Add initial scaffolding for the GN build.

[thakis]

Summary

See "GN build roundtable summary; adding GN build files to the repo" on llvm-dev and cfe-dev for discussion.

In particular, this build is completely unsupported. People adding new files to LLVM are not expected to update the GN build files, and reviewers are not supposed to request the gn build files to be updated.

This adds just enough to be able to build llvm/lib/Demangle. It requires using a monorepo.

This adds a few build config options you can set in args.gn (gn args out/foo --list for all):

• is_debug = true to enable debug builds (defaults to release)
• llvm_enable_assertions to toggle assertions (defaults to true)
• clang_base_path, if set an absolute path to a locally-built clang to be used as host compiler, e.g. clang_base_path = getenv("HOME") + "/src/chrome/src/third_party/llvm-build/Release+Asserts"

[thakis]

just one symlink

[thakis]

fix bad exec_script_whitelist default

[thakis]

zero symlinks, in exchange for longer gen command

[phosek]

Shouldn't all the GN files have a copyright header?

llvm/utils/gn/.gn

10

The convention is to use the name secondary, i.e. //llvm/utils/gn/secondary.

12

I prefer llvm/contrib/gn as suggested on the list over utils.

llvm/utils/gn/README.rst

91

nit: capitalize GN here and below

llvm/utils/gn/build/BUILD.gn

7

I'd prefer to break these into smaller configs so they can be added/removed independently rather than using nested if/else branches, but it's something that can be probably done later as a cleanup.

llvm/utils/gn/build/buildflags.gni

10

Shouldn't this be llvm_enable_assertions = is_debug?

llvm/utils/gn/build/mac_sdk.gni

5

Make it an argument so it can be overriden if needed? Also maybe call it mac_sdk_path so it's obvious that it's a path.

llvm/utils/gn/build/toolchain/BUILD.gn

4

This should be moved to a separate .gni file, see below.

13

I'd prefer moving these into a template and then define the concrete instance using those templates like most GN builds do. I think it makes it easier to define different toolchains, e.g. for clang and gcc or clang-cl and cl.

Also "posix" is kind of a strange name, Fuchsia also uses the GCC style frontend but we're not POSIX :P I'd slightly prefer using the name Chromium convention to name these after the flag style, i.e. gcc_toolchain or cl_toolchain.

48

We should use response files here to pass the list of inputs.

50

Why not use ar on macOS as well?

52

Can we make ar variable as well? I'd like to change this, e.g. for Clang toolchains to use llvm-ar.

54

nit: I'd just use AR as the name.

56

Can you also set default_output_extension = ".a" and use {{target_output_name}}{{output_extension}} here?

62

Ditto here, use response files.

66

The -Wl,-z,defs should be handled through compiler config, not here.

You also want {{solibs}} beside {{libs}} here.

74

Why do you prefer this over specifying the library with a full path?

77

nit: I'd combine this branch with the one above.

104

Response file here as well.

llvm/utils/gn/build/toolchain/compiler.gni

4

Can we split this into a separate file, e.g. goma.gni? This is a one we use in Fuchsia: https://fuchsia.googlesource.com/build/+/master/toolchain/goma.gni

llvm/utils/gn/tree/llvm/lib/Demangle/BUILD.gn

3

Setting output_name is generally frowned upon as an anti-pattern that should be only used when absolutely necessary, I'd prefer to just set the name of the target to LLVMDemangle.

[thakis]

address comments

[thakis]

Thanks for the thorough review! I replied "not needed yet" to several items, I hope that's fine.

I added a few comments to the .gn file, and I realized that I can put the root BUILD.gn file in gn/secondary and then I don't need to set root, so I did that.

Shouldn't all the GN files have a copyright header?

Maybe! But all the cmake files don't have them either, so I'm not sure.

llvm/utils/gn/.gn

10

Done. I think guessing the meaning of a directory called "secondary" is super hard if you don't know about this one line of gn code here, and "tree" is much more self-explanatory, but /shrug.

12

I agree it'd be a good place if there was a contrib dir, but there isn't, so utils is a better place imo. (My reading of the thread is that someone said that some projects have a contrib dir and that would be a good place if it existed, and someone else saying that that might be a good place for a few other things too, but nobody really suggested putting this in a hypothetical contrib dir, since it doesn't exist yet.)

