Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -150,6 +150,33 @@
   return true;
 }
 
+static bool isZeroString(Value *Str, CallInst *CI,
+                         const TargetLibraryInfo *TLI) {
+  CallInst *Calloc = dyn_cast<CallInst>(Str);
+  if (!Calloc)
+    return false;
+
+  Function *InnerCallee = Calloc->getCalledFunction();
+  if (!InnerCallee)
+    return false;
+
+  LibFunc Func;
+  if (!TLI->getLibFunc(*InnerCallee, Func) || !TLI->has(Func) ||
+      Func != LibFunc_calloc)
+    return false;
+
+  ConstantInt *N = dyn_cast<ConstantInt>(Calloc->getOperand(0));
+  ConstantInt *Size = dyn_cast<ConstantInt>(Calloc->getOperand(1));
+
+  if (!N || !Size)
+    return false;
+
+  if (N->getZExtValue() == 0 || Size->getZExtValue() == 0)
+    return false;
+
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // String and Memory Library Call Optimizations
 //===----------------------------------------------------------------------===//
@@ -159,6 +186,11 @@
   Value *Dst = CI->getArgOperand(0);
   Value *Src = CI->getArgOperand(1);
 
+  // strcat(calloc(...), str) -> strcpy(calloc(...), str))
+  if (isZeroString(Dst, CI, TLI))
+    return emitStrCpy(Dst, Src, B, TLI);
+
+
   // See if we can get the length of the input string.
   uint64_t Len = GetStringLength(Src);
   if (Len == 0)
@@ -204,6 +236,12 @@
   else
     return nullptr;
 
+  // strncat(calloc(...), str, len) -> strncpy(calloc(...), str, len))
+  if (isZeroString(Dst, CI, TLI)) {
+    B.CreateMemCpy(Dst, 1, Src, 1, ConstantInt::get(DL.getIntPtrType(CI->getContext()), Len));
+    return Dst;
+  }
+
   // See if we can get the length of the input string.
   uint64_t SrcLen = GetStringLength(Src);
   if (SrcLen == 0)
@@ -447,6 +485,10 @@
                                                unsigned CharSize) {
   Value *Src = CI->getArgOperand(0);
 
