Index: lib/Target/TargetLoweringObjectFile.cpp
===================================================================
--- lib/Target/TargetLoweringObjectFile.cpp
+++ lib/Target/TargetLoweringObjectFile.cpp
@@ -54,11 +54,24 @@
   delete Mang;
 }
 
+static bool isNullOrUndef(const Constant *C) {
+  // Check that the constant isn't all zeros or undefs.
+  if (C->isNullValue() || isa<UndefValue>(C))
+    return true;
+  if (!isa<ConstantAggregate>(C))
+    return false;
+  for (unsigned i = 0, e = C->getNumOperands(); i != e; ++i) {
+    if (!isNullOrUndef(cast<Constant>(C->getOperand(i))))
+      return false;
+  }
+  return true;
+}
+
 static bool isSuitableForBSS(const GlobalVariable *GV, bool NoZerosInBSS) {
   const Constant *C = GV->getInitializer();
 
   // Must have zero initializer.
-  if (!C->isNullValue())
+  if (!isNullOrUndef(C))
     return false;
 
   // Leave constant zeros in readonly constant sections, so they can be shared.
Index: test/CodeGen/X86/undef-globals-bss.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/undef-globals-bss.ll
@@ -0,0 +1,12 @@
+; RUN: llc < %s | FileCheck %s
+target triple = "x86_64-apple-darwin"
+
+; CHECK: .zerofill __DATA,__bss,_vals,8000000,2 ## @vals
+@vals = internal unnamed_addr global [2000000 x i32] undef, align 4
+
+; CHECK: .zerofill __DATA,__bss,_struct,8000040,3 ## @struct
+@struct = internal global { i1, [8000000 x i8], [7 x i8], i64, { [4 x i32], { i8 }, i1 } }
+                { i1 false, [8000000 x i8] zeroinitializer, [7 x i8] undef, i64 0,
+                        { [4 x i32], { i8 }, i1 }
+                        { [4 x i32] zeroinitializer, { i8 } { i8 undef }, i1 false }
+                }, align 8