Index: llvm/trunk/lib/CodeGen/MIRParser/CMakeLists.txt
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/CMakeLists.txt
+++ llvm/trunk/lib/CodeGen/MIRParser/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_llvm_library(LLVMMIRParser
+  MILexer.cpp
   MIParser.cpp
   MIRParser.cpp
   )
Index: llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
+++ llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
@@ -0,0 +1,65 @@
+//===- MILexer.h - Lexer for machine instructions -------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file declares the function that lexes the machine instruction source
+// string.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIB_CODEGEN_MIRPARSER_MILEXER_H
+#define LLVM_LIB_CODEGEN_MIRPARSER_MILEXER_H
+
+#include "llvm/ADT/StringRef.h"
+#include "llvm/ADT/STLExtras.h"
+#include <functional>
+
+namespace llvm {
+
+class Twine;
+
+/// A token produced by the machine instruction lexer.
+struct MIToken {
+  enum TokenKind {
+    // Markers
+    Eof,
+    Error,
+
+    // Identifier tokens
+    Identifier
+  };
+
+private:
+  TokenKind Kind;
+  StringRef Range;
+
+public:
+  MIToken(TokenKind Kind, StringRef Range) : Kind(Kind), Range(Range) {}
+
+  TokenKind kind() const { return Kind; }
+
+  bool isError() const { return Kind == Error; }
+
+  bool is(TokenKind K) const { return Kind == K; }
+
+  bool isNot(TokenKind K) const { return Kind != K; }
+
+  StringRef::iterator location() const { return Range.begin(); }
+
+  StringRef stringValue() const { return Range; }
+};
+
+/// Consume a single machine instruction token in the given source and return
+/// the remaining source string.
+StringRef lexMIToken(
+    StringRef Source, MIToken &Token,
+    function_ref<void(StringRef::iterator, const Twine &)> ErrorCallback);
+
+} // end namespace llvm
+
+#endif
Index: llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
+++ llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
@@ -0,0 +1,87 @@
+//===- MILexer.cpp - Machine instructions lexer implementation ----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the lexing of machine instructions.
+//
+//===----------------------------------------------------------------------===//
+
+#include "MILexer.h"
+#include "llvm/ADT/Twine.h"
+#include <cctype>
+
+using namespace llvm;
+
+namespace {
+
+/// This class provides a way to iterate and get characters from the source
+/// string.
+class Cursor {
+  const char *Ptr;
+  const char *End;
+
+public:
+  explicit Cursor(StringRef Str) {
+    Ptr = Str.data();
+    End = Ptr + Str.size();
+  }
+
+  bool isEOF() const { return Ptr == End; }
+
+  char peek() const { return isEOF() ? 0 : *Ptr; }
+
+  void advance() { ++Ptr; }
+
+  StringRef remaining() const { return StringRef(Ptr, End - Ptr); }
+
+  StringRef upto(Cursor C) const {
+    assert(C.Ptr >= Ptr && C.Ptr <= End);
+    return StringRef(Ptr, C.Ptr - Ptr);
+  }
+
+  StringRef::iterator location() const { return Ptr; }
+};
+
+} // end anonymous namespace
+
+/// Skip the leading whitespace characters and return the updated cursor.
+static Cursor skipWhitespace(Cursor C) {
+  while (isspace(C.peek()))
+    C.advance();
+  return C;
+}
+
+static bool isIdentifierChar(char C) {
+  return isalpha(C) || isdigit(C) || C == '_' || C == '-' || C == '.';
+}
+
+static Cursor lexIdentifier(Cursor C, MIToken &Token) {
+  auto Range = C;
+  while (isIdentifierChar(C.peek()))
+    C.advance();
+  Token = MIToken(MIToken::Identifier, Range.upto(C));
+  return C;
+}
+
+StringRef llvm::lexMIToken(
+    StringRef Source, MIToken &Token,
+    function_ref<void(StringRef::iterator Loc, const Twine &)> ErrorCallback) {
+  auto C = skipWhitespace(Cursor(Source));
+  if (C.isEOF()) {
+    Token = MIToken(MIToken::Eof, C.remaining());
+    return C.remaining();
+  }
+
+  auto Char = C.peek();
+  if (isalpha(Char) || Char == '_')
+    return lexIdentifier(C, Token).remaining();
+  Token = MIToken(MIToken::Error, C.remaining());
+  ErrorCallback(C.location(),
+                Twine("unexpected character '") + Twine(Char) + "'");
+  return C.remaining();
+}
Index: llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
+++ llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
@@ -12,6 +12,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "MIParser.h"
+#include "MILexer.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
@@ -29,7 +30,8 @@
   SourceMgr &SM;
   MachineFunction &MF;
   SMDiagnostic &Error;
-  StringRef Source;
+  StringRef Source, CurrentSource;
+  MIToken Token;
   /// Maps from instruction names to op codes.
   StringMap<unsigned> Names2InstrOpCodes;
 
