[NFC][LLVM][SVE] Refactor predicate register ASM constraint parsing t…

…o use std::optional.

llvm/lib/Target/AArch64/AArch64ISelLowering.cpp

@@ -10163,14 +10163,15 @@ const char *AArch64TargetLowering::LowerXConstraint(EVT ConstraintVT) const {
   return "r";
 }
 
-enum PredicateConstraint { Uph, Upl, Upa, Invalid };
+enum class PredicateConstraint { Uph, Upl, Upa };
 
-static PredicateConstraint parsePredicateConstraint(StringRef Constraint) {
-  return StringSwitch<PredicateConstraint>(Constraint)
+static std::optional<PredicateConstraint>
+parsePredicateConstraint(StringRef Constraint) {
+  return StringSwitch<std::optional<PredicateConstraint>>(Constraint)
       .Case("Uph", PredicateConstraint::Uph)
       .Case("Upl", PredicateConstraint::Upl)
       .Case("Upa", PredicateConstraint::Upa)
-      .Default(PredicateConstraint::Invalid);
+      .Default(std::nullopt);
 }
 
 static const TargetRegisterClass *
@@ -10180,8 +10181,6 @@ getPredicateRegisterClass(PredicateConstraint Constraint, EVT VT) {
     return nullptr;
 
   switch (Constraint) {
-  default:
-    return nullptr;
   case PredicateConstraint::Uph:
     return VT == MVT::aarch64svcount ? &AArch64::PNR_p8to15RegClass
                                      : &AArch64::PPR_p8to15RegClass;
@@ -10192,6 +10191,8 @@ getPredicateRegisterClass(PredicateConstraint Constraint, EVT VT) {
     return VT == MVT::aarch64svcount ? &AArch64::PNRRegClass
                                      : &AArch64::PPRRegClass;
   }
+
+  llvm_unreachable("Missing PredicateConstraint!");
 }
 
 // The set of cc code supported is from
@@ -10289,9 +10290,8 @@ AArch64TargetLowering::getConstraintType(StringRef Constraint) const {
     case 'S': // A symbolic address
       return C_Other;
     }
-  } else if (parsePredicateConstraint(Constraint) !=
-             PredicateConstraint::Invalid)
-      return C_RegisterClass;
+  } else if (parsePredicateConstraint(Constraint))
+    return C_RegisterClass;
   else if (parseConstraintCode(Constraint) != AArch64CC::Invalid)
     return C_Other;
   return TargetLowering::getConstraintType(Constraint);
@@ -10325,7 +10325,7 @@ AArch64TargetLowering::getSingleConstraintMatchWeight(
     weight = CW_Constant;
     break;
   case 'U':
-    if (parsePredicateConstraint(constraint) != PredicateConstraint::Invalid)
+    if (parsePredicateConstraint(constraint))
       weight = CW_Register;
     break;
   }
@@ -10382,9 +10382,9 @@ AArch64TargetLowering::getRegForInlineAsmConstraint(
       break;
     }
   } else {
-    PredicateConstraint PC = parsePredicateConstraint(Constraint);
-    if (const TargetRegisterClass *RegClass = getPredicateRegisterClass(PC, VT))
-      return std::make_pair(0U, RegClass);
+    if (const auto PC = parsePredicateConstraint(Constraint))
+      if (const auto *RegClass = getPredicateRegisterClass(*PC, VT))
+        return std::make_pair(0U, RegClass);
   }
   if (StringRef("{cc}").equals_insensitive(Constraint) ||
       parseConstraintCode(Constraint) != AArch64CC::Invalid)