Anyhoo, this is a bit of a bikeshed and easy to change later if someone feels we need contrib in addition to tools and utils. For now, this location is as good as most others imho, and it's also one dir suggested in that thread :-)

llvm/utils/gn/build/BUILD.gn

7

Ack. I'm happy with folks tweaking this to their heart's content later on.

llvm/utils/gn/build/buildflags.gni

10

It's at least intentional; from README.rst: "By default, you get a release build with assertions enabled that targets the host arch". That's imho the best config for day-to-day development, which is what we're targeting here. I don't feel strongly about this, but it's imho the best default.

llvm/utils/gn/build/mac_sdk.gni

5

We can make it an argument once someone needs to override it. Until then, YAGNI :-)

But I did rename it, that's a better name.

llvm/utils/gn/build/toolchain/BUILD.gn

4

Replied below.

13

I expect that this will be necessary once we do cross builds, bootstrap builds, etc. Until then, I'd like to keep this as-is if that's alright – then blame history will show where the template came from and why it's there.

Re "posix": We used to have LLVM_ON_WIN32 and LLVM_ON_POSIX (we still have the latter), so these names were consistent with that. I don't care much about the name, and it's easy to change (referenced in 2 places I think).

48

LLVM is small enough that they aren't needed (at least for the parts I've hooked up), and if you can get away without them that's preferable: You can just copy failing link commands around without having to pass -d keeprsp to ninja, and it's easier to see the full link command.

50

Last I checked (a few years ago), libtool -static was considerably faster than ar.

I just re-checked and it's still a tiny bit faster. It's _much_ faster if I don't rm the file first, which points out that the ar branch below doesn't rm first -- so thanks for the comment, the other branch had a bug :-) Fixed!

52

Sure, but let's do that when we need it?

54

Sure, done.

56

I haven't found a place where I had to override this yet.

62

Same answer, don't need it (yet?) and more convenient to not use them when not needed.

66

I consider -Wl,-z,defs not being the default a bug :-) Unless something wants to disable that, it can stay here.

{{solibs}} isn't here because I haven't implemented the llvm-as-lots-of-shared-objects, and so nothing needs that atm.

74

It's how I write my commands by hand (cc foo.cc -lbar), and it's what chromium uses. No strong preference though.

104

(same answer)

llvm/utils/gn/build/toolchain/compiler.gni

4

If you use goma, you need to use some compiler locally that's available remotely, i.e. you'll have to set clang_base_path. So it's not clear to me if that should be in goma.gni or here. If it's in goma.gni, then I'd have to include that here for the is_clang default value below. If it's here, then I'd have to put an awkward "see clang_base_path in that other file" comment in goma.gni. So I figured one file is simpler, especially since it's just a single arg.

llvm/utils/gn/tree/llvm/lib/Demangle/BUILD.gn

3

I don't frown upon it :-)

The options here are:

1. Set the target name to LLVMDemangle. This means we'd have to refer to everything as "llvm/lib/foo:LLVMFoo" instead of "llvm/lib/foo", which is annoying.

2. Don't set output_name and have output files different from the cmake build. IIRC, llvm-config returns the names of these files and something tests for that, so that's kind of annoying too.

3. As-is, which has no downside other than apparently someone frowning upon it.

[phosek]

llvm/utils/gn/build/toolchain/BUILD.gn

52

Why not use llvm-ar if the clang_base_path is set? I don't think there's any reason not to?

170

Can we use lld-link if clang_base_path is set?

llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn

2 ↗

(On Diff #173848)

Setting output_name explicitly is generally considered an anti-pattern and should be used only when absolutely necessary. I think we should just "LLVMDemangle" as the target name.

[thakis]

address comments

llvm/utils/gn/build/toolchain/BUILD.gn

52

My use case for clang_base_path is using goma with chromium's clang package, which doesn't include llvm-ar.

170

Sure, why not :-)

llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn

2 ↗

(On Diff #173848)

I had replied to this at https://reviews.llvm.org/D54345?id=173428#inline-480457

[thakis]

llvm/utils/gn/build/toolchain/BUILD.gn

13

Turns out this is a lie, it's LLVM_ON_UNIX. I'll rename this to "unix".

