diff --git a/lib/Target/X86/AsmParser/X86AsmParser.cpp b/lib/Target/X86/AsmParser/X86AsmParser.cpp
index d8650e06045..4fbe7f68e6d 100644
--- a/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -10,20 +10,21 @@
 #include "llvm/Target/TargetAsmParser.h"
 #include "X86.h"
 #include "X86Subtarget.h"
-#include "llvm/ADT/SmallString.h"
-#include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringSwitch.h"
-#include "llvm/ADT/Twine.h"
+#include "llvm/Target/TargetRegistry.h"
+#include "llvm/Target/TargetAsmParser.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCExpr.h"
 #include "llvm/MC/MCInst.h"
 #include "llvm/MC/MCParser/MCAsmLexer.h"
 #include "llvm/MC/MCParser/MCAsmParser.h"
 #include "llvm/MC/MCParser/MCParsedAsmOperand.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/StringSwitch.h"
+#include "llvm/ADT/Twine.h"
 #include "llvm/Support/SourceMgr.h"
 #include "llvm/Support/raw_ostream.h"
-#include "llvm/Target/TargetRegistry.h"
-#include "llvm/Target/TargetAsmParser.h"
 using namespace llvm;
 
 namespace {
@@ -373,14 +374,18 @@ bool X86ATTAsmParser::ParseRegister(unsigned &RegNo,
   // validation later, so maybe there is no need for this here.
   RegNo = MatchRegisterName(Tok.getString());
 
+  // If the match failed, try the register name as lowercase.
+  if (RegNo == 0)
+    RegNo = MatchRegisterName(LowercaseString(Tok.getString()));
+  
   // FIXME: This should be done using Requires<In32BitMode> and
   // Requires<In64BitMode> so "eiz" usage in 64-bit instructions
   // can be also checked.
   if (RegNo == X86::RIZ && !Is64Bit)
     return Error(Tok.getLoc(), "riz register in 64-bit mode only");
 
-  // Parse %st(1) and "%st" as "%st(0)"
-  if (RegNo == 0 && Tok.getString() == "st") {
+  // Parse "%st" as "%st(0)" and "%st(1)", which is multiple tokens.
+  if (RegNo == 0 && (Tok.getString() == "st" || Tok.getString() == "ST")) {
     RegNo = X86::ST0;
     EndLoc = Tok.getLoc();
     Parser.Lex(); // Eat 'st'
diff --git a/test/MC/AsmParser/X86/x86_instructions.s b/test/MC/AsmParser/X86/x86_instructions.s
index 207de850e7d..a37af85b858 100644
--- a/test/MC/AsmParser/X86/x86_instructions.s
+++ b/test/MC/AsmParser/X86/x86_instructions.s
@@ -316,3 +316,6 @@ enter $0x7ace,$0x7f
 // CHECK: encoding: [0xd9,0x18]
 fstp	(%eax)
 
+// rdar://8456364
+// CHECK: movw	%cs, %ax
+mov %CS, %ax