diff --git a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
--- a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -3619,6 +3619,33 @@
   }
   }
 
+  const MCInstrDesc &MCID = MII.get(Inst.getOpcode());
+  // Check that we aren't mixing AH/BH/CH/DH with REX prefix. We only need to
+  // check this with the legacy encoding, VEX/EVEX/XOP don't use REX.
+  if ((MCID.TSFlags & X86II::EncodingMask) == 0) {
+    MCPhysReg HReg = X86::NoRegister;
+    bool UsesRex = MCID.TSFlags & X86II::REX_W;
+    unsigned NumOps = Inst.getNumOperands();
+    for (unsigned i = 0; i != NumOps; ++i) {
+      const MCOperand &MO = Inst.getOperand(i);
+      if (!MO.isReg())
+        continue;
+      unsigned Reg = MO.getReg();
+      if (Reg == X86::AH || Reg == X86::BH || Reg == X86::CH || Reg == X86::DH)
+        HReg = Reg;
+      if (X86II::isX86_64NonExtLowByteReg(Reg) ||
+          X86II::isX86_64ExtendedReg(Reg))
+        UsesRex = true;
+    }
+
+    if (UsesRex && HReg != X86::NoRegister) {
+      StringRef RegName = X86IntelInstPrinter::getRegisterName(HReg);
+      return Error(Ops[0]->getStartLoc(),
+                   "can't encode '" + RegName + "' in an instruction requiring "
+                   "REX prefix.");
+    }
+  }
+
   return false;
 }
 
diff --git a/llvm/test/MC/X86/encoder-fail.s b/llvm/test/MC/X86/encoder-fail.s
--- a/llvm/test/MC/X86/encoder-fail.s
+++ b/llvm/test/MC/X86/encoder-fail.s
@@ -1,3 +1,3 @@
-// RUN: not --crash llvm-mc -triple x86_64-unknown-unknown --show-encoding %s 2>&1 | FileCheck %s
-// CHECK: LLVM ERROR: Cannot encode high byte register in REX-prefixed instruction
+// RUN: not llvm-mc -triple x86_64-unknown-unknown --show-encoding %s 2>&1 | FileCheck %s
+// CHECK: error: can't encode 'dh' in an instruction requiring REX prefix.
  movzx %dh, %rsi