Index: llvm/trunk/utils/gn/build/BUILDCONFIG.gn
===================================================================
--- llvm/trunk/utils/gn/build/BUILDCONFIG.gn
+++ llvm/trunk/utils/gn/build/BUILDCONFIG.gn
@@ -33,6 +33,13 @@
   current_os = target_os
 }
 
+if (target_cpu == "") {
+  target_cpu = host_cpu
+}
+if (current_cpu == "") {
+  current_cpu = target_cpu
+}
+
 if (host_os == "win") {
   host_toolchain = "//llvm/utils/gn/build/toolchain:win"
 } else {
Index: llvm/trunk/utils/gn/build/toolchain/BUILD.gn
===================================================================
--- llvm/trunk/utils/gn/build/toolchain/BUILD.gn
+++ llvm/trunk/utils/gn/build/toolchain/BUILD.gn
@@ -153,6 +153,7 @@
 
   toolchain_args = {
     current_os = host_os
+    current_cpu = host_cpu
   }
 }
 
@@ -171,6 +172,7 @@
 
     toolchain_args = {
       current_os = "android"
+      current_cpu = "arm64"
     }
 
     libcxx_path = "$android_ndk_path/sources/cxx-stl/llvm-libc++"
@@ -293,5 +295,6 @@
 
   toolchain_args = {
     current_os = "win"
+    current_cpu = host_cpu
   }
 }
Index: llvm/trunk/utils/gn/secondary/clang/test/BUILD.gn
===================================================================
--- llvm/trunk/utils/gn/secondary/clang/test/BUILD.gn
+++ llvm/trunk/utils/gn/secondary/clang/test/BUILD.gn
@@ -52,7 +52,7 @@
     # builds exist, to make sure it's a toolchain var.
     "CMAKE_CXX_COMPILER=c++",
     "ENABLE_BACKTRACES=1",
-    "LLVM_HOST_TRIPLE=$llvm_host_triple",
+    "LLVM_HOST_TRIPLE=$llvm_current_triple",
     "LLVM_LIT_TOOLS_DIR=",  # Intentionally empty, matches cmake build.
     "LLVM_USE_SANITIZER=",
     "PYTHON_EXECUTABLE=$python_path",
Index: llvm/trunk/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn
===================================================================
--- llvm/trunk/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn
+++ llvm/trunk/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn
@@ -322,7 +322,7 @@
     "LINK_POLLY_INTO_TOOLS=",
     "LLVM_DEFAULT_TARGET_TRIPLE=$llvm_target_triple",
     "LLVM_HAS_ATOMICS=1",
-    "LLVM_HOST_TRIPLE=$llvm_host_triple",
+    "LLVM_HOST_TRIPLE=$llvm_current_triple",
     "LLVM_NATIVE_ARCH=$native_target",
     "LLVM_NATIVE_ASMPARSER=1",
     "LLVM_NATIVE_ASMPRINTER=1",
Index: llvm/trunk/utils/gn/secondary/llvm/test/BUILD.gn
===================================================================
--- llvm/trunk/utils/gn/secondary/llvm/test/BUILD.gn
+++ llvm/trunk/utils/gn/secondary/llvm/test/BUILD.gn
@@ -54,7 +54,7 @@
 
     "LLVM_ENABLE_FFI=0",
     "LLVM_HAVE_OPT_VIEWER_MODULES=0",
-    "LLVM_HOST_TRIPLE=$llvm_host_triple",
+    "LLVM_HOST_TRIPLE=$llvm_current_triple",
     "LLVM_LIBRARY_DIR=" + rebase_path("$root_out_dir/lib"),
     "LLVM_LINK_LLVM_DYLIB=0",
     "LLVM_LIT_TOOLS_DIR=",  # Intentionally empty, matches cmake build.
Index: llvm/trunk/utils/gn/secondary/llvm/triples.gni
===================================================================
--- llvm/trunk/utils/gn/secondary/llvm/triples.gni
+++ llvm/trunk/utils/gn/secondary/llvm/triples.gni
@@ -1,16 +1,22 @@
-if (host_cpu == "x64") {
-  if (host_os == "linux") {
-    llvm_host_triple = "x86_64-unknown-linux-gnu"
-  } else if (host_os == "mac") {
-    llvm_host_triple = "x86_64-apple-darwin"
-  } else if (host_os == "win") {
-    llvm_host_triple = "x86_64-pc-windows"
+if (current_cpu == "x64") {
+  if (current_os == "linux") {
+    llvm_current_triple = "x86_64-unknown-linux-gnu"
+  } else if (current_os == "mac") {
+    llvm_current_triple = "x86_64-apple-darwin"
+  } else if (current_os == "win") {
+    llvm_current_triple = "x86_64-pc-windows"
   }
-} else {
-  assert(false, "unimplemented host_cpu " + host_cpu)
+} else if (current_cpu == "arm64") {
+  if (current_os == "android") {
+    llvm_current_triple = "aarch64-linux-android21"
+  }
+}
+
+if (!defined(llvm_current_triple)) {
+  assert(false, "unimplemented cpu/os " + current_cpu + "/" + current_os)
 }
 
 declare_args() {
   # The default target triple.
-  llvm_target_triple = llvm_host_triple
+  llvm_target_triple = llvm_current_triple
 }