diff --git a/llvm/include/llvm/MC/SubtargetFeature.h b/llvm/include/llvm/MC/SubtargetFeature.h
--- a/llvm/include/llvm/MC/SubtargetFeature.h
+++ b/llvm/include/llvm/MC/SubtargetFeature.h
@@ -40,14 +40,11 @@
 class FeatureBitset {
   static_assert((MAX_SUBTARGET_FEATURES % 64) == 0,
                 "Should be a multiple of 64!");
-  // This cannot be a std::array, operator[] is not constexpr until C++17.
-  uint64_t Bits[MAX_SUBTARGET_WORDS] = {};
+  std::array<uint64_t, MAX_SUBTARGET_WORDS> Bits{};
 
 protected:
-  constexpr FeatureBitset(const std::array<uint64_t, MAX_SUBTARGET_WORDS> &B) {
-    for (unsigned I = 0; I != B.size(); ++I)
-      Bits[I] = B[I];
-  }
+  constexpr FeatureBitset(const std::array<uint64_t, MAX_SUBTARGET_WORDS> &B)
+      : Bits{B} {}
 
 public:
   constexpr FeatureBitset() = default;
@@ -103,7 +100,7 @@
   }
 
   constexpr FeatureBitset &operator^=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] ^= RHS.Bits[I];
     }
     return *this;
@@ -115,7 +112,7 @@
   }
 
   constexpr FeatureBitset &operator&=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] &= RHS.Bits[I];
     }
     return *this;
@@ -127,7 +124,7 @@
   }
 
   constexpr FeatureBitset &operator|=(const FeatureBitset &RHS) {
-    for (unsigned I = 0, E = array_lengthof(Bits); I != E; ++I) {
+    for (unsigned I = 0, E = Bits.size(); I != E; ++I) {
       Bits[I] |= RHS.Bits[I];
     }
     return *this;