diff --git a/README.md b/README.md
index d3d8fe0151..43ca2c5c04 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,7 @@ For other versions, check the git tags of this repository.
  * all: Deleted unused files.
  * LLVM: Support .lib extension for static zstd.
  * LLVM: Copied CMake modules from out of tree.
+ * LLVM: Portable handling of .def linker flag
  * Clang: Disable building of libclang-cpp.so.
  * LLD: Added additional include directory to Zig's libunwind.
  * zlib: Delete the ability to build a shared library.
@@ -59,6 +60,20 @@ significantly affect how long it takes to build:
 
 When it succeeds, output can be found in `out/zig-<triple>-<cpu>/`.
 
+## Windows Build Instructions
+
+Bootstrapping on Windows with MSVC is also possible via `build.bat`, which
+takes the same arguments as `build` above.
+
+This script requires that the "C++ CMake tools for Windows" component be
+installed via the Visual Studio installer.
+
+The script must be run within the `Developer Command Prompt for VS 2019` shell:
+
+```
+build.bat <arch>-<os>-<abi> <mcpu>
+```
+
 ### Supported Triples
 
 If you try a "not tested" one and find a problem please file an issue,
diff --git a/build.bat b/build.bat
new file mode 100644
index 0000000000..cae6f660de
--- /dev/null
+++ b/build.bat
@@ -0,0 +1,258 @@
+@echo off
+
+SETLOCAL EnableDelayedExpansion
+if NOT DEFINED VSCMD_VER (
+   echo error: this script must be run within the visual studio developer command prompt
+   exit /b 1
+)
+
+where ninja >nul 2>nul
+if %ERRORLEVEL% neq 0 (
+   echo error: this script requires ninja to be installed, as the Visual Studio cmake generator doesn't support alternate compilers
+   exit /b %ERRORLEVEL%
+)
+
+if "%1" == "" (set "TARGET=x86_64-windows-gnu") ELSE (set TARGET=%~1)
+if "%2" == "" (set "MCPU=native") ELSE (set MCPU=%~2)
+
+set TARGET_ABI=
+set TARGET_OS_CMAKE=
+FOR /F "tokens=2,3 delims=-" %%i IN ("%TARGET%") DO (
+  IF "%%i"=="macos" set "TARGET_OS_CMAKE=Darwin"
+  IF "%%i"=="freebsd" set "TARGET_OS_CMAKE=FreeBSD"
+  IF "%%i"=="windows" set "TARGET_OS_CMAKE=Windows"
+  IF "%%i"=="linux" set "TARGET_OS_CMAKE=Linux"
+  set TARGET_ABI=%%j
+)
+
+set OUTDIR=out-win
+set ROOTDIR=%~dp0
+set "ROOTDIR_CMAKE=%ROOTDIR:\=/%"
+set ZIG_VERSION=0.11.0-dev.995+7350f0d9b
+
+set JOBS_ARG=
+
+pushd %ROOTDIR%
+
+rem Build zlib for the host
+mkdir "%ROOTDIR%%OUTDIR%\build-zlib-host"
+cd "%ROOTDIR%%OUTDIR%\build-zlib-host"
+cmake "%ROOTDIR%/zlib" ^
+  -G "Ninja" ^
+  -DCMAKE_INSTALL_PREFIX="%ROOTDIR%/%OUTDIR%/host" ^
+  -DCMAKE_PREFIX_PATH="%ROOTDIR%/%OUTDIR%/host" ^
+  -DCMAKE_BUILD_TYPE=Release ^
+  -DCMAKE_USER_MAKE_RULES_OVERRIDE="%ROOTDIR%/zig/cmake/c_flag_overrides.cmake"
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+cmake --build . %JOBS_ARG% --target install
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+rem Build the libraries for Zig to link against, as well as native `llvm-tblgen` using msvc
+mkdir "%ROOTDIR%%OUTDIR%\build-llvm-host"
+cd "%ROOTDIR%%OUTDIR%\build-llvm-host"
+cmake "%ROOTDIR%/llvm" ^
+  -G "Ninja" ^
+  -DLLVM_ENABLE_PROJECTS="lld;clang" ^
+  -DLLVM_ENABLE_LIBXML2=OFF ^
+  -DLLVM_ENABLE_ZSTD=OFF ^
+  -DCMAKE_INSTALL_PREFIX="%ROOTDIR%/%OUTDIR%/host" ^
+  -DCMAKE_PREFIX_PATH="%ROOTDIR%/%OUTDIR%/host" ^
+  -DLLVM_INCLUDE_TESTS=OFF ^
