diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index dbaadd824a1..af8649499b2 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -81,8 +81,16 @@ void llvm::ComputeMaskedBits(Value *V, const APInt &Mask,
   // The address of an aligned GlobalValue has trailing zeros.
   if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
     unsigned Align = GV->getAlignment();
-    if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) 
-      Align = TD->getPrefTypeAlignment(GV->getType()->getElementType());
+    if (Align == 0 && TD && GV->getType()->getElementType()->isSized()) {
+      const Type *ObjectType = GV->getType()->getElementType();
+      // If the object is defined in the current Module, we'll be giving
+      // it the preferred alignment. Otherwise, we have to assume that it
+      // may only have the minimum ABI alignment.
+      if (!GV->isDeclaration() && !GV->mayBeOverridden())
+        Align = TD->getPrefTypeAlignment(ObjectType);
+      else
+        Align = TD->getABITypeAlignment(ObjectType);
+    }
     if (Align > 0)
       KnownZero = Mask & APInt::getLowBitsSet(BitWidth,
                                               CountTrailingZeros_32(Align));
diff --git a/test/Transforms/InstCombine/align-external.ll b/test/Transforms/InstCombine/align-external.ll
new file mode 100644
index 00000000000..38be3143eda
--- /dev/null
+++ b/test/Transforms/InstCombine/align-external.ll
@@ -0,0 +1,22 @@
+; RUN: llvm-as < %s | opt -instcombine | llvm-dis | FileCheck %s
+
+; Don't assume that external global variables have their preferred
+; alignment. They may only have the ABI minimum alignment.
+
+; CHECK: %s = shl i64 %a, 3
+; CHECK: %r = or i64 %s, ptrtoint (i32* @A to i64)
+; CHECK: %q = add i64 %r, 1
+; CHECK: ret i64 %q
+
+target datalayout = "-i32:8:32"
+
+@A = external global i32
+@B = external global i32
+
+define i64 @foo(i64 %a) {
+  %t = ptrtoint i32* @A to i64
+  %s = shl i64 %a, 3
+  %r = or i64 %t, %s
+  %q = add i64 %r, 1
+  ret i64 %q
+}