[thakis]

rename toolchain to "unix"

[phosek]

llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn

2 ↗

(On Diff #173848)

I missed that reply (Phabricator no longer shows that file because it was moved).

There are two downsides to manually specifying output_name:

1. You have to manually type it in each target which is also kind of annoying.
2. It's difficult to derive the output name from the target name which may be needed when dealing with packaging, copy rules, etc. (this is potentially YAGNI, but it's something we ran into Fuchsia on multiple occasions).

[thakis]

llvm/utils/gn/secondary/llvm/lib/Demangle/BUILD.gn

2 ↗

(On Diff #173848)

I volunteer to change things around if we should need it for packaging or something at some point. For now, I'd prefer to keep this as-is since all the build files I've written currently use this pattern and there isn't really a reason for changing it at this point.

[phosek]

LGTM

llvm/utils/gn/.gn

+# FIXME: Once it's possible to add files to the root directory of the
+# monorepo, move this file to there.
+# Until then, you manually need to create a symlink in the monorepo root to this
+# file.
+
+buildconfig = "//llvm/utils/gn/build/BUILDCONFIG.gn"
+
+secondary_source = "//llvm/utils/gn/tree/"

llvm/utils/gn/BUILD.gn

+# FIXME: Once it's possible to add files to the root directory of the
+# monorepo, move this file to there.
+# Until then, you manually need to create a symlink in the monorepo root to this
+# file.
+
+group("default") {
+  deps = [
+    "//llvm/lib/Demangle",
+  ]
+}
+
+# A pool called "console" in the root BUILD.gn is magic and represents ninja's
+# built-in console pool. (Requires a GN with `gn --version` >= 552353.)
+pool("console") {
+  depth = 1
+}

llvm/utils/gn/README.rst

+=====================
+Building LLVM with GN
+=====================
+
+.. contents::
+   :local:
+
+.. _Introduction:
+
+Introduction
+============
+
+*Warning* The GN build is experimental and best-effort. It might not work,
+and if you use it you're expected to feel comfortable to unbreak it if
+necessary. LLVM's official build system is CMake, if in doubt use that.
+If you add files, you're expected to update the CMake build but you don't need
+to update GN build files. Reviewers should not ask authors to update GN build
+files. Keeping the GN build files up-to-date is on the people who use the GN
+build.
+
+*Another Warning* Right now, we're in the process of getting the GN build
+checked in. As of this writing, it's not yet functional at all. Check back
+in a week!
+
+`GN <https://gn.googlesource.com/gn/>`_ is another metabuild system. It always
+creates ninja files, but it can create some IDE projects (MSVC, Xcode, ...)
+which then shell out to ninja for the actual build.
+
+Its main features are that GN is very fast (it currently produces ninja files
+for LLVM's build in 35ms on the author's laptop, compared to 66s for CMake) --
+a 2000x difference), and since it's so fast it doesn't aggressively cache,
+making it possible to switch e.g. between release and debug builds in one build
+directory.
+
+It is arguable easier to configure than the CMake build, and has native support
+for building with multiple toolchains in one build directory. The build
+description is declarative-ish, allowing GN to print it in a json format that
+can fairly easily be converted to other metabuild system inputs.
+
+The main motivation behind the GN build is that some people find it more
+convenient for day-to-day hacking on LLVM than CMake. Distribution, building
+just parts of LLVM, and embedding the LLVM GN build from other builds are a
+non-goal for the GN build.
+
