Index: llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ llvm/trunk/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -2360,7 +2360,7 @@
     Name == "lock" || Name == "rep" ||
     Name == "repe" || Name == "repz" ||
     Name == "repne" || Name == "repnz" ||
-    Name == "rex64" || Name == "data16";
+    Name == "rex64" || Name == "data16" || Name == "data32";
 
   bool CurlyAsEndOfStatement = false;
   // This does the actual operand parsing.  Don't parse any more if we have a
Index: llvm/trunk/lib/Target/X86/InstPrinter/X86ATTInstPrinter.cpp
===================================================================
--- llvm/trunk/lib/Target/X86/InstPrinter/X86ATTInstPrinter.cpp
+++ llvm/trunk/lib/Target/X86/InstPrinter/X86ATTInstPrinter.cpp
@@ -61,6 +61,17 @@
     OS << "\tcallq\t";
     printPCRelImm(MI, 0, OS);
   }
+  // data16 and data32 both have the same encoding of 0x66. While data32 is
+  // valid only in 16 bit systems, data16 is valid in the rest.
+  // There seems to be some lack of support of the Requires clause that causes
+  // 0x66 to be interpreted as "data16" by the asm printer.
+  // Thus we add an adjustment here in order to print the "right" instruction.
+  else if (MI->getOpcode() == X86::DATA16_PREFIX &&
+    (STI.getFeatureBits()[X86::Mode16Bit])) {
+    MCInst Data32MI(*MI);
+    Data32MI.setOpcode(X86::DATA32_PREFIX);
+    printInstruction(&Data32MI, OS);
+  }
   // Try to print any aliases first.
   else if (!printAliasInstr(MI, OS))
     printInstruction(MI, OS);
Index: llvm/trunk/lib/Target/X86/X86InstrInfo.td
===================================================================
--- llvm/trunk/lib/Target/X86/X86InstrInfo.td
+++ llvm/trunk/lib/Target/X86/X86InstrInfo.td
@@ -1967,7 +1967,12 @@
                      Requires<[In64BitMode]>;
 
 // Data16 instruction prefix
-def DATA16_PREFIX : I<0x66, RawFrm, (outs),  (ins), "data16", []>;
+def DATA16_PREFIX : I<0x66, RawFrm, (outs),  (ins), "data16", []>, 
+                     Requires<[Not16BitMode]>;
+
+// Data instruction prefix
+def DATA32_PREFIX : I<0x66, RawFrm, (outs),  (ins), "data32", []>, 
+                     Requires<[In16BitMode]>;
 
