.ConvertTypeForMemRecursive(Field->getType());
+    
     // Add the bit field info.
     LLVMBitFields.push_back(
       LLVMBitFieldInfo(Field, ComputeBitFieldInfo(Types, Field, 0, FieldSize)));
-    return Types.ConvertTypeForMemRecursive(Field->getType());
+    return FieldTy;
   }
   
   // This is a regular union field.

clang/test/CodeGenCXX/bitfield-layout.cpp

@@ -1,9 +1,17 @@
 // RUN: %clang_cc1 %s -triple=x86_64-apple-darwin10 -emit-llvm -o - | FileCheck %s
 
-// CHECK: = type { i32, [4 x i8] }
+// CHECK: %union.Test1 = type { i32, [4 x i8] }
 union Test1 {
   int a;
   int b: 39;
-};
+} t1;
 
-Test1 t1;
+// CHECK: %union.Test2 = type { i8 }
+union Test2 {
+  int : 6;
+} t2;
+
+// CHECK: %union.Test3 = type { [2 x i8] }
+union Test3 {
+  int : 9;
+} t3;