Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -1465,8 +1465,12 @@
     * ``m``: Mips mangling: Private symbols get a ``$`` prefix.
     * ``o``: Mach-O mangling: Private symbols get ``L`` prefix. Other
       symbols get a ``_`` prefix.
-    * ``w``: Windows COFF prefix:  Similar to Mach-O, but stdcall and fastcall
-      functions also get a suffix based on the frame size.
+    * ``w``: Windows COFF prefix:  Similar to Mach-O, but stdcall, fastcall, and
+      vectorcall functions also get a suffix based on the parameter pack size.
+
+    Prefixing symbols with the ``\01`` character disables any mangling.
+    Prefixing non-private symbols with the ``?`` character disables any
+    mangling.
 ``n<size1>:<size2>:<size3>...``
     This specifies a set of native integer widths for the target CPU in
     bits. For example, it might contain ``n32`` for 32-bit PowerPC,
Index: docs/ReleaseNotes.rst
===================================================================
--- docs/ReleaseNotes.rst
+++ docs/ReleaseNotes.rst
@@ -44,6 +44,10 @@
 * The minimum required Visual Studio version for building LLVM is now 2013
   Update 4.
 
+* Symbols starting with ``?`` are no longer mangled by LLVM, meaning most
+  symbols used in the Microsoft C++ ABI no longer require ``\01`` prefix
+  escapes.
+
 * ... next change ...
 
 .. NOTE
Index: lib/IR/Mangler.cpp
===================================================================
--- lib/IR/Mangler.cpp
+++ lib/IR/Mangler.cpp
@@ -113,13 +113,22 @@
   }
 
   StringRef Name = GV->getName();
+
   char Prefix = DL->getGlobalPrefix();
 
   // Mangle functions with Microsoft calling conventions specially.  Only do
   // this mangling for x86_64 vectorcall and 32-bit x86.
   const Function *MSFunc = dyn_cast<Function>(GV);
   if (Name.startswith("\01"))
-    MSFunc = nullptr; // Don't mangle when \01 is present.
+    MSFunc = nullptr; // Don't mangle when the first character is '\01'.
+
+  // Don't add prefixes or MS suffixes for symbols starting with '?'.  Add the
+  // private global prefix if necessary, though.
+  if (Name.startswith("?")) {
+    MSFunc = nullptr;
+    Prefix = '\0';
+  }
+
   CallingConv::ID CC =
       MSFunc ? MSFunc->getCallingConv() : (unsigned)CallingConv::C;
   if (!DL->hasMicrosoftFastStdCallMangling() &&
Index: test/CodeGen/X86/mangle-question-mark.ll
===================================================================
--- /dev/null
+++ test/CodeGen/X86/mangle-question-mark.ll
@@ -0,0 +1,63 @@
+; The purpose of this test is to see if the MC layer properly handles symbol
+; names needing quoting on MS/Windows. This code is generated by clang when
+; using -cxx-abi microsoft. At some point, we'd like to stop using the '\01'
+; escape and just use a leading '?'.
+
+; RUN: llc -mtriple i686-pc-win32 < %s | FileCheck %s --check-prefix=COFF
+; RUN: llc -mtriple i686-linux-gnu < %s | FileCheck %s --check-prefix=ELF
+; RUN: llc -mtriple i686-apple-darwin < %s | FileCheck %s --check-prefix=MACHO
+
+; Currently all object files allow escaping private symbols, but eventually we
+; might want to reject that.
+
+; COFF: calll "?withescape@A@@QBEXXZ"
+; COFF: calll "?withquestion@A@@QBEXXZ"
+; COFF: calll "L?privatequestion@A@@QBEXXZ"
+; COFF: calll "L?privatequestionfast@A@@QBEXXZ"
+; COFF: calll "?escapedprivate@A@@QBEXXZ"
+
+; ELF: calll "?withescape@A@@QBEXXZ"
+; ELF: calll "?withquestion@A@@QBEXXZ"
+; ELF: calll ".L?privatequestion@A@@QBEXXZ"
+; ELF: calll ".L?privatequestionfast@A@@QBEXXZ"
+; ELF: calll "?escapedprivate@A@@QBEXXZ"
+
+; On MachO, everything gets the local prefix due to stubbing.
+
+; MACHO: calll "L?withescape@A@@QBEXXZ$stub"
+; MACHO: calll "L?withquestion@A@@QBEXXZ$stub"
+; MACHO: calll "l?privatequestion@A@@QBEXXZ"
+; MACHO: calll "l?privatequestionfast@A@@QBEXXZ"
+; MACHO: calll "?escapedprivate@A@@QBEXXZ"
+
+; Check the stubs.
+
+; MACHO: .indirect_symbol "?withescape@A@@QBEXXZ"
+; MACHO: .indirect_symbol "?withquestion@A@@QBEXXZ"
+
+%struct.A = type {}
+
+define i32 @main() {
+entry:
+  tail call void @"\01?withescape@A@@QBEXXZ"(%struct.A* null)
+  tail call void @"?withquestion@A@@QBEXXZ"(%struct.A* null)
+  tail call void @"?privatequestion@A@@QBEXXZ"(%struct.A* null)
+  tail call x86_fastcallcc void @"?privatequestionfast@A@@QBEXXZ"(%struct.A* null)
+  tail call void @"\01?escapedprivate@A@@QBEXXZ"(%struct.A* null)
+  ret i32 0
+}
+
+declare void @"\01?withescape@A@@QBEXXZ"(%struct.A*)
+declare void @"?withquestion@A@@QBEXXZ"(%struct.A*)
+
+define private void @"?privatequestion@A@@QBEXXZ"(%struct.A*) {
+  ret void
+}
+
+define private x86_fastcallcc void @"?privatequestionfast@A@@QBEXXZ"(%struct.A*) {
+  ret void
+}
+
+define private void @"\01?escapedprivate@A@@QBEXXZ"(%struct.A*) {
+  ret void
+}
Index: test/MC/COFF/symbol-mangling.ll
===================================================================
--- test/MC/COFF/symbol-mangling.ll
+++ /dev/null
@@ -1,17 +0,0 @@
-; The purpose of this test is to see if the MC layer properly handles symbol
-; names needing quoting on MS/Windows. This code is generated by clang when
-; using -cxx-abi microsoft.
-
-; RUN: llc -filetype=asm -mtriple i686-pc-win32 %s -o - | FileCheck %s
-
-; CHECK: ?sayhi@A@@QBEXXZ
-
-%struct.A = type {}
-
-define i32 @main() {
-entry:
-  tail call void @"\01?sayhi@A@@QBEXXZ"(%struct.A* null)
-  ret i32 0
-}
-
-declare void @"\01?sayhi@A@@QBEXXZ"(%struct.A*)
Index: test/MC/COFF/tricky-names.ll
===================================================================
--- test/MC/COFF/tricky-names.ll
+++ test/MC/COFF/tricky-names.ll
@@ -3,14 +3,16 @@
 
