Index: lib/Target/X86/Disassembler/X86DisassemblerDecoder.h
===================================================================
--- lib/Target/X86/Disassembler/X86DisassemblerDecoder.h
+++ lib/Target/X86/Disassembler/X86DisassemblerDecoder.h
@@ -546,10 +546,13 @@
 
   // Prefix state
 
+  // TODO: we're able to get rid off the following 2 arrays
   // 1 if the prefix byte corresponding to the entry is present; 0 if not
   uint8_t prefixPresent[0x100];
   // contains the location (for use with the reader) of the prefix byte
   uint64_t prefixLocations[0x100];
+  // The possible mandatory prefix
+  uint8_t mandatory_prefix;
   // The value of the vector extension prefix(EVEX/VEX/XOP), if present
   uint8_t vectorExtensionPrefix[4];
   // The type of the vector extension prefix
Index: lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
===================================================================
--- lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
+++ lib/Target/X86/Disassembler/X86DisassemblerDecoder.cpp
@@ -277,6 +277,12 @@
   insn->dlog(insn->dlogArg, buffer);
 }
 
+static bool isREX(struct InternalInstruction *insn, uint8_t prefix) {
+  if (insn->mode == MODE_64BIT)
+    return prefix >= 0x40 && prefix <= 0x4f;
+  return false;
+}
+
 /*
  * setPrefixPresent - Marks that a particular prefix is present at a particular
  *   location.
@@ -290,6 +296,38 @@
                                     uint8_t prefix,
                                     uint64_t location)
 {
+  uint8_t nextByte;
+  switch (prefix) {
+  case 0xf2:
+  case 0xf3:
+    if (lookAtByte(insn, &nextByte))
+      break;
+    // TODO:
+    //  1. There could be several 0x66
+    //  2. if (nextByte == 0x66) and nextNextByte != 0x0f then
+    //      it's not mandatory prefix
+    //  3. if (nextByte >= 0x40 && nextByte <= 0x4f) it's REX and we need
+    //     0x0f exactly after it to be mandatory prefix
+    if (isREX(insn, nextByte) || nextByte == 0x0f || nextByte == 0x66) {
+      // The last of 0xf2 /0xf3 is mandatory prefix
+      insn->mandatory_prefix = prefix;
+      insn->necessaryPrefixLocation = location;
+      break;
+    }
+    break;
+  case 0x66:
+    if (lookAtByte(insn, &nextByte))
+      break;
+    // 0x66 can't overwrite existing mandatory prefix and should be ignored
+    if (!insn->mandatory_prefix &&
+        (nextByte == 0x0f || isREX(insn, nextByte))) {
+      insn->mandatory_prefix = prefix;
+      insn->necessaryPrefixLocation = location;
+    }
+    break;
+  }
+  if (!insn->mandatory_prefix)
+    insn->necessaryPrefixLocation = location;
   insn->prefixPresent[prefix] = 1;
   insn->prefixLocations[prefix] = location;
 }
@@ -343,13 +381,10 @@
      * If the byte is a LOCK/REP/REPNE prefix and not a part of the opcode, then
      * break and let it be disassembled as a normal "instruction".
      */
-    if (insn->readerCursor - 1 == insn->startLocation && byte == 0xf0)
+    if (insn->readerCursor - 1 == insn->startLocation && byte == 0xf0) // LOCK
       break;
 
-    if (insn->readerCursor - 1 == insn->startLocation
-        && (byte == 0xf2 || byte == 0xf3)
-        && !lookAtByte(insn, &nextByte))
-    {
+    if ((byte == 0xf2 || byte == 0xf3) && !lookAtByte(insn, &nextByte)) {
       /*
        * If the byte is 0xf2 or 0xf3, and any of the following conditions are
        * met:
@@ -357,9 +392,8 @@
        * - it is followed by an xchg instruction
        * then it should be disassembled as a xacquire/xrelease not repne/rep.
        */
-      if ((byte == 0xf2 || byte == 0xf3) &&
-          ((nextByte == 0xf0) ||
-          ((nextByte & 0xfe) == 0x86 || (nextByte & 0xf8) == 0x90)))
+      if (((nextByte == 0xf0) ||
+           ((nextByte & 0xfe) == 0x86 || (nextByte & 0xf8) == 0x90)))
         insn->xAcquireRelease = true;
       /*
        * Also if the byte is 0xf3, and the following condition is met:
@@ -378,7 +412,13 @@
           return -1;
         unconsumeByte(insn);
       }
-      if (nextByte != 0x0f && nextByte != 0x90)
+      // If the current byte can't be mandatory prefix then it's a simple repeat
+      // prefix and should be elaborated as a separated instr
+      if (nextByte != 0x0f &&
+          // We can have f2 f3 f2 66 f2 0f (in any order) as a valid set
+          // of prefixes with the last one as a mandatory prefix (SSE/SSE2...)
+          nextByte != 0xf2 && nextByte != 0xf3 && nextByte != 0x66 &&
+          nextByte != 0x90 && !isREX(insn, nextByte))
         break;
     }
 
