diff --git a/llvm/utils/gn/README.rst b/llvm/utils/gn/README.rst
index 97ce5e7e72bd..852cffaed37f 100644
--- a/llvm/utils/gn/README.rst
+++ b/llvm/utils/gn/README.rst
@@ -78,9 +78,9 @@ for the `gen` command. The full GN reference is also `available online
 GN has an autoformatter: `git ls-files '*.gn' '*.gni' | xargs -n 1 gn format`
 after making GN build changes is your friend.
 
-To not put `BUILD.gn` into the main tree, they are all below `utils/gn/tree`.
-For example, the build file for `llvm/lib/Support` is in
-`utils/gn/tree/llvm/lib/Support`.
+To not put `BUILD.gn` into the main tree, they are all below
+`utils/gn/secondary`.  For example, the build file for `llvm/lib/Support` is in
+`utils/gn/secondary/llvm/lib/Support`.
 
 .. _Philosophy:
 
diff --git a/llvm/utils/gn/build/BUILD.gn b/llvm/utils/gn/build/BUILD.gn
index 0d407dd28ac2..0f369ec73951 100644
--- a/llvm/utils/gn/build/BUILD.gn
+++ b/llvm/utils/gn/build/BUILD.gn
@@ -1,7 +1,6 @@
 import("//llvm/utils/gn/build/buildflags.gni")
 import("//llvm/utils/gn/build/mac_sdk.gni")
 import("//llvm/utils/gn/build/toolchain/compiler.gni")
-import("//llvm/utils/gn/build/enable_threads.gni")
 
 config("compiler_defaults") {
   # FIXME: Don't define this globally here.
@@ -140,14 +139,3 @@ config("warn_covered_switch_default") {
     cflags = [ "-Wcovered-switch-default" ]
   }
 }
-
-config("pthread_config") {
-  visibility = [ ":pthread" ]
-  libs = [ "pthread" ]
-}
-
-group("pthread") {
-  if (llvm_enable_threads && host_os != "win") {
-    public_configs = [ ":pthread_config" ]
-  }
-}
diff --git a/llvm/utils/gn/build/libs/pthread/BUILD.gn b/llvm/utils/gn/build/libs/pthread/BUILD.gn
new file mode 100644
index 000000000000..51e7f1f31fa8
--- /dev/null
+++ b/llvm/utils/gn/build/libs/pthread/BUILD.gn
@@ -0,0 +1,12 @@
+import("//llvm/utils/gn/build/libs/pthread/enable.gni")
+
+config("pthread_config") {
+  visibility = [ ":pthread" ]
+  libs = [ "pthread" ]
+}
+
+group("pthread") {
+  if (llvm_enable_threads && host_os != "win") {
+    public_configs = [ ":pthread_config" ]
+  }
+}
diff --git a/llvm/utils/gn/build/libs/terminfo/BUILD.gn b/llvm/utils/gn/build/libs/terminfo/BUILD.gn
new file mode 100644
index 000000000000..10003d61c4df
--- /dev/null
+++ b/llvm/utils/gn/build/libs/terminfo/BUILD.gn
@@ -0,0 +1,12 @@
+import("//llvm/utils/gn/build/libs/terminfo/enable.gni")
+
+config("terminfo_config") {
+  visibility = [ ":terminfo" ]
+  libs = [ "ncurses" ]
+}
+
+group("terminfo") {
+  if (llvm_enable_terminfo) {
+    public_configs = [ ":terminfo_config" ]
+  }
+}
diff --git a/llvm/utils/gn/build/libs/terminfo/enable.gni b/llvm/utils/gn/build/libs/terminfo/enable.gni
new file mode 100644
index 000000000000..79ea2b601857
--- /dev/null
+++ b/llvm/utils/gn/build/libs/terminfo/enable.gni
@@ -0,0 +1,4 @@
+declare_args() {
+  # Whether to link against terminfo.
+  llvm_enable_terminfo = false
+}
diff --git a/llvm/utils/gn/build/libs/xml/BUILD.gn b/llvm/utils/gn/build/libs/xml/BUILD.gn
new file mode 100644
index 000000000000..eb60982384cf
--- /dev/null
+++ b/llvm/utils/gn/build/libs/xml/BUILD.gn
@@ -0,0 +1,19 @@
+import("//llvm/utils/gn/build/libs/xml/enable.gni")
+import("//llvm/utils/gn/build/mac_sdk.gni")
+
+config("xml_config") {
+  visibility = [ ":xml" ]
+  defines = [ "LLVM_LIBXML2_ENABLED" ]
+  libs = [ "xml2" ]
+  if (host_os == "mac") {
+    include_dirs = [ "$mac_sdk/usr/include/libxml2" ]
+  } else {
+    include_dirs = [ "/usr/include/libxml2" ]
+  }
+}
+
+group("xml") {
+  if (llvm_enable_libxml2) {
+    public_configs = [ ":xml_config" ]
+  }
+}
diff --git a/llvm/utils/gn/build/libs/xml/enable.gni b/llvm/utils/gn/build/libs/xml/enable.gni
new file mode 100644
index 000000000000..3ef357b4fd2f
--- /dev/null
+++ b/llvm/utils/gn/build/libs/xml/enable.gni
@@ -0,0 +1,4 @@
+declare_args() {
+  # Whether to include code that links against libxml2.