 // Repeat string operation instruction prefixes
 // These uses the DF flag in the EFLAGS register to inc or dec ECX
Index: llvm/trunk/test/MC/X86/data-prefix-fail.s
===================================================================
--- llvm/trunk/test/MC/X86/data-prefix-fail.s
+++ llvm/trunk/test/MC/X86/data-prefix-fail.s
@@ -0,0 +1,25 @@
+// RUN: not llvm-mc -triple x86_64-unknown-unknown --show-encoding %s 2> %t.err | FileCheck --check-prefix=64 %s
+// RUN: FileCheck --check-prefix=ERR64 < %t.err %s
+// RUN: not llvm-mc -triple i386-unknown-unknown --show-encoding %s 2> %t.err | FileCheck --check-prefix=32 %s
+// RUN: FileCheck --check-prefix=ERR32 < %t.err %s
+// RUN: not llvm-mc -triple i386-unknown-unknown-code16 --show-encoding %s 2> %t.err | FileCheck --check-prefix=16 %s
+// RUN: FileCheck --check-prefix=ERR16 < %t.err %s
+
+// ERR64: error: instruction requires: 16-bit mode
+// ERR32: error: instruction requires: 16-bit mode
+// 16: data32
+// 16: encoding: [0x66]
+// 16: lgdtw 0
+// 16: encoding: [0x0f,0x01,0x16,0x00,0x00]
+data32 lgdt 0
+
+// 64: data16
+// 64: encoding: [0x66]
+// 64: lgdtq 0
+// 64: encoding: [0x0f,0x01,0x14,0x25,0x00,0x00,0x00,0x00]
+// 32: data16
+// 32: encoding: [0x66]
+// 32: lgdtl 0
+// 32: encoding: [0x0f,0x01,0x15,0x00,0x00,0x00,0x00]
+// ERR16: error: instruction requires: Not 16-bit mode
+data16 lgdt 0
Index: llvm/trunk/test/MC/X86/data-prefix16.s
===================================================================
--- llvm/trunk/test/MC/X86/data-prefix16.s
+++ llvm/trunk/test/MC/X86/data-prefix16.s
@@ -0,0 +1,9 @@
+# RUN: llvm-mc -triple i386-unknown-unknown-code16 -filetype=obj %s -o - | llvm-objdump -triple i386-unknown-unknown-code16 -d - | FileCheck %s
+
+# CHECK: 66 0f 01 16 00 00
+# CHECK: lgdtl 0
+data32 lgdt 0
+
+# CHECK: 66
+# CHECK: data32
+data32
Index: llvm/trunk/test/MC/X86/data-prefix32.s
===================================================================
--- llvm/trunk/test/MC/X86/data-prefix32.s
+++ llvm/trunk/test/MC/X86/data-prefix32.s
@@ -0,0 +1,9 @@
+# RUN: llvm-mc -triple=i386-unknown-unknown -filetype=obj %s -o - | llvm-objdump -triple=i386-unknown-unknown -d - | FileCheck %s
+
+# CHECK: 66 0f 01 15 00 00 00 00
+# CHECK: lgdtw 0
+data16 lgdt 0
+
+# CHECK: 66
+# CHECK: data16
+data16
Index: llvm/trunk/test/MC/X86/data-prefix64.s
===================================================================
--- llvm/trunk/test/MC/X86/data-prefix64.s
+++ llvm/trunk/test/MC/X86/data-prefix64.s
@@ -0,0 +1,9 @@
+# RUN: llvm-mc -triple=x86_64-unknown-unknown -filetype=obj %s -o - | llvm-objdump -triple=x86_64-unknown-unknown -d - | FileCheck %s
+
+# CHECK: 66 0f 01 14 25 00 00 00 00
+# CHECK: lgdtq 0
+data16 lgdt 0
+
+# CHECK: 66
+# CHECK: data16
+data16
Index: llvm/trunk/test/MC/X86/x86-16.s
===================================================================
--- llvm/trunk/test/MC/X86/x86-16.s
+++ llvm/trunk/test/MC/X86/x86-16.s
@@ -959,3 +959,13 @@
 // CHECK: lretl
 // CHECK: encoding: [0x66,0xcb]
 lretl
+
+// CHECK: data32
+// CHECK: encoding: [0x66]
+data32
+
+// CHECK: data32
+// CHECK: encoding: [0x66]
+// CHECK: lgdtw 4(%eax)
+// CHECK:  encoding: [0x67,0x0f,0x01,0x50,0x04]
+data32 lgdt 4(%eax)
Index: llvm/trunk/test/MC/X86/x86-32.s
===================================================================
--- llvm/trunk/test/MC/X86/x86-32.s
+++ llvm/trunk/test/MC/X86/x86-32.s
@@ -1079,3 +1079,13 @@
 // CHECK: lretw
 // CHECK: encoding: [0x66,0xcb]
 lretw
+
+// CHECK: data16
+// CHECK: encoding: [0x66]
+data16
+
+// CHECK: data16
+// CHECK: encoding: [0x66]
+// CHECK: lgdtl 4(%eax)
+// CHECK:  encoding: [0x0f,0x01,0x50,0x04]
+data16 lgdt 4(%eax)
Index: llvm/trunk/test/MC/X86/x86-64.s
===================================================================
--- llvm/trunk/test/MC/X86/x86-64.s
+++ llvm/trunk/test/MC/X86/x86-64.s
@@ -1119,6 +1119,12 @@
 rex64 // CHECK: rex64 # encoding: [0x48]
 data16 // CHECK: data16 # encoding: [0x66]
 
+// CHECK: data16
+// CHECK: encoding: [0x66]
+// CHECK: lgdtq 4(%rax)
+// CHECK:  encoding: [0x0f,0x01,0x50,0x04]
+data16 lgdt 4(%rax)
+
 // PR8855
 movq 18446744073709551615,%rbx   // CHECK: movq	-1, %rbx
 
Index: llvm/trunk/utils/TableGen/X86DisassemblerTables.cpp
===================================================================
--- llvm/trunk/utils/TableGen/X86DisassemblerTables.cpp
+++ llvm/trunk/utils/TableGen/X86DisassemblerTables.cpp
@@ -879,6 +879,10 @@
                                            newInfo.name == "XCHG64ar"))
           continue; // special case for XCHG*ar and NOOP
 
+        if (previousInfo.name == "DATA16_PREFIX" &&
+            newInfo.name == "DATA32_PREFIX")
+          continue; // special case for data16 and data32
+
         if (outranks(previousInfo.insnContext, newInfo.insnContext))
           continue;