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

Arm64EC uses a special name mangling mode that adds `$$h` between the
symbol name and its type. In MSVC's name mangling `@` is used to
separate the name and type BUT it is also used for other purposes, such
as the separator between paths in a fully qualified name.

The original algorithm was quite fragile and made assumptions that
didn't hold true for all MSVC mangled symbols, so instead of trying to
improve this algorithm we are now using the demangler to indicate where
the insertion point should be (i.e., to parse the fully-qualified name
and return the current string offset).

Also fixed `isArm64ECMangledFunctionName` to search for `@$$h` since the
`$$h` must always be after a `@`.

Fixes #115231

Author: dpaoliello

llvm/include/llvm/Demangle/Demangle.h

@@ -10,6 +10,7 @@
 #define LLVM_DEMANGLE_DEMANGLE_H
 
 #include <cstddef>
+#include <optional>
 #include <string>
 #include <string_view>
 
@@ -54,6 +55,9 @@ enum MSDemangleFlags {
 char *microsoftDemangle(std::string_view mangled_name, size_t *n_read,
                         int *status, MSDemangleFlags Flags = MSDF_None);
 
+std::optional<size_t>
+getArm64ECInsertionPointInMangledName(std::string_view MangledName);
+
 // Demangles a Rust v0 mangled symbol.
 char *rustDemangle(std::string_view MangledName);
 

llvm/include/llvm/Demangle/MicrosoftDemangle.h

@@ -9,6 +9,7 @@
 #ifndef LLVM_DEMANGLE_MICROSOFTDEMANGLE_H
 #define LLVM_DEMANGLE_MICROSOFTDEMANGLE_H
 
+#include "llvm/Demangle/Demangle.h"
 #include "llvm/Demangle/MicrosoftDemangleNodes.h"
 
 #include <cassert>
@@ -141,6 +142,9 @@ enum class FunctionIdentifierCodeGroup { Basic, Under, DoubleUnder };
 // It has a set of functions to parse mangled symbols into Type instances.
 // It also has a set of functions to convert Type instances to strings.
 class Demangler {
+  friend std::optional<size_t>
+  llvm::getArm64ECInsertionPointInMangledName(std::string_view MangledName);
+
 public:
   Demangler() = default;
   virtual ~Demangler() = default;

llvm/include/llvm/IR/Mangler.h

@@ -64,7 +64,7 @@ std::optional<std::string> getArm64ECDemangledFunctionName(StringRef Name);
 /// Check if an ARM64EC function name is mangled.
 bool inline isArm64ECMangledFunctionName(StringRef Name) {
   return Name[0] == '#' ||
-         (Name[0] == '?' && Name.find("$$h") != StringRef::npos);
+         (Name[0] == '?' && Name.find("@$$h") != StringRef::npos);
 }
 
 } // End llvm namespace

llvm/lib/Demangle/MicrosoftDemangle.cpp

@@ -24,6 +24,7 @@
 #include <array>
 #include <cctype>
 #include <cstdio>
+#include <optional>
 #include <string_view>
 #include <tuple>
 
@@ -2428,6 +2429,24 @@ void Demangler::dumpBackReferences() {
     std::printf("\n");
 }
 
+std::optional<size_t>
+llvm::getArm64ECInsertionPointInMangledName(std::string_view MangledName) {
+  std::string_view ProcessedName{MangledName};
+
+  // We only support this for MSVC-style C++ symbols.
+  if (!consumeFront(ProcessedName, '?'))
+    return std::nullopt;
+
+  // The insertion point is just after the name of the symbol, so parse that to
+  // remove it from the processed name.
+  Demangler D;
+  D.demangleFullyQualifiedSymbolName(ProcessedName);
+  if (D.Error)
+    return std::nullopt;
+
+  return MangledName.length() - ProcessedName.length();
+}
+
 char *llvm::microsoftDemangle(std::string_view MangledName, size_t *NMangled,
                               int *Status, MSDemangleFlags Flags) {
   Demangler D;

llvm/lib/IR/Mangler.cpp

@@ -14,6 +14,7 @@
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/Twine.h"
+#include "llvm/Demangle/Demangle.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
@@ -299,21 +300,17 @@ std::optional<std::string> llvm::getArm64ECMangledFunctionName(StringRef Name) {
     return std::optional<std::string>(("#" + Name).str());
   }
 