 ; Check that we can roundtrip these names through our assembler.
 ; RUN: llc -mtriple=i686-pc-win32 %s -o - | llvm-mc -triple i686-pc-win32 -filetype=obj | llvm-readobj -t | FileCheck %s --check-prefix=READOBJ
+; RUN: llc -mtriple=x86_64-pc-win32 %s -o - | llvm-mc -triple x86_64-pc-win32 -filetype=obj | llvm-readobj -t | FileCheck %s --check-prefix=READOBJ
 
+; We don't need the \01 prefix for MSVC symbols using ?.
 
-@"\01??__E_Generic_object@?$_Error_objects@H@std@@YAXXZ" = global i32 0
+@"??__E_Generic_object@?$_Error_objects@H@std@@YAXXZ" = global i32 0
 @"\01__ZL16ExceptionHandlerP19_EXCEPTION_POINTERS@4" = global i32 0
 @"\01@foo.bar" = global i32 0
 
-define weak i32 @"\01??_B?$num_put@_WV?$back_insert_iterator@V?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@@std@@@std@@51"() section ".text" {
-  %a = load i32, i32* @"\01??__E_Generic_object@?$_Error_objects@H@std@@YAXXZ"
+define weak i32 @"??_B?$num_put@_WV?$back_insert_iterator@V?$basic_string@_WU?$char_traits@_W@std@@V?$allocator@_W@2@@std@@@std@@@std@@51"() section ".text" {
+  %a = load i32, i32* @"??__E_Generic_object@?$_Error_objects@H@std@@YAXXZ"
   %b = load i32, i32* @"\01__ZL16ExceptionHandlerP19_EXCEPTION_POINTERS@4"
   %c = load i32, i32* @"\01@foo.bar"
   %x = add i32 %a, %b