Index: lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
===================================================================
--- lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -165,6 +165,7 @@
                       OperandVector &Operands);
 
   bool parseDirectiveArch(SMLoc L);
+  bool parseDirectiveArchExtension(SMLoc L);
   bool parseDirectiveCPU(SMLoc L);
   bool parseDirectiveInst(SMLoc L);
 
@@ -5031,6 +5032,8 @@
     parseDirectiveInst(Loc);
   else if (IDVal == ".cfi_negate_ra_state")
     parseDirectiveCFINegateRAState();
+  else if (IDVal == ".arch_extension")
+    parseDirectiveArchExtension(Loc);
   else if (IsMachO) {
     if (IDVal == MCLOHDirectiveName())
       parseDirectiveLOH(IDVal, Loc);
@@ -5151,6 +5154,49 @@
   return false;
 }
 
+/// parseDirectiveArchExtension
+///   ::= .arch_extension [no]feature
+bool AArch64AsmParser::parseDirectiveArchExtension(SMLoc L) {
+  MCAsmParser &Parser = getParser();
+
+  if (getLexer().isNot(AsmToken::Identifier))
+    return Error(getLexer().getLoc(), "expected architecture extension name");
+
+  StringRef Name = Parser.getTok().getString();
+  SMLoc ExtLoc = Parser.getTok().getLoc();
+  Lex();
+
+  if (parseToken(AsmToken::EndOfStatement,
+                 "unexpected token in '.arch_extension' directive"))
+    return true;
+
+  bool EnableFeature = true;
+  if (Name.startswith_lower("no")) {
+    EnableFeature = false;
+    Name = Name.substr(2);
+  }
+
+  MCSubtargetInfo &STI = copySTI();
+  FeatureBitset Features = STI.getFeatureBits();
+  for (const auto &Extension : ExtensionMap) {
+    if (Extension.Name != Name)
+      continue;
+
+    if (Extension.Features.none())
+      return Error(ExtLoc, "unsupported architectural extension: " + Name);
+
+    FeatureBitset ToggleFeatures = EnableFeature
+                                       ? (~Features & Extension.Features)
+                                       : (Features & Extension.Features);
+    uint64_t Features =
+        ComputeAvailableFeatures(STI.ToggleFeature(ToggleFeatures));
+    setAvailableFeatures(Features);
+    return false;
+  }
+
+  return Error(ExtLoc, "unknown architectural extension: " + Name);
+}
+
 static SMLoc incrementLoc(SMLoc L, int Offset) {
   return SMLoc::getFromPointer(L.getPointer() + Offset);
 }
Index: test/MC/AArch64/directive-arch_extension-nosimd.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/directive-arch_extension-nosimd.s
@@ -0,0 +1,6 @@
+// RUN: not llvm-mc -triple aarch64 -filetype asm -o - %s 2>&1 | FileCheck %s
+
+	.arch_extension nosimd
+
+	add v0.8b, v0.8b, v0.8b
+// CHECK: error: instruction requires: neon
Index: test/MC/AArch64/directive-arch_extension-simd.s
===================================================================
--- /dev/null
+++ test/MC/AArch64/directive-arch_extension-simd.s
@@ -0,0 +1,6 @@
+// RUN: llvm-mc -triple aarch64 -mattr=-simd -filetype asm -o - %s | FileCheck %s
+
+	.arch_extension simd
+
+	add v0.8b, v0.8b, v0.8b
+// CHECK: add v0.8b, v0.8b, v0.8b