-  // Insert the ARM64EC "$$h" tag after the mangled function name.
+  // If the name contains $$h, then it is already mangled.
   if (Name.contains("$$h"))
     return std::nullopt;
-  size_t InsertIdx = Name.find("@@");
-  size_t ThreeAtSignsIdx = Name.find("@@@");
-  if (InsertIdx != std::string::npos && InsertIdx != ThreeAtSignsIdx) {
-    InsertIdx += 2;
-  } else {
-    InsertIdx = Name.find("@");
-    if (InsertIdx != std::string::npos)
-      InsertIdx++;
-  }
+
+  // Ask the demangler where we should insert "$$h".
+  auto InsertIdx = getArm64ECInsertionPointInMangledName(Name);
+  if (!InsertIdx)
+    return std::nullopt;
 
   return std::optional<std::string>(
-      (Name.substr(0, InsertIdx) + "$$h" + Name.substr(InsertIdx)).str());
+      (Name.substr(0, *InsertIdx) + "$$h" + Name.substr(*InsertIdx)).str());
 }
 
 std::optional<std::string>

llvm/unittests/IR/ManglerTest.cpp

@@ -172,4 +172,81 @@ TEST(ManglerTest, GOFF) {
             "L#foo");
 }
 
+TEST(ManglerTest, Arm64EC) {
+  constexpr std::string_view Arm64ECNames[] = {
+      // Basic C name.
+      "#Foo",
+
+      // Basic C++ name.
+      "?foo@@$$hYAHXZ",
+
+      // Regression test: https://github.com/llvm/llvm-project/issues/115231
+      "?GetValue@?$Wrapper@UA@@@@$$hQEBAHXZ",
+
+      // Symbols from:
+      // ```
+      // namespace A::B::C::D {
+      // struct Base {
+      //   virtual int f() { return 0; }
+      // };
+      // }
+      // struct Derived : public A::B::C::D::Base {
+      //   virtual int f() override { return 1; }
+      // };
+      // A::B::C::D::Base* MakeObj() { return new Derived(); }
+      // ```
+      // void * __cdecl operator new(unsigned __int64)
+      "??2@$$hYAPEAX_K@Z",
+      // public: virtual int __cdecl A::B::C::D::Base::f(void)
+      "?f@Base@D@C@B@A@@$$hUEAAHXZ",
+      // public: __cdecl A::B::C::D::Base::Base(void)
+      "??0Base@D@C@B@A@@$$hQEAA@XZ",
+      // public: virtual int __cdecl Derived::f(void)
+      "?f@Derived@@$$hUEAAHXZ",
+      // public: __cdecl Derived::Derived(void)
+      "??0Derived@@$$hQEAA@XZ",
+      // struct A::B::C::D::Base * __cdecl MakeObj(void)
+      "?MakeObj@@$$hYAPEAUBase@D@C@B@A@@XZ",
+
+      // Symbols from:
+      // ```
+      // template <typename T> struct WW { struct Z{}; };
+      // template <typename X> struct Wrapper {
+      //   int GetValue(typename WW<X>::Z) const;
+      // };
+      // struct A { };
+      // template <typename X> int Wrapper<X>::GetValue(typename WW<X>::Z) const
+      // { return 3; }
+      // template class Wrapper<A>;
+      // ```
+      // public: int __cdecl Wrapper<struct A>::GetValue(struct WW<struct
+      // A>::Z)const
+      "?GetValue@?$Wrapper@UA@@@@$$hQEBAHUZ@?$WW@UA@@@@@Z",
+  };
+
+  for (const auto &Arm64ECName : Arm64ECNames) {
+    // Check that this is a mangled name.
+    EXPECT_TRUE(isArm64ECMangledFunctionName(Arm64ECName))
+        << "Test case: " << Arm64ECName;
+    // Refuse to mangle it again.
+    EXPECT_FALSE(getArm64ECMangledFunctionName(Arm64ECName).has_value())
+        << "Test case: " << Arm64ECName;
+
+    // Demangle.
+    auto Arm64Name = getArm64ECDemangledFunctionName(Arm64ECName);
+    EXPECT_TRUE(Arm64Name.has_value()) << "Test case: " << Arm64ECName;
+    // Check that it is not mangled.
+    EXPECT_FALSE(isArm64ECMangledFunctionName(Arm64Name.value()))
+        << "Test case: " << Arm64ECName;
+    // Refuse to demangle it again.
+    EXPECT_FALSE(getArm64ECDemangledFunctionName(Arm64Name.value()).has_value())
+        << "Test case: " << Arm64ECName;
+
+    // Round-trip.
+    auto RoundTripArm64ECName =
+        getArm64ECMangledFunctionName(Arm64Name.value());
+    EXPECT_EQ(RoundTripArm64ECName, Arm64ECName);
+  }
+}
+
 } // end anonymous namespace