diff --git a/lldb/source/Core/Mangled.cpp b/lldb/source/Core/Mangled.cpp
--- a/lldb/source/Core/Mangled.cpp
+++ b/lldb/source/Core/Mangled.cpp
@@ -108,7 +108,7 @@
 // Local helpers for different demangling implementations.
 static char *GetMSVCDemangledStr(const char *M) {
   char *demangled_cstr = llvm::microsoftDemangle(
-      M, nullptr, nullptr, nullptr, nullptr,
+      M, nullptr, nullptr,
       llvm::MSDemangleFlags(
           llvm::MSDF_NoAccessSpecifier | llvm::MSDF_NoCallingConvention |
           llvm::MSDF_NoMemberType | llvm::MSDF_NoVariableType));
diff --git a/llvm/include/llvm/Demangle/Demangle.h b/llvm/include/llvm/Demangle/Demangle.h
--- a/llvm/include/llvm/Demangle/Demangle.h
+++ b/llvm/include/llvm/Demangle/Demangle.h
@@ -46,15 +46,9 @@
 /// success, or nullptr on error.
 /// If n_read is non-null and demangling was successful, it receives how many
 /// bytes of the input string were consumed.
-/// buf can point to a *n_buf bytes large buffer where the demangled name is
-/// stored. If the buffer is too small, it is grown with realloc(). If buf is
-/// nullptr, then this malloc()s memory for the result.
-/// *n_buf stores the size of buf on input if buf is non-nullptr, and it
-/// receives the size of the demangled string on output if n_buf is not nullptr.
 /// status receives one of the demangle_ enum entries above if it's not nullptr.
 /// Flags controls various details of the demangled representation.
-char *microsoftDemangle(const char *mangled_name, size_t *n_read, char *buf,
-                        size_t *n_buf, int *status,
+char *microsoftDemangle(const char *mangled_name, size_t *n_read, int *status,
                         MSDemangleFlags Flags = MSDF_None);
 
 // Demangles a Rust v0 mangled symbol.
diff --git a/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp b/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
--- a/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
+++ b/llvm/lib/DebugInfo/Symbolize/Symbolize.cpp
@@ -687,7 +687,7 @@
     // Only do MSVC C++ demangling on symbols starting with '?'.
     int status = 0;
     char *DemangledName = microsoftDemangle(
-        Name.c_str(), nullptr, nullptr, nullptr, &status,
+        Name.c_str(), nullptr, &status,
         MSDemangleFlags(MSDF_NoAccessSpecifier | MSDF_NoCallingConvention |
                         MSDF_NoMemberType | MSDF_NoReturnType));
     if (status != 0)
diff --git a/llvm/lib/Demangle/Demangle.cpp b/llvm/lib/Demangle/Demangle.cpp
--- a/llvm/lib/Demangle/Demangle.cpp
+++ b/llvm/lib/Demangle/Demangle.cpp
@@ -36,8 +36,7 @@
   if (S[0] == '_' && nonMicrosoftDemangle(S + 1, Result))
     return Result;
 
-  if (char *Demangled =
-          microsoftDemangle(S, nullptr, nullptr, nullptr, nullptr)) {
+  if (char *Demangled = microsoftDemangle(S, nullptr, nullptr)) {
     Result = Demangled;
     std::free(Demangled);
     return Result;
diff --git a/llvm/lib/Demangle/MicrosoftDemangle.cpp b/llvm/lib/Demangle/MicrosoftDemangle.cpp
--- a/llvm/lib/Demangle/MicrosoftDemangle.cpp
+++ b/llvm/lib/Demangle/MicrosoftDemangle.cpp
@@ -2398,7 +2398,6 @@
 }
 
 char *llvm::microsoftDemangle(const char *MangledName, size_t *NMangled,
-                              char *Buf, size_t *N,
                               int *Status, MSDemangleFlags Flags) {
   Demangler D;
 
@@ -2423,14 +2422,13 @@
     OF = OutputFlags(OF | OF_NoVariableType);
 
   int InternalStatus = demangle_success;
+  char *Buf;
   if (D.Error)
     InternalStatus = demangle_invalid_mangled_name;
   else {
-    OutputBuffer OB(Buf, N);
+    OutputBuffer OB;
     AST->output(OB, OF);
     OB += '\0';
-    if (N != nullptr)
-      *N = OB.getCurrentPosition();
     Buf = OB.getBuffer();
   }
 
diff --git a/llvm/tools/llvm-microsoft-demangle-fuzzer/llvm-microsoft-demangle-fuzzer.cpp b/llvm/tools/llvm-microsoft-demangle-fuzzer/llvm-microsoft-demangle-fuzzer.cpp
--- a/llvm/tools/llvm-microsoft-demangle-fuzzer/llvm-microsoft-demangle-fuzzer.cpp
+++ b/llvm/tools/llvm-microsoft-demangle-fuzzer/llvm-microsoft-demangle-fuzzer.cpp
@@ -14,7 +14,6 @@
 
 extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
   std::string NullTerminatedString((const char *)Data, Size);
-  free(llvm::microsoftDemangle(NullTerminatedString.c_str(), nullptr, nullptr,
-                               nullptr, nullptr));
+  free(llvm::microsoftDemangle(NullTerminatedString.c_str(), nullptr, nullptr));
   return 0;
 }
diff --git a/llvm/tools/llvm-objdump/COFFDump.cpp b/llvm/tools/llvm-objdump/COFFDump.cpp
--- a/llvm/tools/llvm-objdump/COFFDump.cpp
+++ b/llvm/tools/llvm-objdump/COFFDump.cpp
@@ -849,8 +849,7 @@
            << Name;
     if (Demangle && Name.startswith("?")) {
       int Status = -1;
-      char *DemangledSymbol =
-          microsoftDemangle(Name.data(), nullptr, nullptr, nullptr, &Status);
+      char *DemangledSymbol = microsoftDemangle(Name.data(), nullptr, &Status);
 
       if (Status == 0 && DemangledSymbol) {
         outs() << " (" << StringRef(DemangledSymbol) << ")";
diff --git a/llvm/tools/llvm-undname/llvm-undname.cpp b/llvm/tools/llvm-undname/llvm-undname.cpp
--- a/llvm/tools/llvm-undname/llvm-undname.cpp
+++ b/llvm/tools/llvm-undname/llvm-undname.cpp
@@ -75,8 +75,7 @@
     Flags = MSDemangleFlags(Flags | MSDF_NoVariableType);
 
   size_t NRead;
-  char *ResultBuf =
-      microsoftDemangle(S.c_str(), &NRead, nullptr, nullptr, &Status, Flags);
+  char *ResultBuf = microsoftDemangle(S.c_str(), &NRead, &Status, Flags);
   if (Status == llvm::demangle_success) {
     outs() << ResultBuf << "\n";
     outs().flush();