Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -811,27 +811,6 @@
   return CI->getArgOperand(0);
 }
 
-// TODO: Does this belong in BuildLibCalls or should all of those similar
-// functions be moved here?
-static Value *emitCalloc(Value *Num, Value *Size, const AttributeList &Attrs,
-                         IRBuilder<> &B, const TargetLibraryInfo &TLI) {
-  LibFunc Func;
-  if (!TLI.getLibFunc("calloc", Func) || !TLI.has(Func))
-    return nullptr;
-
-  Module *M = B.GetInsertBlock()->getModule();
-  const DataLayout &DL = M->getDataLayout();
-  IntegerType *PtrType = DL.getIntPtrType((B.GetInsertBlock()->getContext()));
-  Value *Calloc = M->getOrInsertFunction("calloc", Attrs, B.getInt8PtrTy(),
-                                         PtrType, PtrType);
-  CallInst *CI = B.CreateCall(Calloc, { Num, Size }, "calloc");
-
-  if (const auto *F = dyn_cast<Function>(Calloc->stripPointerCasts()))
-    CI->setCallingConv(F->getCallingConv());
-
-  return CI;
-}
-
 /// Fold memset[_chk](malloc(n), 0, n) --> calloc(1, n).
 static Value *foldMallocMemset(CallInst *Memset, IRBuilder<> &B,
                                const TargetLibraryInfo &TLI) {
@@ -889,6 +868,13 @@
   return CI->getArgOperand(0);
 }
 
+Value *LibCallSimplifier::optimizeRealloc(CallInst *CI, IRBuilder<> &B) {
+  if (isa<ConstantPointerNull>(CI->getArgOperand(0)))
+    return emitMalloc(CI->getArgOperand(1), B, DL, TLI);
+
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 // Math Library Optimizations
 //===----------------------------------------------------------------------===//
@@ -2080,6 +2066,8 @@
       return optimizeMemMove(CI, Builder);
     case LibFunc_memset:
       return optimizeMemSet(CI, Builder);
+    case LibFunc_realloc:
+      return optimizeRealloc(CI, Builder);
     case LibFunc_wcslen:
       return optimizeWcslen(CI, Builder);
     default:
Index: test/Transforms/InstCombine/realloc.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/realloc.ll
@@ -0,0 +1,24 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+declare i8* @realloc(i8*, i64) #1
+declare noalias i8* @malloc(i64) #1
+
+
+define i8* @realloc_null_ptr() #0 {
+; CHECK-LABEL: @realloc_null_ptr(
+; CHECK-NEXT:    [[MALLOC:%.*]] = call i8* @malloc(i64 100)
+; CHECK-NEXT:    ret i8* [[MALLOC]]
+;
+  %malloc = call i8* @malloc(i64 100)
+  ret i8* %malloc
+}
+
+define i8* @realloc_unknown_ptr(i8* %ptr) #0 {
+; CHECK-LABEL: @realloc_unknown_ptr(
+; CHECK-NEXT:    [[CALL:%.*]] = call i8* @realloc(i8* [[PTR:%.*]], i64 100)
+; CHECK-NEXT:    ret i8* [[CALL]]
+;
+  %call = call i8* @realloc(i8* %ptr, i64 100) #2
+  ret i8* %call
+}