Index: llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
+++ llvm/trunk/lib/CodeGen/MIRParser/MILexer.h
@@ -118,7 +118,8 @@
     IRBlock,
     NamedIRValue,
     IRValue,
-    QuotedIRValue // `<constant value>`
+    QuotedIRValue, // `<constant value>`
+    SubRegisterIndex
   };
 
 private:
Index: llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
+++ llvm/trunk/lib/CodeGen/MIRParser/MILexer.cpp
@@ -325,6 +325,15 @@
   return maybeLexIndex(C, Token, "%const.", MIToken::ConstantPoolItem);
 }
 
+static Cursor maybeLexSubRegisterIndex(Cursor C, MIToken &Token,
+                                       ErrorCallbackType ErrorCallback) {
+  const StringRef Rule = "%subreg.";
+  if (!C.remaining().startswith(Rule))
+    return None;
+  return lexName(C, Token, MIToken::SubRegisterIndex, Rule.size(),
+                 ErrorCallback);
+}
+
 static Cursor maybeLexIRBlock(Cursor C, MIToken &Token,
                               ErrorCallbackType ErrorCallback) {
   const StringRef Rule = "%ir-block.";
@@ -570,6 +579,8 @@
     return R.remaining();
   if (Cursor R = maybeLexConstantPoolItem(C, Token))
     return R.remaining();
+  if (Cursor R = maybeLexSubRegisterIndex(C, Token, ErrorCallback))
+    return R.remaining();
   if (Cursor R = maybeLexIRBlock(C, Token, ErrorCallback))
     return R.remaining();
   if (Cursor R = maybeLexIRValue(C, Token, ErrorCallback))
Index: llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
+++ llvm/trunk/lib/CodeGen/MIRParser/MIParser.cpp
@@ -144,6 +144,7 @@
   bool parseGlobalValue(GlobalValue *&GV);
   bool parseGlobalAddressOperand(MachineOperand &Dest);
   bool parseConstantPoolIndexOperand(MachineOperand &Dest);
+  bool parseSubRegisterIndexOperand(MachineOperand &Dest);
   bool parseJumpTableIndexOperand(MachineOperand &Dest);
   bool parseExternalSymbolOperand(MachineOperand &Dest);
   bool parseMDNode(MDNode *&Node);
@@ -1237,6 +1238,17 @@
   return false;
 }
 
+bool MIParser::parseSubRegisterIndexOperand(MachineOperand &Dest) {
+  assert(Token.is(MIToken::SubRegisterIndex));
+  StringRef Name = Token.stringValue();
+  unsigned SubRegIndex = getSubRegIndex(Token.stringValue());
+  if (SubRegIndex == 0)
+    return error(Twine("unknown subregister index '") + Name + "'");
+  lex();
+  Dest = MachineOperand::CreateImm(SubRegIndex);
+  return false;
+}
+
 bool MIParser::parseMDNode(MDNode *&Node) {
   assert(Token.is(MIToken::exclaim));
   auto Loc = Token.location();
@@ -1482,6 +1494,8 @@
     return parseJumpTableIndexOperand(Dest);
   case MIToken::ExternalSymbol:
     return parseExternalSymbolOperand(Dest);
+  case MIToken::SubRegisterIndex:
+    return parseSubRegisterIndexOperand(Dest);
   case MIToken::exclaim:
     return parseMetadataOperand(Dest);
   case MIToken::kw_cfi_same_value:
Index: llvm/trunk/test/CodeGen/MIR/X86/subregister-index-operands.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/subregister-index-operands.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/subregister-index-operands.mir
@@ -0,0 +1,32 @@
+# RUN: llc -march=x86-64 -start-after machine-sink -stop-after machine-sink -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that the MIR parser parses and prints subregisters index
+# operands correctly.
+
+--- |
+
+  define zeroext i1 @t(i1 %c) {
+  entry:
+    ret i1 %c
+  }
+
+...
+---
+# CHECK-LABEL: name: t
+# CHECK: %0 = INSERT_SUBREG %edi, %al, {{[0-9]+}}
+# CHECK: %1 = EXTRACT_SUBREG %eax, {{[0-9]+}}
+# CHECK: %ax = REG_SEQUENCE %1, {{[0-9]+}}, %1, {{[0-9]+}}
+name:            t
+isSSA:           true
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr32 }
+  - { id: 1, class: gr8 }
+body: |
+  bb.0.entry:
+    liveins: %edi, %eax
+    %0 = INSERT_SUBREG %edi, %al, %subreg.sub_8bit
+    %1 = EXTRACT_SUBREG %eax, %subreg.sub_8bit_hi
+    %ax = REG_SEQUENCE %1, %subreg.sub_8bit, %1, %subreg.sub_8bit_hi
+    RETQ %ax
+...
+
Index: llvm/trunk/test/CodeGen/MIR/X86/unknown-subregister-index-op.mir
===================================================================
--- llvm/trunk/test/CodeGen/MIR/X86/unknown-subregister-index-op.mir
+++ llvm/trunk/test/CodeGen/MIR/X86/unknown-subregister-index-op.mir
@@ -0,0 +1,26 @@
+# RUN: not llc -march=x86-64 -start-after machine-sink -stop-after machine-sink -o /dev/null %s 2>&1 | FileCheck %s
+# This test ensures that an error is reported when an unknown subregister index
+# is encountered.
+
+--- |
+
+  define zeroext i1 @t(i1 %c) {
+  entry:
+    ret i1 %c
+  }
+
+...
+---
+name:            t
+isSSA:           true
+tracksRegLiveness: true
+registers:
+  - { id: 0, class: gr32 }
+  - { id: 1, class: gr8 }
+  - { id: 2, class: gr8 }
+body: |
+  bb.0.entry:
+    ; CHECK: [[@LINE+1]]:35: unknown subregister index 'bit8'
+    %0 = INSERT_SUBREG %edi, %al, %subreg.bit8
+    RETQ %0
+...