+  -DLLVM_INCLUDE_GO_TESTS=OFF ^
+  -DLLVM_INCLUDE_EXAMPLES=OFF ^
+  -DLLVM_INCLUDE_BENCHMARKS=OFF ^
+  -DLLVM_ENABLE_BINDINGS=OFF ^
+  -DLLVM_ENABLE_OCAMLDOC=OFF ^
+  -DLLVM_ENABLE_Z3_SOLVER=OFF ^
+  -DCLANG_BUILD_TOOLS=OFF ^
+  -DCLANG_INCLUDE_DOCS=OFF ^
+  -DLLVM_INCLUDE_DOCS=OFF ^
+  -DLLVM_USE_CRT_RELEASE=MT ^
+  -DCMAKE_BUILD_TYPE=Release
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+cmake --build . %JOBS_ARG% --target install
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+rem Build an x86_64-windows-msvc zig using msvc, linking against LLVM/Clang/LLD/zlib built by msvc
+mkdir "%ROOTDIR%%OUTDIR%\build-zig-host"
+cd "%ROOTDIR%%OUTDIR%\build-zig-host"
+cmake "%ROOTDIR%/zig" ^
+  -G "Ninja" ^
+  -DCMAKE_INSTALL_PREFIX="%ROOTDIR_CMAKE%%OUTDIR%/host" ^
+  -DCMAKE_PREFIX_PATH="%ROOTDIR_CMAKE%%OUTDIR%/host" ^
+  -DCMAKE_BUILD_TYPE=Release ^
+  -DZIG_STATIC=ON ^
+  -DZIG_STATIC_ZSTD=OFF ^
+  -DZIG_TARGET_TRIPLE=x86_64-windows-msvc ^
+  -DZIG_TARGET_MCPU=baseline
+
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+cmake --build . %JOBS_ARG% --target install
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+IF "%TARGET_ABI%"=="msvc" (
+   echo Building a target with the msvc ABI isn't supported yet
+   exit /b
+)
+
+set ZIG=%ROOTDIR%%OUTDIR%\host\bin\zig.exe
+set "ZIG=%ZIG:\=/%"
+
+rem CMP0091=NEW is required in order for the CMAKE_MSVC_RUNTIME_LIBRARY value to be respected,
+rem which we need to be set to MultiThreaded when building msvc ABI targets
+
+rem Cross compile zlib for the target
+mkdir "%ROOTDIR%%OUTDIR%\build-zlib-%TARGET%-%MCPU%"
+cd "%ROOTDIR%%OUTDIR%\build-zlib-%TARGET%-%MCPU%"
+cmake "%ROOTDIR%/zlib" ^
+  -G "Ninja" ^
+  -DCMAKE_INSTALL_PREFIX="%ROOTDIR_CMAKE%%OUTDIR%/%TARGET%-%MCPU%" ^
+  -DCMAKE_PREFIX_PATH="%ROOTDIR_CMAKE%%OUTDIR%/%TARGET%-%MCPU%" ^
+  -DCMAKE_BUILD_TYPE=Release ^
+  -DCMAKE_CROSSCOMPILING=True ^
+  -DCMAKE_SYSTEM_NAME="%TARGET_OS_CMAKE%" ^
+  -DCMAKE_C_COMPILER="%ZIG%;cc;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_CXX_COMPILER="%ZIG%;c++;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_ASM_COMPILER="%ZIG%;cc;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_RC_COMPILER="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-rc.exe" ^
+  -DCMAKE_AR="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-ar.exe" ^
+  -DCMAKE_RANLIB="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-ranlib.exe" ^
+  -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded ^
+  -DCMAKE_POLICY_DEFAULT_CMP0091=NEW
+cmake --build . %JOBS_ARG% --target install
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+rem Cross compile zstd for the target
+mkdir "%ROOTDIR%%OUTDIR%\%TARGET%-%MCPU%\lib"
+copy "%ROOTDIR%\zstd\lib\zstd.h" "%ROOTDIR%%OUTDIR%\%TARGET%-%MCPU%\include\zstd.h"
+cd "%ROOTDIR%%OUTDIR%\%TARGET%-%MCPU%\lib"
+%ZIG% build-lib ^
+  --name zstd ^
+  -target %TARGET% ^
+  -mcpu=%MCPU% ^
+  -fstrip ^
+  -OReleaseFast ^
+  -lc ^
+  "%ROOTDIR%\zstd\lib\decompress\zstd_ddict.c" ^
+  "%ROOTDIR%\zstd\lib\decompress\zstd_decompress.c" ^
+  "%ROOTDIR%\zstd\lib\decompress\huf_decompress.c" ^
+  "%ROOTDIR%\zstd\lib\decompress\huf_decompress_amd64.S" ^
+  "%ROOTDIR%\zstd\lib\decompress\zstd_decompress_block.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstdmt_compress.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_opt.c" ^