+  llvm_enable_libxml2 = host_os != "win"
+}
diff --git a/llvm/utils/gn/build/libs/zlib/BUILD.gn b/llvm/utils/gn/build/libs/zlib/BUILD.gn
new file mode 100644
index 000000000000..5085578e760e
--- /dev/null
+++ b/llvm/utils/gn/build/libs/zlib/BUILD.gn
@@ -0,0 +1,12 @@
+import("//llvm/utils/gn/build/libs/zlib/enable.gni")
+
+config("zlib_config") {
+  visibility = [ ":zlib" ]
+  libs = [ "z" ]
+}
+
+group("zlib") {
+  if (llvm_enable_zlib) {
+    public_configs = [ ":zlib_config" ]
+  }
+}
diff --git a/llvm/utils/gn/build/libs/zlib/enable.gni b/llvm/utils/gn/build/libs/zlib/enable.gni
new file mode 100644
index 000000000000..9626301378bd
--- /dev/null
+++ b/llvm/utils/gn/build/libs/zlib/enable.gni
@@ -0,0 +1,4 @@
+declare_args() {
+  # Whether to include code that links against zlib.
+  llvm_enable_zlib = host_os != "win"
+}
diff --git a/llvm/utils/gn/build/toolchain/BUILD.gn b/llvm/utils/gn/build/toolchain/BUILD.gn
index f9b31e4dfb4e..927a53d9d631 100644
--- a/llvm/utils/gn/build/toolchain/BUILD.gn
+++ b/llvm/utils/gn/build/toolchain/BUILD.gn
@@ -50,7 +50,8 @@ toolchain("unix") {
     } else {
       # Remove the output file first so that ar doesn't try to modify the
       # existing file.
-      command = "rm -f {{output}} && ar rcsDT {{arflags}} -o {{output}} {{inputs}}"
+      command =
+          "rm -f {{output}} && ar rcsDT {{arflags}} -o {{output}} {{inputs}}"
     }
     description = "AR {{output}}"
     outputs = [
diff --git a/llvm/utils/gn/build/write_cmake_config.py b/llvm/utils/gn/build/write_cmake_config.py
new file mode 100644
index 000000000000..408ffcdbfc08
--- /dev/null
+++ b/llvm/utils/gn/build/write_cmake_config.py
@@ -0,0 +1,109 @@
+"""Processes a foo.h.cmake file and writes foo.h.
+
+Takes a list of KEY=VALUE pairs (where VALUE can be empty).
+
+Handles these types of lines (note that FOO= sets the value of FOO to the empty
+string, which is falsy, but FOO=0 sets it to '0' which is truthy):
+
+1.) #cmakedefine01 FOO
+    Checks if key FOO is set to a truthy value, and depending on that prints
+    one of the following two lines:
+
+        #define FOO 1
+        #define FOO 0
+
+2.) #cmakedefine FOO
+    Checks if key FOO is set to a truthy in value, and depending on that prints
+    one of the following two lines:
+
+        #define FOO
+        /* #undef FOO */
+
+3.) #cmakedefine FOO asdf${BAR}jkl
+    Checks if key FOO is set to a truthy values, and if so replaces all
+    variable references in `asdf${BAR}jkl` with their value and prints that
+    (else it prints the same undef line as the previous form):
+
+        #define FOO asdfBAR_VALjkl
+        /* #undef FOO */
+
+4.) #define FOO asdf{BAR}jkl
+    Always gets its variable values filled in, independent of FOO's value being
+    set:
+
+        #define FOO asdfBAR_VALjkl
+
+Fails if any of the KEY=VALUE arguments aren't needed for processing the
+.h.cmake file, or if the .h.cmake file has unreplaces ${VAR} references after
+processing all values.
