Index: include/llvm/Transforms/Utils/BuildLibCalls.h
===================================================================
--- include/llvm/Transforms/Utils/BuildLibCalls.h
+++ include/llvm/Transforms/Utils/BuildLibCalls.h
@@ -120,6 +120,14 @@
   /// Size is an 'intptr_t', and File is a pointer to FILE.
   Value *emitFWrite(Value *Ptr, Value *Size, Value *File, IRBuilder<> &B,
                     const DataLayout &DL, const TargetLibraryInfo *TLI);
+
+  /// Emit a call to the malloc function.
+  Value *emitMalloc(Value *Len, IRBuilder<> &B, const DataLayout &DL,
+                    const TargetLibraryInfo *TLI);
+
+  /// Emit a call to the calloc function.
+  Value *emitCalloc(Value *Num, Value *Size, const AttributeList &Attrs,
+                    IRBuilder<> &B, const TargetLibraryInfo &TLI);
 }
 
 #endif
Index: include/llvm/Transforms/Utils/SimplifyLibCalls.h
===================================================================
--- include/llvm/Transforms/Utils/SimplifyLibCalls.h
+++ include/llvm/Transforms/Utils/SimplifyLibCalls.h
@@ -124,6 +124,7 @@
   Value *optimizeMemCpy(CallInst *CI, IRBuilder<> &B);
   Value *optimizeMemMove(CallInst *CI, IRBuilder<> &B);
   Value *optimizeMemSet(CallInst *CI, IRBuilder<> &B);
+  Value *optimizeRealloc(CallInst *CI, IRBuilder<> &B);
   Value *optimizeWcslen(CallInst *CI, IRBuilder<> &B);
   // Wrapper for all String/Memory Library Call Optimizations
   Value *optimizeStringMemoryLibCall(CallInst *CI, IRBuilder<> &B);
Index: lib/Transforms/Utils/BuildLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/BuildLibCalls.cpp
+++ lib/Transforms/Utils/BuildLibCalls.cpp
@@ -1037,3 +1037,39 @@
     CI->setCallingConv(Fn->getCallingConv());
   return CI;
 }
+
+Value *llvm::emitMalloc(Value *Len, IRBuilder<> &B, const DataLayout &DL,
+                        const TargetLibraryInfo *TLI) {
+  if (!TLI->has(LibFunc_malloc))
+    return nullptr;
+
+  Module *M = B.GetInsertBlock()->getModule();
+  LLVMContext &Context = B.GetInsertBlock()->getContext();
+  Value *Malloc = M->getOrInsertFunction("malloc", B.getInt8PtrTy(),
+                                         DL.getIntPtrType(Context));
+  inferLibFuncAttributes(*M->getFunction("malloc"), *TLI);
+  CallInst *CI = B.CreateCall(Malloc, Len, "malloc");
+
+  if (const Function *F = dyn_cast<Function>(Malloc->stripPointerCasts()))
+    CI->setCallingConv(F->getCallingConv());
+
+  return CI;
+}
+
+Value *llvm::emitCalloc(Value *Num, Value *Size, const AttributeList &Attrs,
+                        IRBuilder<> &B, const TargetLibraryInfo &TLI) {
+  if (!TLI->has(LibFunc_calloc)
+    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;
+}
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
+}