diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -761,10 +761,13 @@
 }
 
 SizeOffsetType ObjectSizeOffsetVisitor::visitGlobalVariable(GlobalVariable &GV){
-  if (!GV.hasDefinitiveInitializer())
+  // Make sure we have an exact definition that cannot be changed.
+  if (!GV.isDefinitionExact())
     return unknown();
-
+  // Weed out any flexible array types.
   APInt Size(IntTyBits, DL.getTypeAllocSize(GV.getValueType()));
+  if (Size.isNullValue())
+    return unknown();
   return std::make_pair(align(Size, GV.getAlignment()), Zero);
 }
 
diff --git a/llvm/test/Analysis/BasicAA/bug.23540.ll b/llvm/test/Analysis/BasicAA/bug.23540.ll
--- a/llvm/test/Analysis/BasicAA/bug.23540.ll
+++ b/llvm/test/Analysis/BasicAA/bug.23540.ll
@@ -4,8 +4,11 @@
 
 @c = external global i32
 
+; Since there are no sizes and both accesses are based on the same global they
+; (can) partially overlap.
+
 ; CHECK-LABEL: f
-; CHECK: MayAlias: i32* %arrayidx, i32* %arrayidx6
+; CHECK: PartialAlias: i32* %arrayidx, i32* %arrayidx6
 define void @f() {
   %idxprom = zext i32 undef to i64
   %add4 = add i32 0, 1