+"""
+
+import argparse
+import os
+import re
+import sys
+
+
+def main():
+    parser = argparse.ArgumentParser(
+                 epilog=__doc__,
+                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    parser.add_argument('input', help='input file')
+    parser.add_argument('values', nargs='*', help='several KEY=VALUE pairs')
+    parser.add_argument('-o', '--output', required=True,
+                        help='output file')
+    args = parser.parse_args()
+
+    values = {}
+    for value in args.values:
+        key, val = value.split('=', 1)
+        values[key] = val
+    unused_values = set(values.keys())
+
+    # Matches e.g. '${CLANG_FOO}' and captures CLANG_FOO in group 1.
+    var_re = re.compile(r'\$\{([^}]*)\}')
+
+    in_lines = open(args.input).readlines()
+    out_lines = []
+    for in_line in in_lines:
+        def repl(m):
+            unused_values.discard(m.group(1))
+            return values[m.group(1)]
+        if in_line.startswith('#cmakedefine01 '):
+            _, var = in_line.split()
+            out_lines.append('#define %s %d\n' % (var, 1 if values[var] else 0))
+            unused_values.discard(var)
+        elif in_line.startswith('#cmakedefine '):
+            _, var = in_line.split(None, 1)
+            try:
+                var, val = var.split(None, 1)
+            except:
+                var, val = var.rstrip(), '\n'
+            if values[var]:
+                out_lines.append('#define %s %s' % (var,
+                                                    var_re.sub(repl, val)))
+            else:
+                out_lines.append('/* #undef %s */\n' % var)
+            unused_values.discard(var)
+        else:
+            # In particular, handles `#define FOO ${FOO}` lines.
+            out_lines.append(var_re.sub(repl, in_line))
+
+    if unused_values:
+        print >>sys.stderr, 'Unused --values args:'
+        print >>sys.stderr, '    ', '\n    '.join(unused_values)
+        return 1
+
+    output = ''.join(out_lines)
+
+    leftovers = var_re.findall(output)
+    if leftovers:
+        print >>sys.stderr, 'unprocessed values:\n', '\n'.join(leftovers)
+        return 1
+
+    if not os.path.exists(args.output) or open(args.output).read() != output:
+        open(args.output, 'w').write(output)
+
+
+if __name__ == '__main__':
+    sys.exit(main())
diff --git a/llvm/utils/gn/secondary/BUILD.gn b/llvm/utils/gn/secondary/BUILD.gn
index 35547d9d5b6f..4b6e17ff722a 100644
--- a/llvm/utils/gn/secondary/BUILD.gn
+++ b/llvm/utils/gn/secondary/BUILD.gn
@@ -1,6 +1,6 @@
 group("default") {
   deps = [
-    "//llvm/lib/Demangle",
+    "//llvm/tools/llvm-undname",
   ]
 }
 
diff --git a/llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn b/llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn
new file mode 100644
index 000000000000..1aa41e90f8d2
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/include/llvm/Config/BUILD.gn
@@ -0,0 +1,354 @@
+import("//llvm/lib/Target/targets.gni")
+import("//llvm/triples.gni")
+import("//llvm/utils/gn/build/buildflags.gni")
+import("//llvm/utils/gn/build/libs/pthread/enable.gni")
+import("//llvm/utils/gn/build/libs/terminfo/enable.gni")
+import("//llvm/utils/gn/build/libs/xml/enable.gni")
+import("//llvm/utils/gn/build/libs/zlib/enable.gni")
+import("//llvm/version.gni")
+
+# Contains actions to create config.h, llvm-config.h and abi-breaking.h.
+# Other than in the cmake build, these are created at build time, not at
+# config time. That way, they can run in parallel, and this pattern means that
+# e.g. creating the clang config header (which happens in another gn file)
+# doesn't block building lld.
+
+# The headers are created by processing the foo.h.cmake files as input,
+# to keep the gn build close to the cmake build.
+
+# Other than in the cmake build, header generation doesn't do any feature
+# checking. See also "Philosophy" in llvm/utils/gn/README.rst.
+
+# FIXME: Several of the config settings go in a global config header but
+# are only used in a single translation unit -- so toggling that value
+# causes a full rebuild when it really only has to rebuild a single file.
+# Instead monolithing config headers, investigate using something like
+# https://is.gd/buildflag_header_gni instead (needs to be done in both the
+# cmake build and here).
+
+# FIXME: This hardcodes a bunch of settings I never use; some of them should
+# become declare_args if anyone wants to set them.
+
+declare_args() {
+  # Enable additional checks that alter the LLVM C++ ABI.
+  llvm_enable_abi_breaking_checks = llvm_enable_assertions
+
+  # Iterate unordered llvm containers in reverse.