+This is a `good overview of GN <https://docs.google.com/presentation/d/15Zwb53JcncHfEwHpnG_PoIbbzQ3GQi_cpujYwbpcbZo/edit#slide=id.g119d702868_0_12>`_.
+
+.. _Quick start:
+
+Quick start
+===========
+
+*Warning* Right now, we're in the process of getting the GN build checked in.
+As of this writing, it's not yet functional at all.
+
+GN only works in the monorepo layout.
+
+#. Obtain a `gn binary <https://gn.googlesource.com/gn/#getting-started>`_.
+
+#. Set up symlinks to create the two files in the root of the monorepo:
+   `ln -s llvm/utils/gn/{BUILD,}.gn .`
+
+#. In the root of the monorepo, run `gn gen out/gn` (or any other directory)
+
+#. Run e.g. `ninja -C out/gn check-lld` to build all prerequisites for and
+   run the LLD tests.
+
+By default, you get a release build with assertions enabled that targets
+the host arch. You can set various build options by editing `out/gn/args.gn`,
+for example putting `is_debug = true` in there gives you a debug build. Run
+`gn args --list out/gn` to see a list of all possible options. After touching
+`out/gn/args.gn`, just run ninja, it will re-invoke gn before starting the
+build.
+
+GN has extensive built-in help; try e.g. `gn help gen` to see the help
+for the `gen` command. The full GN reference is also `available online
+<https://gn.googlesource.com/gn/+/master/docs/reference.md>`_.
+
+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`.
+
+.. _Philosophy:
+
+Philosophy
+==========
+
+GN believes in using gn arguments to configure the build explicitly, instead
+of implicitly figuring out what to do based on what's available on the current
+system.
+
+configure is used for three classes of feature checks:
+
+- compiler checks. In gn, these could use exec_script to identify the host
+  compiler at gn time. For now the build has explicit toggles for compiler
+  features. (Maybe there could be a script that writes args.gn based on the
+  host compiler).  It's possible we'll use exec_script() for this going forward,
+  but we'd have one exec_script call to identify compiler id and version,
+  and then base gn arg default values of compiler id and version instead of
+  doing one exec_script per feature check.
+  (In theory, the config approach means a new os / compiler just needs to tweak
+  the checks and not the code, but in practice a) new os's / compilers are rare
+  b) they will require code changes anyhow, so the configure tradeoff seems
+  not worth it.)
+
+- library checks. For e.g. like zlib, GN thinks it's better to say "we require
+  zlib, else we error at build time" than silently omitting features. People
+  who really don't want to install zlib can explicitly set the gn arg to turn
+  off zlib.
+
+- header checks (does system header X exist). These are generally not needed
+  (just keying this off the host OS works fine), but if they should become
+  necessary in the future, they should be done at build time and the few
+  targets that need to know if header X exists then depend on that build-time
+  check while everything else can build parallel with it.
+
+- LLVM-specific build toggles (assertions on/off, debug on/off, targets to
+  build, ...). These map cleanly to GN args (which then get copied into
+  config.h in a build step).
+
+For the last two points, it would be nice if LLVM didn't have a single
+`config.h` header, but one header per toggle. That way, when e.g.
+`llvm_enable_terminfo` is toggled, only the 3 files caring about that setting
+would need to be rebuilt, instead of everything including `config.h`.
+
+GN doesn't believe in users setting arbitrary cflags from an environment
+variable, it wants the build to be controlled by .gn files.

