[RISCV] Don't crash if parseNormalizedArchString encounters a multi-letter extension with an unknown prefix. (#91398)

The sorting code previously asserted if a prefix was multiple letters,
but didn't start with s, x, or z.

Replace the assert with an explicit check and sort the multi-letter
extension after the known multi-letter prefixes.

Author: topperc

llvm/lib/Support/RISCVISAUtils.cpp

@@ -24,13 +24,15 @@ using namespace llvm;
 // -Multi-letter extensions starting with 's' in alphabetical order.
 // -(TODO) Multi-letter extensions starting with 'zxm' in alphabetical order.
 // -X extensions in alphabetical order.
+// -Unknown multi-letter 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_Z_EXTENSION = 1 << 6,
-  RF_S_EXTENSION = 1 << 7,
-  RF_X_EXTENSION = 1 << 8,
+  RF_S_EXTENSION = 2 << 6,
+  RF_X_EXTENSION = 3 << 6,
+  RF_UNKNOWN_MULTILETTER_EXTENSION = 4 << 6,
 };
 
 // Get the rank for single-letter extension, lower value meaning higher
@@ -68,8 +70,9 @@ static unsigned getExtensionRank(const std::string &ExtName) {
   case 'x':
     return RF_X_EXTENSION;
   default:
-    assert(ExtName.size() == 1);
-    return singleLetterExtensionRank(ExtName[0]);
+    if (ExtName.size() == 1)
+      return singleLetterExtensionRank(ExtName[0]);
+    return RF_UNKNOWN_MULTILETTER_EXTENSION;
   }
 }
 

llvm/unittests/TargetParser/RISCVISAInfoTest.cpp

@@ -150,6 +150,14 @@ TEST(ParseNormalizedArchString, UpdatesFLenMinVLenMaxELen) {
   EXPECT_EQ(Info.getMaxELenFp(), 64U);
 }
 
+TEST(ParseNormalizedArchString, AcceptsUnknownMultiletter) {
+  auto MaybeISAInfo = RISCVISAInfo::parseNormalizedArchString(
+      "rv64i2p0_f2p0_d2p0_zicsr2p0_ykk1p0");
+  ASSERT_THAT_EXPECTED(MaybeISAInfo, Succeeded());
+  RISCVISAInfo &Info = **MaybeISAInfo;
+  EXPECT_EQ(Info.toString(), "rv64i2p0_f2p0_d2p0_zicsr2p0_ykk1p0");
+}
+
 TEST(ParseArchString, RejectsInvalidChars) {
   for (StringRef Input : {"RV32", "rV64", "rv32i2P0", "rv64i2p0_A2p0"}) {
     EXPECT_EQ(toString(RISCVISAInfo::parseArchString(Input, true).takeError()),