[llvm] [llvm][aarch64] Fix Arm64EC name mangling algorithm (PR #115567)

[Daniel Paoliello via llvm-commits]

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@@ -302,14 +302,23 @@ std::optional<std::string> llvm::getArm64ECMangledFunctionName(StringRef Name) {
   // Insert the ARM64EC "$$h" tag after the mangled function name.
   if (Name.contains("$$h"))
     return std::nullopt;
-  size_t InsertIdx = Name.find("@@");
-  size_t ThreeAtSignsIdx = Name.find("@@@");
-  if (InsertIdx != std::string::npos && InsertIdx != ThreeAtSignsIdx) {
+
+  // The last 4 characters of the symbol type may contain a `@@` if the symbol
+  // is returning a qualified type. We don't want to insert `$$h` at that point.
+  auto TrimmedName = Name.drop_back(4);
+
+  // The last `@@` is the separation between the qualified name of the symbol
+  // and its type, which is where we want to insert `$$h`.
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dpaoliello wrote:

MSVC generates the mangled name in the frontend, and that's certainly the most robust choice.

Unfortunately, LLD and import library generation have both taken a dependency on this function so we'd have to detangle that first.

The other option is to use a name demangler to parse the mangled name instead of this adhoc from-the-back parsing.

https://github.com/llvm/llvm-project/pull/115567