+  llvm_enable_reverse_iteration = false
+}
+
+action("abi-breaking") {
+  script = "//llvm/utils/gn/build/write_cmake_config.py"
+
+  sources = [
+    "abi-breaking.h.cmake",
+  ]
+  outputs = [
+    "$target_gen_dir/abi-breaking.h",
+  ]
+  args = [
+    "-o",
+    rebase_path(outputs[0], root_out_dir),
+
+    rebase_path(sources[0], root_out_dir),
+  ]
+
+  if (llvm_enable_abi_breaking_checks) {
+    args += [ "LLVM_ENABLE_ABI_BREAKING_CHECKS=1" ]
+  } else {
+    args += [ "LLVM_ENABLE_ABI_BREAKING_CHECKS=" ]
+  }
+
+  if (llvm_enable_reverse_iteration) {
+    args += [ "LLVM_ENABLE_REVERSE_ITERATION=1" ]
+  } else {
+    args += [ "LLVM_ENABLE_REVERSE_ITERATION=" ]
+  }
+}
+
+action("config") {
+  script = "//llvm/utils/gn/build/write_cmake_config.py"
+
+  public_deps = [
+    ":llvm-config",
+  ]
+
+  sources = [
+    "config.h.cmake",
+  ]
+  outputs = [
+    "$target_gen_dir/config.h",
+  ]
+  args = [
+    "-o",
+    rebase_path(outputs[0], root_out_dir),
+    rebase_path(sources[0], root_out_dir),
+
+    "BUG_REPORT_URL=https://bugs.llvm.org/",
+    "ENABLE_BACKTRACES=1",
+    "ENABLE_CRASH_OVERRIDES=1",
+    "BACKTRACE_HEADER=execinfo.h",
+    "HAVE_CRASHREPORTERCLIENT_H=",
+    "HAVE_DECL_FE_ALL_EXCEPT=1",
+    "HAVE_DECL_FE_INEXACT=1",
+    "LLVM_ENABLE_DIA_SDK=",
+    "LLVM_ENABLE_CRASH_DUMPS=",
+    "HAVE_ERRNO_H=1",
+    "HAVE_FCNTL_H=1",
+    "HAVE_FENV_H=1",
+    "HAVE_FFI_CALL=",
+    "HAVE_FFI_FFI_H=",
+    "HAVE_FFI_H=",
+    "HAVE_LIBPFM=",
+    "HAVE_LIBPSAPI=",
+    "HAVE_MALLCTL=",
+    "HAVE_SIGNAL_H=1",
+    "HAVE_STRERROR=1",
+    "HAVE_SYS_STAT_H=1",
+    "HAVE_SYS_TYPES_H=1",
+    "HAVE__ALLOCA=",
+    "HAVE___ALLOCA=",
+    "HAVE___ASHLDI3=",
+    "HAVE___ASHRDI3=",
+    "HAVE___CHKSTK=",
+    "HAVE___CHKSTK_MS=",
+    "HAVE___CMPDI2=",
+    "HAVE___DIVDI3=",
+    "HAVE___FIXDFDI=",
+    "HAVE___FIXSFDI=",
+    "HAVE___FLOATDIDF=",
+    "HAVE___LSHRDI3=",
+    "HAVE___MAIN=",
+    "HAVE___MODDI3=",
+    "HAVE___UDIVDI3=",
+    "HAVE___UMODDI3=",
+    "HAVE____CHKSTK=",
+    "HAVE____CHKSTK_MS=",
+    "HOST_LINK_VERSION=",
+    "LLVM_TARGET_TRIPLE_ENV=",
+    "LLVM_VERSION_INFO=",
+    "LLVM_VERSION_PRINTER_SHOW_HOST_TARGET_INFO=1",
+    "PACKAGE_BUGREPORT=https://bugs.llvm.org/",
+    "PACKAGE_NAME=LLVM",
+    "PACKAGE_STRING=LLVM ${llvm_version}svn",
+    "PACKAGE_VERSION=${llvm_version}svn",
+    "PACKAGE_VENDOR=",
+    "RETSIGTYPE=void",
+    "LLVM_GISEL_COV_ENABLED=",
+    "LLVM_GISEL_COV_PREFIX=",
+
+    # This is both in llvm-config.h and config.h; llvm-config.h doesn't
+    # define it if it's not set while config.h defines it to empty in that case.