llvm/utils/gn/build/BUILD.gn

+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.
+  if (host_os != "win") {
+    defines = [ "LLVM_ON_UNIX" ]
+  }
+
+  if (!llvm_enable_assertions) {
+    defines += [ "NDEBUG" ]
+  }
+
+  cflags = []
+
+  if (host_os == "mac" && clang_base_path != "") {
+    cflags += [
+      "-isysroot",
+      mac_sdk,
+    ]
+  }
+
+  if (host_os != "win") {
+    if (is_debug) {
+      cflags += [ "-g" ]
+    } else {
+      cflags += [ "-O3" ]
+    }
+    cflags += [ "-fdiagnostics-color" ]
+    cflags_cc = [
+      "-std=c++11",
+      "-fno-exceptions",
+      "-fno-rtti",
+      "-fvisibility-inlines-hidden",
+    ]
+  } else {
+    if (is_debug) {
+      cflags += [ "/Zi" ]
+    } else {
+      cflags += [
+        "/O2",
+        "/Zc:inline",
+      ]
+    }
+    defines += [
+      "_CRT_SECURE_NO_DEPRECATE",
+      "_CRT_SECURE_NO_WARNINGS",
+      "_CRT_NONSTDC_NO_DEPRECATE",
+      "_CRT_NONSTDC_NO_WARNINGS",
+      "_SCL_SECURE_NO_DEPRECATE",
+      "_SCL_SECURE_NO_WARNINGS",
+
+      "_HAS_EXCEPTIONS=0",
+      "_UNICODE",
+      "UNICODE",
+    ]
+    cflags += [
+      "/EHs-c-",
+      "/GR-",
+    ]
+
+    # The MSVC default value (1 MB) is not enough for parsing recursive C++
+    # templates in Clang.
+    ldflags = [ "/STACK:10000000" ]
+  }
+
+  # Warning setup.
+  if (host_os == "win" && !is_clang) {
+    cflags += [
+      # Suppress ''modifier' : used more than once' (__forceinline and inline).
+      "-wd4141",
+
+      # Suppress 'conversion from 'type1' to 'type2', possible loss of data'.
+      "-wd4244",
+
+      # Suppress 'conversion from 'size_t' to 'type', possible loss of data'.
+      "-wd4267",
+
+      # Suppress 'no matching operator delete found'.
+      "-wd4291",
+
+      # Suppress 'noexcept used with no exception handling mode specified'.
+      "-wd4577",
+
+      # Suppress 'destructor was implicitly defined as deleted'.
+      "-wd4624",
+
+      # Suppress 'unsafe mix of type <type> and type <type> in operation'.
+      "-wd4805",
+    ]
+  } else {
+    if (host_os == "win") {
+      cflags += [ "/W4" ]
+    } else {
+      cflags += [
+        "-Wall",
+        "-Wextra",
+      ]
+    }
+    cflags += [
+      "-Wno-unused-parameter",
+      "-Wstring-conversion",
+    ]
+    if (is_clang) {
+      cflags += [ "-Wdelete-non-virtual-dtor" ]
+    }
+    if (is_clang && use_goma) {
+      # goma converts all paths to lowercase on the server, breaking this
+      # warning.
+      cflags += [ "-Wno-nonportable-include-path" ]
+    }
+  }
+}
+
+config("llvm_code") {
+  include_dirs = [
+    "//llvm/include",
+    "$root_gen_dir/llvm/include",
+  ]
+}
+
+config("lld_code") {
+  include_dirs = [
+    "//lld/include",
+    "$root_gen_dir/lld/include",
+  ]
+}
+
+config("clang_code") {
+  include_dirs = [
+    "//clang/include",
+    "$root_gen_dir/clang/include",
+  ]
+}
+
+config("warn_covered_switch_default") {
+  if (is_clang) {
+    cflags = [ "-Wcovered-switch-default" ]
+  }
+}
+
+config("pthread_config") {
+  visibility = [ ":pthread" ]
+  libs = [ "pthread" ]
+}
+
+group("pthread") {
+  if (llvm_enable_threads && host_os != "win") {
+    public_configs = [ ":pthread_config" ]
+  }
+}

llvm/utils/gn/build/BUILDCONFIG.gn

+# All targets will get this list of configs by default.
+# Targets can opt out of a config by removing it from their local configs list.
+# If you're adding global flags and don't need targets to be able to opt out,
+# add the flags to compiler_defaults, not to a new config.
+_shared_binary_target_configs = [
+  "//llvm/utils/gn/build:compiler_defaults",
+  "//llvm/utils/gn/build:llvm_code",
+  "//llvm/utils/gn/build:warn_covered_switch_default",
+]
+
+# Apply that default list to the binary target types.
+set_defaults("executable") {
+  configs = _shared_binary_target_configs
+}
+set_defaults("loadable_module") {
+  configs = _shared_binary_target_configs
+}
+set_defaults("static_library") {
+  configs = _shared_binary_target_configs
+}
+set_defaults("shared_library") {
+  configs = _shared_binary_target_configs
+}
+set_defaults("source_set") {
+  configs = _shared_binary_target_configs
+}
+
+if (host_os == "win") {
+  set_default_toolchain("//llvm/utils/gn/build/toolchain:win")
+} else {
+  set_default_toolchain("//llvm/utils/gn/build/toolchain:posix")
+}

llvm/utils/gn/build/buildflags.gni

+declare_args() {
+  # Whether to build with debug information and without optimizations.
+  is_debug = false
+}
+
+# args that depend on other args must live in a later declare_args() block.
+declare_args() {
+  # Whether to enable assertions.
+  llvm_enable_assertions = true
+}

llvm/utils/gn/build/enable_threads.gni

+declare_args() {
+  # Whether to enable threading.
+  llvm_enable_threads = true
+}

llvm/utils/gn/build/mac_sdk.gni

+# Location of the mac sdk.
+# If that's not fixed, might want to shell out to xcrun at gn time to
+# retrieve this, but for now this seems to do the trick.
+mac_sdk = "/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk"

