diff --git a/doc/extensions.html b/doc/extensions.html
index b048f137..cb9be3f4 100644
--- a/doc/extensions.html
+++ b/doc/extensions.html
@@ -210,7 +210,8 @@ bytecode (e.g. from Lua 5.1) is incompatible and cannot be loaded.
 <p>
 Note: <tt>LJ_GC64</tt> mode requires a different frame layout, which implies
 a different, incompatible bytecode format for ports that use this mode (e.g.
-ARM64). This may be rectified in the future.
+ARM64 or MIPS64) or when explicitly enabled for x64. This may be rectified
+in the future.
 </p>
 
 <h3 id="table_new"><tt>table.new(narray, nhash)</tt> allocates a pre-sized table</h3>
diff --git a/doc/install.html b/doc/install.html
index efeda33c..230e9386 100644
--- a/doc/install.html
+++ b/doc/install.html
@@ -175,6 +175,14 @@ MSVC or WinSDK.</li>
 Please read the instructions given in these files, before changing
 any settings.
 </p>
+<p>
+LuaJIT on x64 currently uses 32 bit GC objects by default.
+<tt>LJ_GC64</tt> mode may be explicitly enabled:
+add <tt>XCFLAGS=-DLUAJIT_ENABLE_GC64</tt> to the make command or run
+<tt>msvcbuild gc64</tt> for MSVC/WinSDK. Please check the note
+about the <a href="extensions.html#string_dump">bytecode format</a>
+differences, too.
+</p>
 
 <h2 id="posix">POSIX Systems (Linux, OSX, *BSD etc.)</h2>
 <h3>Prerequisites</h3>
@@ -584,14 +592,11 @@ intend to load Lua/C modules at runtime.
 </li>
 <li>
 If you're building a 64 bit application on OSX which links directly or
-indirectly against LuaJIT, you need to link your main executable
-with these flags:
+indirectly against LuaJIT which is not built for <tt>LJ_GC64</tt> mode,
+you need to link your main executable with these flags:
 <pre class="code">
 -pagezero_size 10000 -image_base 100000000
 </pre>
-Also, it's recommended to <tt>rebase</tt> all (self-compiled) shared libraries
-which are loaded at runtime on OSX/x64 (e.g. C extension modules for Lua).
-See: <tt>man rebase</tt>
 </li>
 </ul>
 <p>Additional hints for initializing LuaJIT using the C API functions:</p>
diff --git a/src/Makefile b/src/Makefile
index 4e479ae5..21e16a22 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -110,6 +110,9 @@ XCFLAGS=
 #XCFLAGS+= -DLUAJIT_NUMMODE=1
 #XCFLAGS+= -DLUAJIT_NUMMODE=2
 #
+# Enable GC64 mode for x64.
+#XCFLAGS+= -DLUAJIT_ENABLE_GC64
+#
 ##############################################################################
 
 ##############################################################################
diff --git a/src/msvcbuild.bat b/src/msvcbuild.bat
index 4334bbde..f7a1addc 100644
--- a/src/msvcbuild.bat
+++ b/src/msvcbuild.bat
@@ -20,6 +20,7 @@
 @set LJLIB=lib /nologo /nodefaultlib
 @set DASMDIR=..\dynasm
 @set DASM=%DASMDIR%\dynasm.lua
+@set DASC=vm_x86.dasc
 @set LJDLLNAME=lua51.dll
 @set LJLIBNAME=lua51.lib
 @set ALL_LIB=lib_base.c lib_math.c lib_bit.c lib_string.c lib_table.c lib_io.c lib_os.c lib_package.c lib_debug.c lib_jit.c lib_ffi.c
@@ -39,7 +40,12 @@ if exist minilua.exe.manifest^
 @set LJARCH=x86
 @set LJCOMPILE=%LJCOMPILE% /arch:SSE2
 :X64
-minilua %DASM% -LN %DASMFLAGS% -o host\buildvm_arch.h vm_x86.dasc
+@if "%1" neq "gc64" goto :NOGC64
+@shift
+@set DASC=vm_x64.dasc
+@set LJCOMPILE=%LJCOMPILE% /DLUAJIT_ENABLE_GC64
+:NOGC64
+minilua %DASM% -LN %DASMFLAGS% -o host\buildvm_arch.h %DASC%
 @if errorlevel 1 goto :BAD
 
 %LJCOMPILE% /I "." /I %DASMDIR% host\buildvm*.c