+  // strlen(calloc(...))
+  if (isZeroString(Src, CI, TLI))
+    return ConstantInt::get(CI->getType(), 0);
+
   // Constant folding: strlen("xyz") -> 3
   if (uint64_t Len = GetStringLength(Src, CharSize))
     return ConstantInt::get(CI->getType(), Len - 1);
Index: test/Transforms/InstCombine/zero-string.ll
===================================================================
--- test/Transforms/InstCombine/zero-string.ll
+++ test/Transforms/InstCombine/zero-string.ll
@@ -0,0 +1,122 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine  -S | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+
+declare i32 @strlen(i8* nocapture)
+declare noalias i8* @calloc(i32, i32)
+declare noalias i8* @malloc(i32)
+declare i8* @strcat(i8*, i8* nocapture readonly)
+declare i8* @strncat(i8*, i8* nocapture readonly, i32)
+
+define i32 @calloc_strlen() {
+; CHECK-LABEL: @calloc_strlen(
+; CHECK-NEXT:    ret i32 0
+;
+  %call = tail call noalias i8* @calloc(i32 1, i32 10)
+  %call1 = tail call i32 @strlen(i8* %call)
+  ret i32 %call1
+}
+
+define i32 @calloc_strlen_not_const_nmemb(i32 %n) {
+; CHECK-LABEL: @calloc_strlen_not_const_nmemb(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 [[N:%.*]], i32 10)
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i32 @strlen(i8* [[CALL]])
+; CHECK-NEXT:    ret i32 [[CALL1]]
+;
+  %call = tail call noalias i8* @calloc(i32 %n, i32 10)
+  %call1 = tail call i32 @strlen(i8* %call) #4
+  ret i32 %call1
+}
+
+
+define i32 @calloc_strlen_not_const_size(i32 %size) {
+; CHECK-LABEL: @calloc_strlen_not_const_size(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 1, i32 [[SIZE:%.*]])
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i32 @strlen(i8* [[CALL]])
+; CHECK-NEXT:    ret i32 [[CALL1]]
+;
+  %call = tail call noalias i8* @calloc(i32 1, i32 %size)
+  %call1 = tail call i32 @strlen(i8* %call) #4
+  ret i32 %call1
+}
+
+
+define i32 @calloc_strlen_not_const_args(i32 %n, i32 %size) {
+; CHECK-LABEL: @calloc_strlen_not_const_args(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 [[N:%.*]], i32 [[SIZE:%.*]])
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i32 @strlen(i8* [[CALL]])
+; CHECK-NEXT:    ret i32 [[CALL1]]
+;
+  %call = tail call noalias i8* @calloc(i32 %n, i32 %size)
+  %call1 = tail call i32 @strlen(i8* %call) #4
+  ret i32 %call1
+}
+
+
+define i32 @malloc_strlen() {
+; CHECK-LABEL: @malloc_strlen(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @malloc(i32 10)
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i32 @strlen(i8* [[CALL]])
+; CHECK-NEXT:    ret i32 [[CALL1]]
+;
+  %call = tail call noalias i8* @malloc(i32 10)
+  %call1 = tail call i32 @strlen(i8* %call)
+  ret i32 %call1
+}
+
+define i8* @malloc_strcat(i8* %str2) {
+; CHECK-LABEL: @malloc_strcat(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @malloc(i32 10)
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i8* @strcat(i8* [[CALL]], i8* [[STR2:%.*]])
+; CHECK-NEXT:    ret i8* [[CALL1]]
+;
+  %call = tail call noalias i8* @malloc(i32 10) #3
+  %call1 = tail call i8* @strcat(i8* %call, i8* %str2)
+  ret i8* %call1
+}
+
+define i8* @calloc_strcat(i8*  %str2) {
+; CHECK-LABEL: @calloc_strcat(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 1, i32 10)
+; CHECK-NEXT:    [[STRCPY:%.*]] = call i8* @strcpy(i8* [[CALL]], i8* [[STR2:%.*]])
+; CHECK-NEXT:    ret i8* [[STRCPY]]
+;
+  %call = tail call noalias i8* @calloc(i32 1, i32 10)
+  %call1 = call i8* @strcat(i8* %call, i8* %str2)
+  ret i8* %call1
+}
+
+
+define i8* @calloc_strncat(i8* %str2)  {
+; CHECK-LABEL: @calloc_strncat(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 1, i32 10)
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 [[CALL]], i8* align 1 [[STR2:%.*]], i32 10, i1 false)
+; CHECK-NEXT:    ret i8* [[CALL]]
+;
+  %call = tail call noalias i8* @calloc(i32 1, i32 10)
+  %call1 = tail call i8* @strncat(i8* %call, i8* %str2, i32 10)
+  ret i8* %call1
+}
+
+define i8* @malloc_strncat(i8* %str2) {
+; CHECK-LABEL: @malloc_strncat(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @malloc(i32 10)
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i8* @strncat(i8* [[CALL]], i8* [[STR2:%.*]], i32 10)
+; CHECK-NEXT:    ret i8* [[CALL1]]
+;
+  %call = tail call noalias i8* @malloc(i32 10)
+  %call1 = tail call i8* @strncat(i8* %call, i8* %str2, i32 10)
+  ret i8* %call1
+}
+
+define i8* @calloc_strncat_not_const(i8* %str2, i32 %n) {
+; CHECK-LABEL: @calloc_strncat_not_const(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call noalias i8* @calloc(i32 1, i32 10)
+; CHECK-NEXT:    [[CALL1:%.*]] = tail call i8* @strncat(i8* [[CALL]], i8* [[STR2:%.*]], i32 [[N:%.*]])
+; CHECK-NEXT:    ret i8* [[CALL1]]
+;
+  %call = tail call noalias i8* @calloc(i32 1, i32 10) #3
+  %call1 = tail call i8* @strncat(i8* %call, i8* %str2, i32 %n)
+  ret i8* %call1
+}