[RISCV] Simplify RISCVISAInfo::compareExtension. NFCI

[craig.topper]

Summary

Instead of having a separate single letter and multiletter ranking
use a unified rank that assigns multiletter a larger value than
single letter.

Once we've ranked the extensions, then we compare using these ranks.

[kito-cheng]

LGTM

llvm/lib/Support/RISCVISAInfo.cpp

bool RISCVISAInfo::hasExtension(StringRef Ext) const {
   stripExperimentalPrefix(Ext);
 
   if (!isSupportedExtension(Ext))
     return false;
 
   return Exts.count(Ext.str()) != 0;
 }
 
+// We rank extensions in the following order:
+// -Single letter extensions in canonical order.
+// -Unknown single letter extensions in alphabetical order.
+// -Multi-letter extensions starting with 's' in alphabetical order.
+// -Multi-letter extensions starting with 'z' sorted by canonical order of
+//  the second letter then sorted alphabetically.
+// -X extensions in alphabetical order.
+// These flags are used to indicate the category. The first 6 bits store the
+// single letter extension rank for single letter and multi-letter extensions
+// starting with 'z'.
+enum RankFlags {
+  RF_S_EXTENSION = 1 << 6,
+  RF_Z_EXTENSION = 1 << 7,
+  RF_X_EXTENSION = 1 << 8,
+};
+
 // Get the rank for single-letter extension, lower value meaning higher
 // priority.
-static int singleLetterExtensionRank(char Ext) {
+static unsigned singleLetterExtensionRank(char Ext) {
+  assert(Ext >= 'a' && Ext <= 'z');
   switch (Ext) {
   case 'i':
-    return -2;
+    return 0;
   case 'e':
-    return -1;
-  default:
-    break;
+    return 1;
   }
 
   size_t Pos = AllStdExts.find(Ext);
-  int Rank;
-  if (Pos == StringRef::npos)
-    // If we got an unknown extension letter, then give it an alphabetical
-    // order, but after all known standard extensions.
-    Rank = AllStdExts.size() + (Ext - 'a');
-  else
-    Rank = Pos;
-
-  return Rank;
+  if (Pos != StringRef::npos)
+    return Pos + 2; // Skip 'e' and 'i' from above.
+
+  return 2 + AllStdExts.size() + (Ext - 'a');
 }
 
 // Get the rank for multi-letter extension, lower value meaning higher
 // priority/order in canonical order.
-static int multiLetterExtensionRank(const std::string &ExtName) {
-  assert(ExtName.length() >= 2);
-  int HighOrder;
-  int LowOrder = 0;
-  // The order between multi-char extensions: s -> h -> z -> x.
-  char ExtClass = ExtName[0];
-  switch (ExtClass) {
+static unsigned getExtensionRank(const std::string &ExtName) {
+  assert(ExtName.size() >= 1);
+  switch (ExtName[0]) {
   case 's':
-    HighOrder = 0;
-    break;
+    return RF_S_EXTENSION;
   case 'z':
-    HighOrder = 1;
+    assert(ExtName.size() >= 2);
     // `z` extension must be sorted by canonical order of second letter.
     // e.g. zmx has higher rank than zax.
-    LowOrder = singleLetterExtensionRank(ExtName[1]);
-    break;
+    return RF_Z_EXTENSION | singleLetterExtensionRank(ExtName[1]);
   case 'x':
-    HighOrder = 2;
-    break;
+    return RF_X_EXTENSION;
   default:
-    llvm_unreachable("Unknown prefix for multi-char extension");
-    return -1;
+    assert(ExtName.size() == 1);
+    return singleLetterExtensionRank(ExtName[0]);
   }
-
-  return (HighOrder << 8) + LowOrder;
 }
 
 // Compare function for extension.
 // Only compare the extension name, ignore version comparison.
 bool RISCVISAInfo::compareExtension(const std::string &LHS,
                                     const std::string &RHS) {
-  size_t LHSLen = LHS.length();
-  size_t RHSLen = RHS.length();
-  if (LHSLen == 1 && RHSLen != 1)
-    return true;
-
-  if (LHSLen != 1 && RHSLen == 1)
-    return false;
+  unsigned LHSRank = getExtensionRank(LHS);
+  unsigned RHSRank = getExtensionRank(RHS);
 
-  if (LHSLen == 1 && RHSLen == 1)
-    return singleLetterExtensionRank(LHS[0]) <
-           singleLetterExtensionRank(RHS[0]);
-
-  // Both are multi-char ext here.
-  int LHSRank = multiLetterExtensionRank(LHS);
-  int RHSRank = multiLetterExtensionRank(RHS);
+  // If the ranks differ, pick the lower rank.
   if (LHSRank != RHSRank)
     return LHSRank < RHSRank;
 
   // If the rank is same, it must be sorted by lexicographic order.
   return LHS < RHS;
 }
 
 void RISCVISAInfo::toFeatures(