[RISCV] Fix canonical ordering of s* vs z* extensions in RISCVISAInfo

[asb]

Summary

As noted in https://reviews.llvm.org/D148315, the ordering logic for OrderedExtensionMap currently puts s* before z* extensions, but per the ISA manual the correct order should be z* and then s* (with the exception of zxm*, which are ordered after s*).

This patch fixes the ordering and adds a TODO for zxm*. The changes are visible in the test case added in a35e67fc5be654a7efdfa6125343b90f8960a487 which also demonstrates an issue with the ordering of single letter extensions (which isn't addressed in this patch).

[joshua-arch1]

LGTM.

[asb]

Thanks for the reviews - just to additionally note that I've checked vs GCC/binutils and it seems GCC was already sorting in this order (though required order in the -march string matching Clang's prior to D143815).

llvm/lib/Support/RISCVISAInfo.cpp

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.
+// -Multi-letter extensions starting with 's' in alphabetical order.
+// -(TODO) Multi-letter extensions starting with 'zxm' in alphabetical order.
 // -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_Z_EXTENSION = 1 << 6,
+  RF_S_EXTENSION = 1 << 7,
   RF_X_EXTENSION = 1 << 8,
 };
 
 // Get the rank for single-letter extension, lower value meaning higher
 // priority.
 static unsigned singleLetterExtensionRank(char Ext) {
   assert(Ext >= 'a' && Ext <= 'z');
   switch (Ext) {

llvm/unittests/Support/RISCVISAInfoTest.cpp

TEST(OrderedExtensionMap, ExtensionsAreCorrectlyOrdered) {
   for (auto ExtName : {"y", "l", "m", "c", "i", "xfoo", "xbar", "sfoo", "sbar",
                        "zmfoo", "zzfoo", "zfinx", "zicsr"})
     Exts[ExtName] = {1, 0};
 
   std::vector<std::string> ExtNames;
   for (const auto &Ext : Exts)
     ExtNames.push_back(Ext.first);
 
-  // FIXME: z* extensions should be ordered before s* extensions. The current
-  // ordering matches what is documented in RISCVISAInfo, but this doesn't
-  // match the ISA manual.
   // FIXME: 'l' and 'y' should be ordered after 'i', 'm', 'c'.
   EXPECT_THAT(ExtNames,
-              ElementsAre("i", "m", "l", "c", "y", "sbar", "sfoo", "zicsr",
-                          "zmfoo", "zfinx", "zzfoo", "xbar", "xfoo"));
+              ElementsAre("i", "m", "l", "c", "y", "zicsr", "zmfoo", "zfinx",
+                           "zzfoo", "sbar", "sfoo", "xbar", "xfoo"));
 }