@@ -426,11 +466,13 @@
       setPrefixPresent(insn, byte, prefixLocation);
       break;
     case 0x66:  /* Operand-size override */
-      if (prefixGroups[2])
-        dbgprintf(insn, "Redundant Group 3 prefix");
-      prefixGroups[2] = true;
-      hasOpSize = true;
       setPrefixPresent(insn, byte, prefixLocation);
+      if (!insn->mandatory_prefix) {
+        if (prefixGroups[2])
+          dbgprintf(insn, "Redundant Group 3 prefix");
+        prefixGroups[2] = true;
+      }
+      hasOpSize = true;
       break;
     case 0x67:  /* Address-size override */
       if (prefixGroups[3])
@@ -624,10 +666,8 @@
         insn->necessaryPrefixLocation = insn->readerCursor - 2;
 
         dbgprintf(insn, "Found REX prefix 0x%hhx", byte);
-      } else {
+      } else
         unconsumeByte(insn);
-        insn->necessaryPrefixLocation = insn->readerCursor - 1;
-      }
     } else {
       unconsumeByte(insn);
       insn->necessaryPrefixLocation = insn->readerCursor - 1;
@@ -950,8 +990,19 @@
     } else {
       return -1;
     }
+  } else if (insn->mode == MODE_64BIT && !insn->mandatory_prefix) {
+    // If we don't have mandatory prefix we should use "standard" prefixes here
+    if (insn->prefixPresent[0x66])
+      attrMask |= ATTR_OPSIZE;
+    if (insn->prefixPresent[0x67])
+      attrMask |= ATTR_ADSIZE;
+    if (insn->prefixPresent[0xf2])
+      attrMask |= ATTR_XD;
+    if (insn->prefixPresent[0xf3])
+      attrMask |= ATTR_XS;
   } else {
-    if (insn->mode != MODE_16BIT && isPrefixAtLocation(insn, 0x66, insn->necessaryPrefixLocation))
+    if (insn->mode != MODE_16BIT &&
+        isPrefixAtLocation(insn, 0x66, insn->necessaryPrefixLocation))
       attrMask |= ATTR_OPSIZE;
     else if (isPrefixAtLocation(insn, 0x67, insn->necessaryPrefixLocation))
       attrMask |= ATTR_ADSIZE;
Index: test/MC/Disassembler/X86/prefixes.txt
===================================================================
--- test/MC/Disassembler/X86/prefixes.txt
+++ test/MC/Disassembler/X86/prefixes.txt
@@ -1,5 +1,60 @@
 # RUN: llvm-mc --disassemble %s -triple=x86_64 | FileCheck %s
 
+# CHECK: rep
+# CHECK-NEXT: insb    %dx, %es:(%rdi)
+0xf3 0x6c #rep ins
+# CHECK: rep
+# CHECK-NEXT: insl    %dx, %es:(%rdi)
+0xf3 0x6d #rep ins
+# CHECK: rep
+# CHECK-NEXT: movsb   (%rsi), %es:(%rdi)
+0xf3 0xa4 #rep movs
+# CHECK: rep
+# CHECK-NEXT: movsl   (%rsi), %es:(%rdi)
+0xf3 0xa5 #rep movs
+# CHECK: rep
+# CHECK-NEXT: outsb   (%rsi), %dx
+0xf3 0x6e #rep outs
+# CHECK: rep
+# CHECK-NEXT: outsl   (%rsi), %dx
+0xf3 0x6f #rep outs
+# CHECK: rep
+# CHECK-NEXT: lodsb   (%rsi), %al
+0xf3 0xac #rep lods
+# CHECK: rep
+# CHECK-NEXT: lodsl   (%rsi), %eax
+0xf3 0xad #rep lods
+# CHECK: rep
+# CHECK-NEXT: stosb   %al, %es:(%rdi)
+0xf3 0xaa #rep stos
+# CHECK: rep
+# CHECK-NEXT: stosl   %eax, %es:(%rdi)
+0xf3 0xab #rep stos
+# CHECK: rep
+# CHECK-NEXT: cmpsb   %es:(%rdi), (%rsi)
+0xf3 0xa6 #rep cmps
+# CHECK: rep
+# CHECK-NEXT: cmpsl   %es:(%rdi), (%rsi)
+0xf3 0xa7 #repe cmps
+# CHECK: rep
+# CHECK-NEXT: scasb   %es:(%rdi), %al
+0xf3 0xae #repe scas
+# CHECK: rep
+# CHECK-NEXT: scasl   %es:(%rdi), %eax
+0xf3 0xaf #repe scas
+# CHECK: repne
+# CHECK-NEXT: cmpsb   %es:(%rdi), (%rsi)
+0xf2 0xa6 #repne cmps
+# CHECK: repne
+# CHECK-NEXT: cmpsl   %es:(%rdi), (%rsi)
+0xf2 0xa7 #repne cmps
+# CHECK: repne
+# CHECK-NEXT: scasb   %es:(%rdi), %al
+0xf2 0xae #repne scas
+# CHECK: repne
+# CHECK-NEXT: scasl   %es:(%rdi), %eax
+0xf2 0xaf #repne scas
+
 # CHECK: lock
 # CHECK-NEXT:	orl	$16, %fs:776
 0xf0 0x64 0x83 0x0c 0x25 0x08 0x03 0x00 0x00 0x10
@@ -50,7 +105,6 @@
 
 # Test that multiple redundant prefixes work (redundant, but valid x86).
 # CHECK: rep
-# CHECK-NEXT: rep
 # CHECK-NEXT: stosq
 0xf3 0xf3 0x48 0xab