diff --git a/llvm/lib/Analysis/TargetLibraryInfo.cpp b/llvm/lib/Analysis/TargetLibraryInfo.cpp
--- a/llvm/lib/Analysis/TargetLibraryInfo.cpp
+++ b/llvm/lib/Analysis/TargetLibraryInfo.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Support/CommandLine.h"
@@ -940,16 +941,26 @@
   return GlobalValue::dropLLVMManglingEscape(funcName);
 }
 
+static DenseMap<StringRef, LibFunc>
+buildIndexMap(ArrayRef<StringLiteral> StandardNames) {
+  DenseMap<StringRef, LibFunc> Indices;
+  unsigned Idx = 0;
+  Indices.reserve(LibFunc::NumLibFuncs);
+  for (const auto &Func : StandardNames)
+    Indices[Func] = static_cast<LibFunc>(Idx++);
+  return Indices;
+}
+
 bool TargetLibraryInfoImpl::getLibFunc(StringRef funcName, LibFunc &F) const {
   funcName = sanitizeFunctionName(funcName);
   if (funcName.empty())
     return false;
 
-  const auto *Start = std::begin(StandardNames);
-  const auto *End = std::end(StandardNames);
-  const auto *I = std::lower_bound(Start, End, funcName);
-  if (I != End && *I == funcName) {
-    F = (LibFunc)(I - Start);
+  static const DenseMap<StringRef, LibFunc> Indices =
+      buildIndexMap(StandardNames);
+
+  if (auto Loc = Indices.find(funcName); Loc != Indices.end()) {
+    F = Loc->second;
     return true;
   }
   return false;