+  "%ROOTDIR%\zstd\lib\compress\hist.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_ldm.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_fast.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_compress_literals.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_double_fast.c" ^
+  "%ROOTDIR%\zstd\lib\compress\huf_compress.c" ^
+  "%ROOTDIR%\zstd\lib\compress\fse_compress.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_lazy.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_compress.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_compress_sequences.c" ^
+  "%ROOTDIR%\zstd\lib\compress\zstd_compress_superblock.c" ^
+  "%ROOTDIR%\zstd\lib\deprecated\zbuff_compress.c" ^
+  "%ROOTDIR%\zstd\lib\deprecated\zbuff_decompress.c" ^
+  "%ROOTDIR%\zstd\lib\deprecated\zbuff_common.c" ^
+  "%ROOTDIR%\zstd\lib\common\entropy_common.c" ^
+  "%ROOTDIR%\zstd\lib\common\pool.c" ^
+  "%ROOTDIR%\zstd\lib\common\threading.c" ^
+  "%ROOTDIR%\zstd\lib\common\zstd_common.c" ^
+  "%ROOTDIR%\zstd\lib\common\xxhash.c" ^
+  "%ROOTDIR%\zstd\lib\common\debug.c" ^
+  "%ROOTDIR%\zstd\lib\common\fse_decompress.c" ^
+  "%ROOTDIR%\zstd\lib\common\error_private.c" ^
+  "%ROOTDIR%\zstd\lib\dictBuilder\zdict.c" ^
+  "%ROOTDIR%\zstd\lib\dictBuilder\divsufsort.c" ^
+  "%ROOTDIR%\zstd\lib\dictBuilder\fastcover.c" ^
+  "%ROOTDIR%\zstd\lib\dictBuilder\cover.c"
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+rem Ideally we could use ZLIB_USE_STATIC_LIBS here (which would detect zlib correctly),
+rem but this was added in 3.24 and the MSVC-bundled CMake is 3.20. Instead, for the msvc
+rem ABI the zlib path is specified explicitly.
+
+IF "%TARGET_ABI%"=="msvc" (
+  set ZLIB_LIBRARY=-DZLIB_LIBRARY="%ROOTDIR_CMAKE%%OUTDIR%/%TARGET%-%MCPU%/lib/z.lib"
+) else (
+  set ZLIB_LIBRARY=
+)
+rem Cross compile LLVM for the target
+mkdir "%ROOTDIR%%OUTDIR%\build-llvm-%TARGET%-%MCPU%"
+cd "%ROOTDIR%%OUTDIR%\build-llvm-%TARGET%-%MCPU%"
+cmake "%ROOTDIR%/llvm" ^
+  -G "Ninja" ^
+  -DCMAKE_INSTALL_PREFIX="%ROOTDIR_CMAKE%%OUTDIR%/%TARGET%-%MCPU%" ^
+  -DCMAKE_PREFIX_PATH="%ROOTDIR_CMAKE%%OUTDIR%/%TARGET%-%MCPU%" ^
+  -DCMAKE_BUILD_TYPE=Release ^
+  -DCMAKE_CROSSCOMPILING=True ^
+  -DCMAKE_SYSTEM_NAME="%TARGET_OS_CMAKE%" ^
+  -DCMAKE_C_COMPILER="%ZIG%;cc;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_CXX_COMPILER="%ZIG%;c++;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_ASM_COMPILER="%ZIG%;cc;-fno-sanitize=all;-s;-target;%TARGET%;-mcpu=%MCPU%" ^
+  -DCMAKE_RC_COMPILER="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-rc.exe" ^
+  -DCMAKE_AR="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-ar.exe" ^
+  -DCMAKE_RANLIB="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-ranlib.exe" ^
+  -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded ^
+  -DCMAKE_POLICY_DEFAULT_CMP0091=NEW ^
+  -DLLVM_ENABLE_BACKTRACES=OFF ^
+  -DLLVM_ENABLE_BINDINGS=OFF ^
+  -DLLVM_ENABLE_CRASH_OVERRIDES=OFF ^
+  -DLLVM_ENABLE_LIBEDIT=OFF ^
+  -DLLVM_ENABLE_LIBPFM=OFF ^
+  -DLLVM_ENABLE_LIBXML2=OFF ^
+  -DLLVM_ENABLE_OCAMLDOC=OFF ^
+  -DLLVM_ENABLE_PLUGINS=OFF ^
+  -DLLVM_ENABLE_PROJECTS="lld;clang" ^
+  -DLLVM_ENABLE_TERMINFO=OFF ^
+  -DLLVM_ENABLE_Z3_SOLVER=OFF ^
