diff --git a/llvm/lib/MC/MCParser/MasmParser.cpp b/llvm/lib/MC/MCParser/MasmParser.cpp
index c1a451a7119d5..e6123e6e74e43 100644
--- a/llvm/lib/MC/MCParser/MasmParser.cpp
+++ b/llvm/lib/MC/MCParser/MasmParser.cpp
@@ -778,6 +778,19 @@ class MasmParser : public MCAsmParser {
   std::optional<std::string> evaluateBuiltinTextMacro(BuiltinSymbol Symbol,
                                                       SMLoc StartLoc);
 
+  // Generic (target and platform independent) directive parsing.
+  enum BuiltinFunction {
+    BI_NO_FUNCTION, // Placeholder
+    BI_CATSTR,
+  };
+
+  /// Maps builtin name --> BuiltinFunction enum, for builtins handled by this
+  /// class.
+  StringMap<BuiltinFunction> BuiltinFunctionMap;
+
+  bool evaluateBuiltinMacroFunction(BuiltinFunction Function, StringRef Name,
+                                    std::string &Res);
+
   // ".ascii", ".asciz", ".string"
   bool parseDirectiveAscii(StringRef IDVal, bool ZeroTerminated);
 
@@ -959,7 +972,7 @@ class MasmParser : public MCAsmParser {
   bool parseDirectiveEcho(SMLoc DirectiveLoc);
 
   void initializeDirectiveKindMap();
-  void initializeBuiltinSymbolMap();
+  void initializeBuiltinSymbolMaps();
 };
 
 } // end anonymous namespace
@@ -999,7 +1012,7 @@ MasmParser::MasmParser(SourceMgr &SM, MCContext &Ctx, MCStreamer &Out,
 
   initializeDirectiveKindMap();
   PlatformParser->Initialize(*this);
-  initializeBuiltinSymbolMap();
+  initializeBuiltinSymbolMaps();
 
   NumOfMacroInstantiations = 0;
 }
@@ -1084,15 +1097,25 @@ bool MasmParser::expandMacros() {
   }
 
   std::optional<std::string> ExpandedValue;
-  auto BuiltinIt = BuiltinSymbolMap.find(IDLower);
-  if (BuiltinIt != BuiltinSymbolMap.end()) {
+
+  if (auto BuiltinIt = BuiltinSymbolMap.find(IDLower);
+      BuiltinIt != BuiltinSymbolMap.end()) {
     ExpandedValue =
         evaluateBuiltinTextMacro(BuiltinIt->getValue(), Tok.getLoc());
-  } else {
-    auto VarIt = Variables.find(IDLower);
-    if (VarIt != Variables.end() && VarIt->getValue().IsText) {
-      ExpandedValue = VarIt->getValue().TextValue;
+  } else if (auto BuiltinFuncIt = BuiltinFunctionMap.find(IDLower);
+             BuiltinFuncIt != BuiltinFunctionMap.end()) {
+    StringRef Name;
+    if (parseIdentifier(Name)) {
+      return true;
+    }
+    std::string Res;
+    if (evaluateBuiltinMacroFunction(BuiltinFuncIt->getValue(), Name, Res)) {
+      return true;
     }
+    ExpandedValue = Res;
+  } else if (auto VarIt = Variables.find(IDLower);
+             VarIt != Variables.end() && VarIt->getValue().IsText) {
+    ExpandedValue = VarIt->getValue().TextValue;
   }
 
   if (!ExpandedValue)
@@ -3197,6 +3220,18 @@ bool MasmParser::parseTextItem(std::string &Data) {
         continue;
       }
 
+      // Try to resolve as a built-in macro function
+      auto BuiltinFuncIt = BuiltinFunctionMap.find(ID.lower());
+      if (BuiltinFuncIt != BuiltinFunctionMap.end()) {
+        Data.clear();
+        if (evaluateBuiltinMacroFunction(BuiltinFuncIt->getValue(), ID, Data)) {
+          return true;
+        }
+        ID = StringRef(Data);
+        Expanded = true;
+        continue;
+      }
+
       // Try to resolve as a variable text macro
       auto VarIt = Variables.find(ID.lower());
       if (VarIt != Variables.end()) {
@@ -6204,7 +6239,7 @@ bool MasmParser::parseMSInlineAsm(
   return false;
 }
 
