diff --git a/lib/CodeGen/CGExpr.cpp b/lib/CodeGen/CGExpr.cpp
index c5f2387..c52e6bd 100644
--- a/lib/CodeGen/CGExpr.cpp
+++ b/lib/CodeGen/CGExpr.cpp
@@ -843,14 +843,16 @@
 LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E) {
   // The index must always be an integer, which is not an aggregate.  Emit it.
   llvm::Value *Idx = EmitScalarExpr(E->getIdx());
-  
+  QualType IdxTy  = E->getIdx()->getType();
+  bool IdxSigned = IdxTy->isSignedIntegerType();
+
   // If the base is a vector type, then we are forming a vector element lvalue
   // with this subscript.
   if (E->getBase()->getType()->isVectorType()) {
     // Emit the vector as an lvalue to get its address.
     LValue LHS = EmitLValue(E->getBase());
     assert(LHS.isSimple() && "Can only subscript lvalue vectors here!");
-    // FIXME: This should properly sign/zero/extend or truncate Idx to i32.
+    Idx = Builder.CreateIntCast(Idx, llvm::Type::Int32Ty, IdxSigned, "vidx");
     return LValue::MakeVectorElt(LHS.getAddress(), Idx,
       E->getBase()->getType().getCVRQualifiers());
   }
@@ -859,8 +861,6 @@
   llvm::Value *Base = EmitScalarExpr(E->getBase());
   
   // Extend or truncate the index type to 32 or 64-bits.
-  QualType IdxTy  = E->getIdx()->getType();
-  bool IdxSigned = IdxTy->isSignedIntegerType();
   unsigned IdxBitwidth = cast<llvm::IntegerType>(Idx->getType())->getBitWidth();
   if (IdxBitwidth != LLVMPointerWidth)
     Idx = Builder.CreateIntCast(Idx, llvm::IntegerType::get(LLVMPointerWidth),
diff --git a/test/CodeGen/vector.c b/test/CodeGen/vector.c
index 2e753b4..1084f6d 100644
--- a/test/CodeGen/vector.c
+++ b/test/CodeGen/vector.c
@@ -11,3 +11,11 @@
 
 typedef int vty __attribute((vector_size(16)));
 int a() { vty b; return b[2LL]; }
+
+// PR4339
+typedef float vec4 __attribute__((vector_size(16)));
+
+void vac ( vec4* a, char b, float c )
+{
+	(*a)[b] = c;
+}