+    "LLVM_DEFAULT_TARGET_TRIPLE=$llvm_target_triple",
+  ]
+
+  if (host_os == "linux") {
+    args += [
+      "HAVE_FUTIMENS=1",
+      "HAVE_LINK_H=1",
+      "HAVE_LSEEK64=1",
+      "HAVE_MALLINFO=1",
+      "HAVE_POSIX_FALLOCATE=1",
+      "HAVE_SCHED_GETAFFINITY=1",
+      "HAVE_CPU_COUNT=1",
+      "HAVE_VALGRIND_VALGRIND_H=1",
+    ]
+  } else {
+    args += [
+      "HAVE_FUTIMENS=",
+      "HAVE_LINK_H=",
+      "HAVE_LSEEK64=",
+      "HAVE_MALLINFO=",
+      "HAVE_POSIX_FALLOCATE=",
+      "HAVE_SCHED_GETAFFINITY=",
+      "HAVE_CPU_COUNT=",
+      "HAVE_VALGRIND_VALGRIND_H=",
+    ]
+  }
+
+  if (host_os == "mac") {
+    args += [
+      "HAVE_CRASHREPORTER_INFO=1",
+      "HAVE_DECL_ARC4RANDOM=1",
+      "HAVE_DLADDR=1",
+      "HAVE_LIBEDIT=1",
+      "HAVE_MALLOC_H=",
+      "HAVE_MACH_MACH_H=1",
+      "HAVE_MALLOC_MALLOC_H=1",
+      "HAVE_MALLOC_ZONE_STATISTICS=1",
+      "HAVE_LIBXAR=1",
+    ]
+  } else {
+    args += [
+      "HAVE_CRASHREPORTER_INFO=",
+      "HAVE_DECL_ARC4RANDOM=",
+      "HAVE_DLADDR=",
+      "HAVE_LIBEDIT=",
+      "HAVE_MACH_MACH_H=",
+      "HAVE_MALLOC_H=1",
+      "HAVE_MALLOC_MALLOC_H=",
+      "HAVE_MALLOC_ZONE_STATISTICS=",
+      "HAVE_LIBXAR=",
+    ]
+  }
+
+  if (host_os == "win") {
+    args += [
+      "HAVE_BACKTRACE=",
+      "HAVE_DECL_STRERROR_S=1",
+      "HAVE_DLFCN_H=",
+      "HAVE_DLOPEN=",
+      "HAVE_FUTIMES=",
+      "HAVE_GETPAGESIZE=",
+      "HAVE_GETRLIMIT=",
+      "HAVE_GETRUSAGE=",
+      "HAVE_ISATTY=",
+      "HAVE_LIBPTHREAD=",
+      "HAVE_PTHREAD_GETNAME_NP=",
+      "HAVE_PTHREAD_SETNAME_NP=",
+      "HAVE_LIBZ=",
+      "HAVE_POSIX_SPAWN=",
+      "HAVE_PREAD=",
+      "HAVE_PTHREAD_GETSPECIFIC=",
+      "HAVE_PTHREAD_H=",
+      "HAVE_PTHREAD_MUTEX_LOCK=",
+      "HAVE_PTHREAD_RWLOCK_INIT=",
+      "HAVE_REALPATH=",
+      "HAVE_SBRK=",
+      "HAVE_SETENV=",
+      "HAVE_SETRLIMIT=",
+      "HAVE_SIGALTSTACK=",
+      "HAVE_STRERROR_R=",
+      "HAVE_SYSCONF=",
+      "HAVE_SYS_IOCTL_H=",
+      "HAVE_SYS_MMAN_H=",
+      "HAVE_SYS_PARAM_H=",
+      "HAVE_SYS_RESOURCE_H=",
+      "HAVE_SYS_TIME_H=",
+      "HAVE_TERMIOS_H=",
+      "HAVE_UNISTD_H=",
+      "HAVE_ZLIB_H=",
+      "HAVE__CHSIZE_S=1",
+      "HAVE__UNWIND_BACKTRACE=",
+      "stricmp=_stricmp",
+      "strdup=_strdup",
+    ]
+  } else {
+    # POSIX-y system defaults.
