diff --git a/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h b/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
--- a/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
+++ b/llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h
@@ -346,6 +346,33 @@
     llvm_unreachable("unknown fusion type");
   }
 
+  /// \returns true if the instruction with given opcode is a prefix.
+  inline bool isPrefix(unsigned Opcode) {
+    switch (Opcode) {
+    default:
+      return false;
+      // segment override prefix
+    case X86::CS_PREFIX:
+    case X86::DS_PREFIX:
+    case X86::ES_PREFIX:
+    case X86::FS_PREFIX:
+    case X86::GS_PREFIX:
+    case X86::SS_PREFIX:
+      // operand-size override prefix
+    case X86::DATA16_PREFIX:
+      // lock and repeat prefix
+    case X86::LOCK_PREFIX:
+    case X86::REPNE_PREFIX:
+    case X86::REP_PREFIX:
+      // rex64 prefix
+    case X86::REX64_PREFIX:
+      // acquire and release prefix
+    case X86::XACQUIRE_PREFIX:
+    case X86::XRELEASE_PREFIX:
+      return true;
+    }
+  }
+
   /// Defines the possible values of the branch boundary alignment mask.
   enum AlignBranchBoundaryKind : uint8_t {
     AlignBranchNone = 0,