diff --git a/llvm/utils/gn/build/toolchain/BUILD.gn b/llvm/utils/gn/build/toolchain/BUILD.gn
--- a/llvm/utils/gn/build/toolchain/BUILD.gn
+++ b/llvm/utils/gn/build/toolchain/BUILD.gn
@@ -9,6 +9,8 @@
 assert(!use_goma || goma_dir != "",
        "set `goma_dir` to the output of `goma_ctl goma_dir` in your args.gn")
 
+unix_copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
+
 template("unix_toolchain") {
   toolchain(target_name) {
     # https://groups.google.com/a/chromium.org/g/gn-dev/c/F_lv5T-tNDM
@@ -144,9 +146,8 @@
       default_output_dir = "{{root_out_dir}}/bin"
     }
 
-    copy_command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
     tool("copy") {
-      command = copy_command
+      command = unix_copy_command
       description = "COPY {{source}} {{output}}"
     }
 
@@ -364,14 +365,23 @@
     }
 
     tool("copy") {
-      # GN hands out slash-using paths, but cmd's copy needs backslashes.
-      # Use cmd's %foo:a=b% substitution feature to convert.
-      command = "cmd /c set source=\"{{source}}\" & set output=\"{{output}}\" & call copy /Y %source:/=\% %output:\=/% > nul"
+      if (host_os == "win") {
+        # GN hands out slash-using paths, but cmd's copy needs backslashes.
+        # Use cmd's %foo:a=b% substitution feature to convert.
+        command = "cmd /c set source=\"{{source}}\" & set output=\"{{output}}\" & call copy /Y %source:/=\% %output:\=/% > nul"
+      } else {
+        command = unix_copy_command
+      }
+
       description = "COPY {{source}} {{output}}"
     }
 
     tool("stamp") {
-      command = "cmd /c type nul > {{output}}"
+      if (host_os == "win") {
+        command = "cmd /c type nul > {{output}}"
+      } else {
+        command = "touch {{output}}"
+      }
       description = "STAMP {{output}}"
     }
   }
@@ -384,11 +394,14 @@
   }
 }
 
-win_toolchain("stage2_win") {
+win_toolchain("stage2_win_x64") {
   toolchain_args = {
-    current_os = host_os
-    current_cpu = host_cpu
+    current_os = "win"
+    current_cpu = "x64"
 
+    if (host_os != "win") {
+      sysroot = win_sysroot
+    }
     clang_base_path = root_build_dir
     use_goma = false
   }
@@ -400,9 +413,12 @@
 
 win_toolchain("stage2_win_x86") {
   toolchain_args = {
-    current_os = host_os
+    current_os = "win"
     current_cpu = "x86"
 
+    if (host_os != "win") {
+      sysroot = win_sysroot
+    }
     clang_base_path = root_build_dir
     use_goma = false
   }
diff --git a/llvm/utils/gn/build/toolchain/compiler.gni b/llvm/utils/gn/build/toolchain/compiler.gni
--- a/llvm/utils/gn/build/toolchain/compiler.gni
+++ b/llvm/utils/gn/build/toolchain/compiler.gni
@@ -13,6 +13,9 @@
   # Set this to the path to Android NDK r21. If set, cross compilation targeting
   # Android will be enabled.
   android_ndk_path = ""
+
+  # Set this to the path of the Win SDK. Only used for cross compilation. If set, cross compilation targeting Windows will be enabled.
+  win_sysroot = ""
 }
 
 declare_args() {
@@ -23,3 +26,7 @@
   # Set this to true to link with LLD instead of the default linker.
   use_lld = clang_base_path != ""
 }
+
+assert(
+    !(host_os == "win" && win_sysroot != ""),
+    "win_sysroot should only be used for cross compilation, use sysroot on Windows")
diff --git a/llvm/utils/gn/build/toolchain/target_flags.gni b/llvm/utils/gn/build/toolchain/target_flags.gni
--- a/llvm/utils/gn/build/toolchain/target_flags.gni
+++ b/llvm/utils/gn/build/toolchain/target_flags.gni
@@ -44,5 +44,5 @@
 }
 
 if (current_cpu == "x86") {
-  target_flags = [ "-m32" ]
+  target_flags += [ "-m32" ]
 }
diff --git a/llvm/utils/gn/secondary/compiler-rt/BUILD.gn b/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
--- a/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
+++ b/llvm/utils/gn/secondary/compiler-rt/BUILD.gn
@@ -5,11 +5,13 @@
 # In the GN build, compiler-rt is always built by just-built clang and lld.
 # FIXME: For macOS and iOS builds, depend on lib in all needed target arch
 # toolchains and then lipo them together for the final output.
-if (current_os == "win") {
-  supported_toolchains = [ "//llvm/utils/gn/build/toolchain:stage2_win" ]
+supported_toolchains = []
+if (current_os == "win" || win_sysroot != "") {
+  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_win_x64" ]
   supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_win_x86" ]
-} else {
-  supported_toolchains = [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
+}
+if (current_os != "win") {
+  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
 }
 supported_toolchains += supported_android_toolchains
 if (llvm_build_AArch64) {
diff --git a/llvm/utils/gn/secondary/compiler-rt/test/asan/BUILD.gn b/llvm/utils/gn/secondary/compiler-rt/test/asan/BUILD.gn
--- a/llvm/utils/gn/secondary/compiler-rt/test/asan/BUILD.gn
+++ b/llvm/utils/gn/secondary/compiler-rt/test/asan/BUILD.gn
@@ -48,8 +48,9 @@
 if (host_os == "linux") {
   supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_unix" ]
 } else if (host_os == "win") {
-  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_win" ]
+  supported_toolchains += [ "//llvm/utils/gn/build/toolchain:stage2_win_64" ]
 }
+
 group("asan") {
   deps = []
   foreach(toolchain, supported_toolchains) {