[IR][ARM64EC][NFC] Clean up and document ARM64EC mangling helpers.

Author: cjacek

llvm/include/llvm/IR/Mangler.h

@@ -53,7 +53,12 @@ void emitLinkerFlagsForGlobalCOFF(raw_ostream &OS, const GlobalValue *GV,
 void emitLinkerFlagsForUsedCOFF(raw_ostream &OS, const GlobalValue *GV,
                                 const Triple &T, Mangler &M);
 
+/// Returns the ARM64EC mangled function name unless the input is already
+/// mangled.
 std::optional<std::string> getArm64ECMangledFunctionName(StringRef Name);
+
+/// Returns the ARM64EC demangled function name, unless the input is not
+/// mangled.
 std::optional<std::string> getArm64ECDemangledFunctionName(StringRef Name);
 
 } // End llvm namespace

llvm/lib/IR/Mangler.cpp

@@ -291,39 +291,41 @@ void llvm::emitLinkerFlagsForUsedCOFF(raw_ostream &OS, const GlobalValue *GV,
 }
 
 std::optional<std::string> llvm::getArm64ECMangledFunctionName(StringRef Name) {
-  bool IsCppFn = Name[0] == '?';
-  if (IsCppFn && Name.contains("$$h"))
-    return std::nullopt;
-  if (!IsCppFn && Name[0] == '#')
-    return std::nullopt;
+  if (Name[0] != '?') {
+    // For non-C++ symbols, prefix the name with "#" unless it's already
+    // mangled.
+    if (Name[0] == '#')
+      return std::nullopt;
+    return std::optional<std::string>(("#" + Name).str());
+  }
 
-  StringRef Prefix = "$$h";
+  // Insert the ARM64EC "$$h" tag after the mangled function name.
+  if (Name.contains("$$h"))
+    return std::nullopt;
   size_t InsertIdx = 0;
-  if (IsCppFn) {
-    InsertIdx = Name.find("@@");
-    size_t ThreeAtSignsIdx = Name.find("@@@");
-    if (InsertIdx != std::string::npos && InsertIdx != ThreeAtSignsIdx) {
-      InsertIdx += 2;
-    } else {
-      InsertIdx = Name.find("@");
-      if (InsertIdx != std::string::npos)
-        InsertIdx++;
-    }
+  InsertIdx = Name.find("@@");
+  size_t ThreeAtSignsIdx = Name.find("@@@");
+  if (InsertIdx != std::string::npos && InsertIdx != ThreeAtSignsIdx) {
+    InsertIdx += 2;
   } else {
-    Prefix = "#";
+    InsertIdx = Name.find("@");
+    if (InsertIdx != std::string::npos)
+      InsertIdx++;
   }
 
   return std::optional<std::string>(
-      (Name.substr(0, InsertIdx) + Prefix + Name.substr(InsertIdx)).str());
+      (Name.substr(0, InsertIdx) + "$$h" + Name.substr(InsertIdx)).str());
 }
 
 std::optional<std::string>
 llvm::getArm64ECDemangledFunctionName(StringRef Name) {
+  // For non-C++ names, drop the "#" prefix.
   if (Name[0] == '#')
     return std::optional<std::string>(Name.substr(1));
   if (Name[0] != '?')
     return std::nullopt;
 
+  // Drop the ARM64EC "$$h" tag.
   std::pair<StringRef, StringRef> Pair = Name.split("$$h");
   if (Pair.second.empty())
     return std::nullopt;