Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -178,11 +178,25 @@
   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 CallInst *setNonNullParam(CallInst *CI, unsigned ArgNo) {
+  if (!CI->paramHasAttr(ArgNo, Attribute::NonNull))
+    CI->addParamAttr(ArgNo, Attribute::NonNull);
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 // String and Memory Library Call Optimizations
 //===----------------------------------------------------------------------===//
 
 Value *LibCallSimplifier::optimizeStrCat(CallInst *CI, IRBuilder<> &B) {
+  setNonNullParam(CI, 0);
+  setNonNullParam(CI, 1);
   // Extract some information from the instruction
   Value *Dst = CI->getArgOperand(0);
   Value *Src = CI->getArgOperand(1);
@@ -221,6 +235,10 @@
 }
 
 Value *LibCallSimplifier::optimizeStrNCat(CallInst *CI, IRBuilder<> &B) {
+  if (isPositiveSize(CI, 2)) {
+    setNonNullParam(CI, 0);
+    setNonNullParam(CI, 1);
+  }
   // Extract some information from the instruction.
   Value *Dst = CI->getArgOperand(0);
   Value *Src = CI->getArgOperand(1);