+    args += [
+      "HAVE_BACKTRACE=1",
+      "HAVE_DECL_STRERROR_S=",
+      "HAVE_DLFCN_H=1",
+      "HAVE_DLOPEN=1",
+      "HAVE_FUTIMES=1",
+      "HAVE_GETPAGESIZE=1",
+      "HAVE_GETRLIMIT=1",
+      "HAVE_GETRUSAGE=1",
+      "HAVE_ISATTY=1",
+      "HAVE_LIBPTHREAD=1",
+      "HAVE_PTHREAD_GETNAME_NP=1",
+      "HAVE_PTHREAD_SETNAME_NP=1",
+      "HAVE_LIBZ=1",
+      "HAVE_POSIX_SPAWN=1",
+      "HAVE_PREAD=1",
+      "HAVE_PTHREAD_GETSPECIFIC=1",
+      "HAVE_PTHREAD_H=1",
+      "HAVE_PTHREAD_MUTEX_LOCK=1",
+      "HAVE_PTHREAD_RWLOCK_INIT=1",
+      "HAVE_REALPATH=1",
+      "HAVE_SBRK=1",
+      "HAVE_SETENV=1",
+      "HAVE_SETRLIMIT=1",
+      "HAVE_SIGALTSTACK=1",
+      "HAVE_STRERROR_R=1",
+      "HAVE_SYSCONF=1",
+      "HAVE_SYS_IOCTL_H=1",
+      "HAVE_SYS_MMAN_H=1",
+      "HAVE_SYS_PARAM_H=1",
+      "HAVE_SYS_RESOURCE_H=1",
+      "HAVE_SYS_TIME_H=1",
+      "HAVE_TERMIOS_H=1",
+      "HAVE_UNISTD_H=1",
+      "HAVE_ZLIB_H=1",
+      "HAVE__CHSIZE_S=",
+      "HAVE__UNWIND_BACKTRACE=1",
+      "stricmp=",
+      "strdup=",
+    ]
+  }
+
+  if (host_os == "linux") {
+    args += [ "LTDL_SHLIB_EXT=.so" ]
+  } else if (host_os == "mac") {
+    args += [ "LTDL_SHLIB_EXT=.dylib" ]
+  } else if (host_os == "win") {
+    args += [ "LTDL_SHLIB_EXT=.dll" ]
+  }
+
+  if (llvm_enable_terminfo) {
+    args += [ "HAVE_TERMINFO=1" ]
+  } else {
+    args += [ "HAVE_TERMINFO=" ]
+  }
+
+  if (llvm_enable_zlib) {
+    args += [ "LLVM_ENABLE_ZLIB=1" ]
+  } else {
+    args += [ "LLVM_ENABLE_ZLIB=" ]
+  }
+
+  if (llvm_enable_libxml2) {
+    args += [ "LLVM_LIBXML2_ENABLED=1" ]
+  } else {
+    args += [ "LLVM_LIBXML2_ENABLED=" ]
+  }
+}
+
+action("llvm-config") {
+  script = "//llvm/utils/gn/build/write_cmake_config.py"
+
+  sources = [
+    "llvm-config.h.cmake",
+  ]
+  outputs = [
+    "$target_gen_dir/llvm-config.h",
+  ]
+  args = [
+    "-o",
+    rebase_path(outputs[0], root_out_dir),
+    rebase_path(sources[0], root_out_dir),
+
+    "LLVM_ENABLE_DUMP=",
+    "LINK_POLLY_INTO_TOOLS=",
+    "LLVM_DEFAULT_TARGET_TRIPLE=$llvm_target_triple",
+    "LLVM_HAS_ATOMICS=1",
+    "LLVM_HOST_TRIPLE=$llvm_host_triple",
+    "LLVM_NATIVE_ARCH=$native_target",
+    "LLVM_NATIVE_ASMPARSER=1",
+    "LLVM_NATIVE_ASMPRINTER=1",
+    "LLVM_NATIVE_DISASSEMBLER=1",
+    "LLVM_NATIVE_TARGET=1",
+    "LLVM_NATIVE_TARGETINFO=1",
+    "LLVM_NATIVE_TARGETMC=1",
+    "LLVM_USE_INTEL_JITEVENTS=",
+    "LLVM_USE_OPROFILE=",
+    "LLVM_USE_PERF=",
+    "LLVM_VERSION_MAJOR=$llvm_version_major",
+    "LLVM_VERSION_MINOR=$llvm_version_minor",
+    "LLVM_VERSION_PATCH=$llvm_version_patch",
+    "PACKAGE_VERSION=${llvm_version}svn",
+    "LLVM_FORCE_ENABLE_STATS=",
+  ]
+
+  if (host_os == "win") {
+    args += [ "LLVM_ON_UNIX=" ]
+  } else {
+    args += [ "LLVM_ON_UNIX=1" ]
+  }
+
+  if (llvm_enable_threads) {
+    args += [ "LLVM_ENABLE_THREADS=1" ]
+  } else {
+    args += [ "LLVM_ENABLE_THREADS=" ]
+  }
+}
diff --git a/llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn b/llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn
new file mode 100644
index 000000000000..c51b824859d4
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/lib/Support/BUILD.gn
@@ -0,0 +1,155 @@
+static_library("Support") {
+  output_name = "LLVMSupport"
+  deps = [
+    "//llvm/include/llvm/Config:config",
+    "//llvm/lib/Demangle",
+    "//llvm/utils/gn/build/libs/pthread",
+    "//llvm/utils/gn/build/libs/terminfo",
+    "//llvm/utils/gn/build/libs/zlib",
+  ]
+
+  # public_deps are used for depending on targets that generate headers
+  # which are included in public headers of this target. public_deps means
+  # that targets depending on Support will implicitly be built only after
+  # abi-breaking and llvm-config have been built.