-void MasmParser::initializeBuiltinSymbolMap() {
+void MasmParser::initializeBuiltinSymbolMaps() {
   // Numeric built-ins (supported in all versions)
   BuiltinSymbolMap["@version"] = BI_VERSION;
   BuiltinSymbolMap["@line"] = BI_LINE;
@@ -6216,6 +6251,9 @@ void MasmParser::initializeBuiltinSymbolMap() {
   BuiltinSymbolMap["@filename"] = BI_FILENAME;
   BuiltinSymbolMap["@curseg"] = BI_CURSEG;
 
+  // Function built-ins (supported in all versions)
+  BuiltinFunctionMap["@catstr"] = BI_CATSTR;
+
   // Some built-ins exist only for MASM32 (32-bit x86)
   if (getContext().getSubtargetInfo()->getTargetTriple().getArch() ==
       Triple::x86) {
@@ -6289,6 +6327,48 @@ MasmParser::evaluateBuiltinTextMacro(BuiltinSymbol Symbol, SMLoc StartLoc) {
   llvm_unreachable("unhandled built-in symbol");
 }
 
+bool MasmParser::evaluateBuiltinMacroFunction(BuiltinFunction Function,
+                                              StringRef Name,
+                                              std::string &Res) {
+  if (parseToken(AsmToken::LParen, "invoking macro function '" + Name +
+                                       "' requires arguments in parentheses")) {
+    return true;
+  }
+
+  MCAsmMacroParameters P;
+  switch (Function) {
+  default:
+    return true;
+  case BI_CATSTR:
+    break;
+  }
+  MCAsmMacro M(Name, "", P, {}, true);
+
+  MCAsmMacroArguments A;
+  if (parseMacroArguments(&M, A, AsmToken::RParen) || parseRParen()) {
+    return true;
+  }
+
+  switch (Function) {
+  default:
+    llvm_unreachable("unhandled built-in function");
+  case BI_CATSTR: {
+    for (const MCAsmMacroArgument &Arg : A) {
+      for (const AsmToken &Tok : Arg) {
+        if (Tok.is(AsmToken::String)) {
+          Res.append(Tok.getStringContents());
+        } else {
+          Res.append(Tok.getString());
+        }
+      }
+    }
+    return false;
+  }
+  }
+  llvm_unreachable("unhandled built-in function");
+  return true;
+}
+
 /// Create an MCAsmParser instance.
 MCAsmParser *llvm::createMCMasmParser(SourceMgr &SM, MCContext &C,
                                       MCStreamer &Out, const MCAsmInfo &MAI,
diff --git a/llvm/test/tools/llvm-ml/macro_function.asm b/llvm/test/tools/llvm-ml/macro_function.asm
index c28d7c8c6222c..475c8b52dce79 100644
--- a/llvm/test/tools/llvm-ml/macro_function.asm
+++ b/llvm/test/tools/llvm-ml/macro_function.asm
@@ -103,14 +103,20 @@ expr_recursive_test PROC
   ret
 expr_recursive_test ENDP
 
+expand_as_directive_test @CatStr(P, RO, C)
+; CHECK-LABEL: expand_as_directive_test:
+
+  ret
+expand_as_directive_test ENDP
+
 custom_strcat MACRO arg1, arg2
   EXITM <arg1&arg2>
 ENDM
 
-expand_as_directive_test custom_strcat(P, ROC)
-; CHECK-LABEL: expand_as_directive_test:
+expand_as_directive_custom_test custom_strcat(P, ROC)
+; CHECK-LABEL: expand_as_directive_custom_test:
 
   ret
-expand_as_directive_test ENDP
+expand_as_directive_custom_test ENDP
 
 end