[X86/MC] Factor out common code [NFC]

llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp

@@ -390,22 +390,8 @@ uint8_t X86AsmBackend::determinePaddingPrefix(const MCInst &Inst) const {
   }
   }
 
-  switch (SegmentReg) {
-  case 0:
-    break;
-  case X86::CS:
-    return X86::CS_Encoding;
-  case X86::DS:
-    return X86::DS_Encoding;
-  case X86::ES:
-    return X86::ES_Encoding;
-  case X86::FS:
-    return X86::FS_Encoding;
-  case X86::GS:
-    return X86::GS_Encoding;
-  case X86::SS:
-    return X86::SS_Encoding;
-  }
+  if (SegmentReg != 0)
+    return X86::getSegmentOverridePrefixForReg(SegmentReg);
 
   if (STI.hasFeature(X86::Mode64Bit))
     return X86::CS_Encoding;

llvm/lib/Target/X86/MCTargetDesc/X86BaseInfo.h

@@ -393,6 +393,29 @@ namespace X86 {
     GS_Encoding = 0x65,
     SS_Encoding = 0x36
   };
+
+  /// Given a segment register, return the encoding of the segment override
+  /// prefix for it.
+  inline EncodingOfSegmentOverridePrefix
+  getSegmentOverridePrefixForReg(unsigned Reg) {
+    switch (Reg) {
+    default:
+      llvm_unreachable("Unknown segment register!");
+    case X86::CS:
+      return CS_Encoding;
+    case X86::DS:
+      return DS_Encoding;
+    case X86::ES:
+      return ES_Encoding;
+    case X86::FS:
+      return FS_Encoding;
+    case X86::GS:
+      return GS_Encoding;
+    case X86::SS:
+      return SS_Encoding;
+    }
+  }
+
 } // end namespace X86;
 
 /// X86II - This namespace holds all of the target specific flags that

llvm/lib/Target/X86/MCTargetDesc/X86MCCodeEmitter.cpp

@@ -1262,30 +1262,8 @@ void X86MCCodeEmitter::emitSegmentOverridePrefix(unsigned &CurByte,
                                                  const MCInst &MI,
                                                  raw_ostream &OS) const {
   // Check for explicit segment override on memory operand.
-  switch (MI.getOperand(SegOperand).getReg()) {
-  default:
-    llvm_unreachable("Unknown segment register!");
-  case 0:
-    break;
-  case X86::CS:
-    emitByte(0x2E, CurByte, OS);
-    break;
-  case X86::SS:
-    emitByte(0x36, CurByte, OS);
-    break;
-  case X86::DS:
-    emitByte(0x3E, CurByte, OS);
-    break;
-  case X86::ES:
-    emitByte(0x26, CurByte, OS);
-    break;
-  case X86::FS:
-    emitByte(0x64, CurByte, OS);
-    break;
-  case X86::GS:
-    emitByte(0x65, CurByte, OS);
-    break;
-  }
+  if (unsigned Reg = MI.getOperand(SegOperand).getReg())
+    emitByte(X86::getSegmentOverridePrefixForReg(Reg), CurByte, OS);
 }
 
 /// Emit all instruction prefixes prior to the opcode.