diff --git a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
--- a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -3260,11 +3260,13 @@
   // repz repnz <insn>    ; GAS errors for the use of two similar prefixes
   // lock addq %rax, %rbx ; Destination operand must be of memory type
   // xacquire <insn>      ; xacquire must be accompanied by 'lock'
-  bool isPrefix = StringSwitch<bool>(Name)
-                      .Cases("rex64", "data32", "data16", true)
-                      .Cases("xacquire", "xrelease", true)
-                      .Cases("acquire", "release", isParsingIntelSyntax())
-                      .Default(false);
+  bool IsPrefix =
+      StringSwitch<bool>(Name)
+          .Cases("cs", "ds", "es", "fs", "gs", "ss", true)
+          .Cases("rex64", "data32", "data16", "addr32", "addr16", true)
+          .Cases("xacquire", "xrelease", true)
+          .Cases("acquire", "release", isParsingIntelSyntax())
+          .Default(false);
 
   auto isLockRepeatNtPrefix = [](StringRef N) {
     return StringSwitch<bool>(N)
@@ -3333,7 +3335,7 @@
       Name = Next;
       PatchedName = Name;
       ForcedDataPrefix = X86::Mode32Bit;
-      isPrefix = false;
+      IsPrefix = false;
     }
   }
 
@@ -3350,7 +3352,7 @@
   // prefix juxtaposed with an operation like "lock incl 4(%rax)", because we
   // just want to parse the "lock" as the first instruction and the "incl" as
   // the next one.
-  if (getLexer().isNot(AsmToken::EndOfStatement) && !isPrefix) {
+  if (getLexer().isNot(AsmToken::EndOfStatement) && !IsPrefix) {
     // Parse '*' modifier.
     if (getLexer().is(AsmToken::Star))
       Operands.push_back(X86Operand::CreateToken("*", consumeToken()));
@@ -3387,7 +3389,7 @@
 
   // Consume the EndOfStatement or the prefix separator Slash
   if (getLexer().is(AsmToken::EndOfStatement) ||
-      (isPrefix && getLexer().is(AsmToken::Slash)))
+      (IsPrefix && getLexer().is(AsmToken::Slash)))
     Parser.Lex();
   else if (CurlyAsEndOfStatement)
     // Add an actual EndOfStatement before the curly brace
diff --git a/llvm/lib/Target/X86/X86InstrSystem.td b/llvm/lib/Target/X86/X86InstrSystem.td
--- a/llvm/lib/Target/X86/X86InstrSystem.td
+++ b/llvm/lib/Target/X86/X86InstrSystem.td
@@ -171,6 +171,17 @@
 def GS_PREFIX : I<0x65, PrefixByte, (outs), (ins), "gs", []>;
 } // SchedRW
 
+//===----------------------------------------------------------------------===//
+// Address-size override prefixes.
+//
+
+let SchedRW = [WriteNop] in {
+def ADDR16_PREFIX : I<0x67, PrefixByte, (outs), (ins), "addr16", []>,
+                      Requires<[In32BitMode]>;
+def ADDR32_PREFIX : I<0x67, PrefixByte, (outs), (ins), "addr32", []>,
+                      Requires<[In64BitMode]>;
+} // SchedRW
+
 //===----------------------------------------------------------------------===//
 // Moves to and from segment registers.
 //
diff --git a/llvm/test/MC/X86/addr16-32.s b/llvm/test/MC/X86/addr16-32.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/X86/addr16-32.s
@@ -0,0 +1,23 @@
+# RUN: llvm-mc %s -triple x86_64-linux-gnu -filetype=obj -o - | llvm-objdump -d - | FileCheck %s
+
+.text
+.global foo
+foo:
+	insl
+	gs outsl
+	.code64
+	addr32 insl
+	addr32 gs outsl
+	.code32
+	addr16 insl
+	addr16 gs outsl
+	.code64
+	retq
+
+# CHECK: <foo>:
+# CHECK-NEXT: 6d                            insl   %dx, %es:(%rdi)
+# CHECK-NEXT: 65 6f                         outsl  %gs:(%rsi), %dx
+# CHECK-NEXT: 67 6d                         insl   %dx, %es:(%edi)
+# CHECK-NEXT: 67 65 6f                      outsl  %gs:(%esi), %dx
+# CHECK-NEXT: 67 6d                         insl   %dx, %es:(%edi)
+# CHECK-NEXT: 67 65 6f                      outsl  %gs:(%esi), %dx
diff --git a/llvm/test/MC/X86/segment-prefix.s b/llvm/test/MC/X86/segment-prefix.s
new file mode 100644
--- /dev/null
+++ b/llvm/test/MC/X86/segment-prefix.s
@@ -0,0 +1,20 @@
+# RUN: llvm-mc %s -triple x86_64-linux-gnu -filetype=obj -o - | llvm-objdump -d - | FileCheck %s
+
+.text
+.global foo
+foo:
+	cs outsl
+	ds outsl
+	es outsw
+	fs outsw
+	gs outsl
+	ss outsl
+	retq
+
+# CHECK: <foo>:
+# CHECK-NEXT: 2e 6f                         outsl  %cs:(%rsi), %dx
+# CHECK-NEXT: 3e 6f                         outsl  %ds:(%rsi), %dx
+# CHECK-NEXT: 26 66 6f                      outsw  %es:(%rsi), %dx
+# CHECK-NEXT: 64 66 6f                      outsw  %fs:(%rsi), %dx
+# CHECK-NEXT: 65 6f                         outsl  %gs:(%rsi), %dx
+# CHECK-NEXT: 36 6f                         outsl  %ss:(%rsi), %dx