@@ -37,11 +39,18 @@
   MIParser(SourceMgr &SM, MachineFunction &MF, SMDiagnostic &Error,
            StringRef Source);
 
+  void lex();
+
   /// Report an error at the current location with the given message.
   ///
   /// This function always return true.
   bool error(const Twine &Msg);
 
+  /// Report an error at the given location with the given message.
+  ///
+  /// This function always return true.
+  bool error(StringRef::iterator Loc, const Twine &Msg);
+
   MachineInstr *parse();
 
 private:
@@ -50,31 +59,42 @@
   /// Try to convert an instruction name to an opcode. Return true if the
   /// instruction name is invalid.
   bool parseInstrName(StringRef InstrName, unsigned &OpCode);
+
+  bool parseInstruction(unsigned &OpCode);
 };
 
 } // end anonymous namespace
 
 MIParser::MIParser(SourceMgr &SM, MachineFunction &MF, SMDiagnostic &Error,
                    StringRef Source)
-    : SM(SM), MF(MF), Error(Error), Source(Source) {}
+    : SM(SM), MF(MF), Error(Error), Source(Source), CurrentSource(Source),
+      Token(MIToken::Error, StringRef()) {}
+
+void MIParser::lex() {
+  CurrentSource = lexMIToken(
+      CurrentSource, Token,
+      [this](StringRef::iterator Loc, const Twine &Msg) { error(Loc, Msg); });
+}
+
+bool MIParser::error(const Twine &Msg) { return error(Token.location(), Msg); }
 
-bool MIParser::error(const Twine &Msg) {
+bool MIParser::error(StringRef::iterator Loc, const Twine &Msg) {
   // TODO: Get the proper location in the MIR file, not just a location inside
   // the string.
-  Error =
-      SMDiagnostic(SM, SMLoc(), SM.getMemoryBuffer(SM.getMainFileID())
-                                    ->getBufferIdentifier(),
-                   1, 0, SourceMgr::DK_Error, Msg.str(), Source, None, None);
+  assert(Loc >= Source.data() && Loc <= (Source.data() + Source.size()));
+  Error = SMDiagnostic(
+      SM, SMLoc(),
+      SM.getMemoryBuffer(SM.getMainFileID())->getBufferIdentifier(), 1,
+      Loc - Source.data(), SourceMgr::DK_Error, Msg.str(), Source, None, None);
   return true;
 }
 
 MachineInstr *MIParser::parse() {
-  StringRef InstrName = Source;
+  lex();
+
   unsigned OpCode;
-  if (parseInstrName(InstrName, OpCode)) {
-    error(Twine("unknown machine instruction name '") + InstrName + "'");
+  if (Token.isError() || parseInstruction(OpCode))
     return nullptr;
-  }
 
   // TODO: Parse the rest of instruction - machine operands, etc.
   const auto &MCID = MF.getSubtarget().getInstrInfo()->get(OpCode);
@@ -82,6 +102,15 @@
   return MI;
 }
 
+bool MIParser::parseInstruction(unsigned &OpCode) {
+  if (Token.isNot(MIToken::Identifier))
+    return error("expected a machine instruction");
+  StringRef InstrName = Token.stringValue();
+  if (parseInstrName(InstrName, OpCode))
+    return error(Twine("unknown machine instruction name '") + InstrName + "'");
+  return false;
+}
+
 void MIParser::initNames2InstrOpCodes() {
   if (!Names2InstrOpCodes.empty())
     return;
Index: llvm/trunk/test/CodeGen/MIR/X86/machine-instructions.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/machine-instructions.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/machine-instructions.mir
@@ -20,5 +20,5 @@
      # CHECK:      - IMUL32rri8
      # CHECK-NEXT: - RETQ
      - IMUL32rri8
-     - RETQ
+     - ' RETQ '
 ...
Index: llvm/trunk/test/CodeGen/MIR/X86/missing-instruction.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/missing-instruction.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/missing-instruction.mir
@@ -0,0 +1,18 @@
+# RUN: not llc -march=x86-64 -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+
+--- |
+
+  define void @foo() {
+  entry:
+    ret void
+  }
+
+...
+---
+name:            foo
+body:
+ - name:         entry
+   instructions:
+     # CHECK: 1:1: expected a machine instruction
+     - ''
+...
Index: llvm/trunk/test/CodeGen/MIR/X86/unrecognized-character.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/unrecognized-character.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/unrecognized-character.mir
@@ -0,0 +1,18 @@
+# RUN: not llc -march=x86-64 -start-after branch-folder -stop-after branch-folder -o /dev/null %s 2>&1 | FileCheck %s
+
+--- |
+
+  define void @foo() {
+  entry:
+    ret void
+  }
+
+...
+---
+name:            foo
+body:
+ - name:         entry
+   instructions:
+     # CHECK: 1:1: unexpected character '`'
+     - '` RETQ'
+...