+  -DLLVM_ENABLE_ZLIB=FORCE_ON ^
+  -DLLVM_ENABLE_ZSTD=FORCE_ON ^
+  -DLLVM_USE_STATIC_ZSTD=ON ^
+  -DLLVM_TABLEGEN="%ROOTDIR_CMAKE%%OUTDIR%/host/bin/llvm-tblgen.exe" ^
+  -DLLVM_BUILD_TOOLS=OFF ^
+  -DLLVM_BUILD_STATIC=ON ^
+  -DLLVM_INCLUDE_UTILS=OFF ^
+  -DLLVM_INCLUDE_TESTS=OFF ^
+  -DLLVM_INCLUDE_GO_TESTS=OFF ^
+  -DLLVM_INCLUDE_EXAMPLES=OFF ^
+  -DLLVM_INCLUDE_BENCHMARKS=OFF ^
+  -DLLVM_INCLUDE_DOCS=OFF ^
+  -DLLVM_DEFAULT_TARGET_TRIPLE=%TARGET% ^
+  -DLLVM_TOOL_LLVM_LTO2_BUILD=OFF ^
+  -DLLVM_TOOL_LLVM_LTO_BUILD=OFF ^
+  -DLLVM_TOOL_LTO_BUILD=OFF ^
+  -DLLVM_TOOL_REMARKS_SHLIB_BUILD=OFF ^
+  -DCLANG_TABLEGEN="%ROOTDIR_CMAKE%%OUTDIR%/build-llvm-host/bin/clang-tblgen.exe" ^
+  -DCLANG_BUILD_TOOLS=OFF ^
+  -DCLANG_INCLUDE_DOCS=OFF ^
+  -DCLANG_INCLUDE_TESTS=OFF ^
+  -DCLANG_ENABLE_ARCMT=ON ^
+  -DCLANG_TOOL_CLANG_IMPORT_TEST_BUILD=OFF ^
+  -DCLANG_TOOL_CLANG_LINKER_WRAPPER_BUILD=OFF ^
+  -DCLANG_TOOL_C_INDEX_TEST_BUILD=OFF ^
+  -DCLANG_TOOL_ARCMT_TEST_BUILD=OFF ^
+  -DCLANG_TOOL_C_ARCMT_TEST_BUILD=OFF ^
+  -DCLANG_TOOL_LIBCLANG_BUILD=OFF ^
+  -DZLIB_USE_STATIC_LIBS=ON ^
+  %ZLIB_LIBRARY% ^
+  -DLIBCLANG_BUILD_STATIC=ON
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+cmake --build . %JOBS_ARG% --target install
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+rem Finally, we can cross compile Zig itself, with Zig.
+cd "%ROOTDIR%\zig"
+%ZIG% build ^
+  --prefix "%ROOTDIR%%OUTDIR%\zig-%TARGET%-%MCPU%" ^
+  --search-prefix "%ROOTDIR%%OUTDIR%\%TARGET%-%MCPU%" ^
+  -Dstatic-llvm ^
+  -Drelease ^
+  -Dstrip ^
+  -Dtarget="%TARGET%" ^
+  -Dcpu="%MCPU%" ^
+  -Dversion-string="%ZIG_VERSION%"
+if %ERRORLEVEL% neq 0 exit /b %ERRORLEVEL%
+
+popd
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index 1f0507f395..b565b02595 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -118,7 +118,7 @@ function(add_llvm_symbol_exports target_name export_file)
       set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                    LINK_FLAGS "  -Wl,--version-script,\"${CMAKE_CURRENT_BINARY_DIR}/${native_export_file}\"")
     endif()
-  else()
+  elseif(WIN32)
     set(native_export_file "${target_name}.def")
 
     add_custom_command(OUTPUT ${native_export_file}
@@ -129,7 +129,18 @@ function(add_llvm_symbol_exports target_name export_file)
       COMMENT "Creating export file for ${target_name}")
     set(export_file_linker_flag "${CMAKE_CURRENT_BINARY_DIR}/${native_export_file}")
     if(MSVC)
+      # cl.exe or clang-cl, i.e. MSVC style command line interface
       set(export_file_linker_flag "/DEF:\"${export_file_linker_flag}\"")
+    elseif(CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
+      # clang in msvc mode, calling a link.exe/lld-link style linker
+      set(export_file_linker_flag "-Wl,/DEF:\"${export_file_linker_flag}\"")
+    elseif(MINGW)
+      # ${export_file_linker_flag}, which is the plain file name, works as is
+      # when passed to the compiler driver, which then passes it on to the
+      # linker as an input file.
+      set(export_file_linker_flag "\"${export_file_linker_flag}\"")
+    else()
+      message(FATAL_ERROR "Unsupported Windows toolchain")
     endif()
     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                  LINK_FLAGS " ${export_file_linker_flag}")