+  public_deps = [
+    # abi-breaking.h is also include by public headers in ADT, but ADT has
+    # no target in the gn build.  Since everything depends on Support, this
+    # public_dep does double duty of abi-breaking.h uses in public headers of
+    # both Support and ADT.
+    "//llvm/include/llvm/Config:abi-breaking",
+    "//llvm/include/llvm/Config:llvm-config",
+  ]
+  include_dirs = [
+    "Unix",
+    "Windows",
+  ]
+  sources = [
+    "AMDGPUMetadata.cpp",
+    "APFloat.cpp",
+    "APInt.cpp",
+    "APSInt.cpp",
+    "ARMAttributeParser.cpp",
+    "ARMBuildAttrs.cpp",
+    "ARMWinEH.cpp",
+    "Allocator.cpp",
+    "BinaryStreamError.cpp",
+    "BinaryStreamReader.cpp",
+    "BinaryStreamRef.cpp",
+    "BinaryStreamWriter.cpp",
+    "BlockFrequency.cpp",
+    "BranchProbability.cpp",
+    "COM.cpp",
+    "CachePruning.cpp",
+    "Chrono.cpp",
+    "CodeGenCoverage.cpp",
+    "CommandLine.cpp",
+    "Compression.cpp",
+    "ConvertUTF.cpp",
+    "ConvertUTFWrapper.cpp",
+    "CrashRecoveryContext.cpp",
+    "DAGDeltaAlgorithm.cpp",
+    "DJB.cpp",
+    "DataExtractor.cpp",
+    "Debug.cpp",
+    "DebugCounter.cpp",
+    "DeltaAlgorithm.cpp",
+    "Error.cpp",
+    "ErrorHandling.cpp",
+    "FileCheck.cpp",
+    "FileOutputBuffer.cpp",
+    "FileUtilities.cpp",
+    "FoldingSet.cpp",
+    "FormatVariadic.cpp",
+    "FormattedStream.cpp",
+    "GlobPattern.cpp",
+    "GraphWriter.cpp",
+    "Hashing.cpp",
+    "InitLLVM.cpp",
+    "IntEqClasses.cpp",
+    "IntervalMap.cpp",
+    "ItaniumManglingCanonicalizer.cpp",
+    "JSON.cpp",
+    "JamCRC.cpp",
+    "KnownBits.cpp",
+    "LEB128.cpp",
+    "LineIterator.cpp",
+    "Locale.cpp",
+    "LockFileManager.cpp",
+    "LowLevelType.cpp",
+    "MD5.cpp",
+    "ManagedStatic.cpp",
+    "MathExtras.cpp",
+    "MemoryBuffer.cpp",
+    "NativeFormatting.cpp",
+    "Options.cpp",
+    "Parallel.cpp",
+    "PluginLoader.cpp",
+    "PrettyStackTrace.cpp",
+    "RandomNumberGenerator.cpp",
+    "Regex.cpp",
+    "SHA1.cpp",
+    "ScaledNumber.cpp",
+    "ScopedPrinter.cpp",
+    "SmallPtrSet.cpp",
+    "SmallVector.cpp",
+    "SourceMgr.cpp",
+    "SpecialCaseList.cpp",
+    "Statistic.cpp",
+    "StringExtras.cpp",
+    "StringMap.cpp",
+    "StringPool.cpp",
+    "StringRef.cpp",
+    "StringSaver.cpp",
+    "SymbolRemappingReader.cpp",
+    "SystemUtils.cpp",
+    "TarWriter.cpp",
+    "TargetParser.cpp",
+    "ThreadPool.cpp",
+    "Timer.cpp",
+    "ToolOutputFile.cpp",
+    "TrigramIndex.cpp",
+    "Triple.cpp",
+    "Twine.cpp",
+    "Unicode.cpp",
+    "UnicodeCaseFold.cpp",
+    "VersionTuple.cpp",
+    "WithColor.cpp",
+    "YAMLParser.cpp",
+    "YAMLTraits.cpp",
+    "circular_raw_ostream.cpp",
+    "raw_os_ostream.cpp",
+    "raw_ostream.cpp",
+    "regcomp.c",
+    "regerror.c",
+    "regexec.c",
+    "regfree.c",
+    "regstrlcpy.c",
+    "xxhash.cpp",
+
+    # System
+    "Atomic.cpp",
+    "DynamicLibrary.cpp",
+    "Errno.cpp",
+    "Host.cpp",
+    "Memory.cpp",
+    "Mutex.cpp",
+    "Path.cpp",
+    "Process.cpp",
+    "Program.cpp",
+    "RWMutex.cpp",
+    "Signals.cpp",
+    "TargetRegistry.cpp",
+    "ThreadLocal.cpp",
+    "Threading.cpp",
+    "Valgrind.cpp",
+    "VirtualFileSystem.cpp",
+    "Watchdog.cpp",
+  ]
+
+  libs = []
+
+  if (host_os == "linux") {
+    libs += [ "dl" ]
+  }
+}
diff --git a/llvm/utils/gn/secondary/llvm/lib/Target/targets.gni b/llvm/utils/gn/secondary/llvm/lib/Target/targets.gni
new file mode 100644
index 000000000000..8d8efeb6bc6a
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/lib/Target/targets.gni
@@ -0,0 +1,54 @@
+declare_args() {
+  # The target archs LLVM should support. Defaults to the host arch.
