If a wide bit-field is inside a union its offset should always be 0.

llvm-svn: 101668

2 files changed, 16 insertions, 4 deletions

diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index d1de70a7ab8..a674ad79709 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -640,13 +640,16 @@ void ASTRecordLayoutBuilder::LayoutWideBitField(uint64_t FieldSize,
   // We're not going to use any of the unfilled bits in the last byte.
   UnfilledBitsInLastByte = 0;
 
-  // The bitfield is allocated starting at the next offset aligned appropriately
-  // for T', with length n bits. 
-  uint64_t FieldOffset = llvm::RoundUpToAlignment(DataSize, TypeAlign);
-
+  uint64_t FieldOffset;
+  
   if (IsUnion) {
     DataSize = std::max(DataSize, FieldSize);
+    FieldOffset = 0;
   } else {
+    // The bitfield is allocated starting at the next offset aligned appropriately
+    // for T', with length n bits. 
+    FieldOffset = llvm::RoundUpToAlignment(DataSize, TypeAlign);
+    
     uint64_t NewSizeInBits = FieldOffset + FieldSize;
     
     DataSize = llvm::RoundUpToAlignment(NewSizeInBits, 8);
diff --git a/clang/test/CodeGenCXX/bitfield-layout.cpp b/clang/test/CodeGenCXX/bitfield-layout.cpp
new file mode 100644
index 00000000000..c77c925d876
--- /dev/null
+++ b/clang/test/CodeGenCXX/bitfield-layout.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 %s -triple=x86_64-apple-darwin10 -emit-llvm -o - | FileCheck %s
+
+// CHECK: = type { i32, [4 x i8] }
+union Test1 {
+  int a;
+  int b: 39;
+};
+
+Test1 t1;