Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -178,11 +178,30 @@
   return true;
 }
 
+template <typename T>
+static bool isPositiveSize(T *I, unsigned ArgNo) {
+  Value *Arg = I->getArgOperand(ArgNo);
+  return isa<ConstantInt>(Arg) && !cast<ConstantInt>(Arg)->isZero();
+}
+
+static bool setNonNullParam(CallInst *CI, unsigned ArgNo) {
+  if (!CI->paramHasAttr(ArgNo, Attribute::NonNull)) {
+    CI->addParamAttr(ArgNo, Attribute::NonNull);
+    return true;
+  }
+  return false;
+}
+
 //===----------------------------------------------------------------------===//
 // String and Memory Library Call Optimizations
 //===----------------------------------------------------------------------===//
 
 Value *LibCallSimplifier::optimizeStrCat(CallInst *CI, IRBuilder<> &B) {
+  bool AttrChanged = false;
+  AttrChanged |= setNonNullParam(CI, 0);
+  AttrChanged |= setNonNullParam(CI, 1);
+  if (AttrChanged)
+    return CI;
   // Extract some information from the instruction
   Value *Dst = CI->getArgOperand(0);
   Value *Src = CI->getArgOperand(1);
@@ -244,6 +263,13 @@
   if (SrcLen == 0 || Len == 0)
     return Dst;
 
+  // Non-zero length case
+  bool AttrChanged = false;
+  AttrChanged |= setNonNullParam(CI, 0);
+  AttrChanged |= setNonNullParam(CI, 1);
+  if (AttrChanged)
+    return CI;
+
   // We don't optimize this case.
   if (Len < SrcLen)
     return nullptr;