+  # Set to a list, e.g. `llvm_targets_to_build = [ "X86", "ARM" ]`,
+  # or to the string "all" to get all known targets.
+  llvm_targets_to_build = "host"
+}
+
+if (llvm_targets_to_build == "host") {
+  if (host_cpu == "x86" || host_cpu == "x64") {
+    llvm_targets_to_build = [ "X86" ]
+  } else if (host_cpu == "arm") {
+    llvm_targets_to_build = [ "ARM" ]
+  } else if (host_cpu == "arm64") {
+    llvm_targets_to_build = [ "AArch64" ]
+  } else {
+    assert(false, "add your host_cpu above")
+  }
+} else if (llvm_targets_to_build == "all") {
+  # FIXME: Port the remaining targets.
+  llvm_targets_to_build = [
+    "AArch64",
+    "ARM",
+    "X86",
+  ]
+}
+
+# Validate that llvm_targets_to_build is set to a list of valid targets,
+# and remember which targets are built.
+llvm_build_AArch64 = false
+llvm_build_ARM = false
+llvm_build_X86 = false
+foreach(target, llvm_targets_to_build) {
+  if (target == "AArch64") {
+    llvm_build_AArch64 = true
+  } else if (target == "ARM") {
+    llvm_build_ARM = true
+  } else if (target == "X86") {
+    llvm_build_X86 = true
+  } else {
+    #FIXME : Port the remaining targets.
+    assert(false, "Unknown target '$target'.")
+  }
+}
+
+# FIXME: This should be based off target_cpu once cross compiles work.
+if (host_cpu == "x86" || host_cpu == "x64") {
+  native_target = "X86"
+} else if (host_cpu == "arm") {
+  native_target = "ARM"
+} else if (host_cpu == "arm64") {
+  native_target = "AArch64"
+} else {
+  assert(false, "Unsuppored host_cpu '$host_cpu'.")
+}
diff --git a/llvm/utils/gn/secondary/llvm/tools/llvm-undname/BUILD.gn b/llvm/utils/gn/secondary/llvm/tools/llvm-undname/BUILD.gn
new file mode 100644
index 000000000000..56f1ed11bc49
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/tools/llvm-undname/BUILD.gn
@@ -0,0 +1,9 @@
+executable("llvm-undname") {
+  deps = [
+    "//llvm/lib/Demangle",
+    "//llvm/lib/Support",
+  ]
+  sources = [
+    "llvm-undname.cpp",
+  ]
+}
diff --git a/llvm/utils/gn/secondary/llvm/triples.gni b/llvm/utils/gn/secondary/llvm/triples.gni
new file mode 100644
index 000000000000..b5ff76237218
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/triples.gni
@@ -0,0 +1,12 @@
+if (host_os == "linux") {
+  llvm_host_triple = "x86_64-unknown-linux-gnu"
+} else if (host_os == "mac") {
+  llvm_host_triple = "x86_64-apple-darwin"
+} else if (host_os == "win") {
+  llvm_host_triple = "x86_64-pc-windows"
+}
+
+declare_args() {
+  # The default target triple.
+  llvm_target_triple = llvm_host_triple
+}
diff --git a/llvm/utils/gn/secondary/llvm/version.gni b/llvm/utils/gn/secondary/llvm/version.gni
new file mode 100644
index 000000000000..b64e3f9bdec0
--- /dev/null
+++ b/llvm/utils/gn/secondary/llvm/version.gni
@@ -0,0 +1,4 @@
+llvm_version_major = 8
+llvm_version_minor = 0
+llvm_version_patch = 0
+llvm_version = "$llvm_version_major.$llvm_version_minor.$llvm_version_patch"