llvm/utils/gn/build/toolchain/BUILD.gn

+import("//llvm/utils/gn/build/toolchain/compiler.gni")
+
+declare_args() {
+  # If is_goma is true, the location of the goma client install.
+  if (host_os == "win") {
+    goma_dir = "c:\src\goma\goma-win64"
+  } else {
+    goma_dir = getenv("HOME") + "/goma"
+  }
+}
+
+toolchain("posix") {
+  cc = "cc"
+  cxx = "c++"
+
+  if (clang_base_path != "") {
+    cc = "$clang_base_path/bin/clang"
+    cxx = "$clang_base_path/bin/clang++"
+  }
+
+  ld = cxx  # Don't use goma wrapper for linking.
+  if (use_goma) {
+    cc = "$goma_dir/gomacc $cc"
+    cxx = "$goma_dir/gomacc $cxx"
+  }
+
+  tool("cc") {
+    depfile = "{{output}}.d"
+    command = "$cc -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
+    depsformat = "gcc"
+    description = "CC {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+    ]
+  }
+
+  tool("cxx") {
+    depfile = "{{output}}.d"
+    command = "$cxx -MMD -MF $depfile -o {{output}} -c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
+    depsformat = "gcc"
+    description = "CXX {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.o",
+    ]
+  }
+
+  tool("alink") {
+    if (host_os == "mac") {
+      command = "libtool -static -no_warning_for_no_symbols {{arflags}} -o {{output}} {{inputs}}"
+    } else {
+      command = "ar rcsDT {{arflags}} -o {{output}} {{inputs}}"
+    }
+    description = "LIBTOOL-STATIC {{output}}"
+    outputs = [
+      "{{output_dir}}/{{target_output_name}}.a",
+    ]
+    output_prefix = "lib"
+    default_output_dir = "{{root_out_dir}}/lib"
+  }
+
+  tool("solink") {
+    outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    if (host_os == "linux") {
+      command =
+          "$ld -shared {{ldflags}} -Wl,-z,defs -o $outfile {{libs}} {{inputs}}"
+    } else {
+      command = "$ld -shared {{ldflags}} -o $outfile {{libs}} {{inputs}}"
+    }
+    description = "SOLINK $outfile"
+    outputs = [
+      outfile,
+    ]
+    lib_switch = "-l"
+    output_prefix = "lib"
+    default_output_dir = "{{root_out_dir}}/lib"
+    if (host_os == "mac") {
+      default_output_extension = ".dylib"
+    } else {
+      default_output_extension = ".so"
+    }
+  }
+
+  tool("solink_module") {
+    outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    if (host_os == "linux") {
+      command = "$ld -shared {{ldflags}} -o $outfile {{libs}} {{inputs}}"
+    } else {
+      command = "$ld -shared {{ldflags}} -Wl,-flat_namespace -Wl,-undefined,suppress -o $outfile {{libs}} {{inputs}}"
+    }
+    description = "SOLINK $outfile"
+    outputs = [
+      outfile,
+    ]
+    lib_switch = "-l"
+    default_output_dir = "{{root_out_dir}}/lib"
+    if (host_os == "mac") {
+      default_output_extension = ".dylib"
+    } else {
+      default_output_extension = ".so"
+    }
+  }
+
+  tool("link") {
+    outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    if (host_os == "mac") {
+      command = "$ld {{ldflags}} -o $outfile {{libs}} {{inputs}}"
+    } else {
+      command = "$ld {{ldflags}} -o $outfile {{libs}} -Wl,--start-group {{inputs}} -Wl,--end-group"
+    }
+    description = "LINK $outfile"
+    outputs = [
+      outfile,
+    ]
+    lib_switch = "-l"
+
+    # Setting this allows targets to override the default executable output by
+    # setting output_dir.
+    default_output_dir = "{{root_out_dir}}/bin"
+  }
+
+  tool("copy") {
+    command = "ln -f {{source}} {{output}} 2>/dev/null || (rm -rf {{output}} && cp -af {{source}} {{output}})"
+    description = "COPY {{source}} {{output}}"
+  }
+
+  tool("stamp") {
+    command = "touch {{output}}"
+    description = "STAMP {{output}}"
+  }
+}
+
+toolchain("win") {
+  cl = "cl"
+
+  if (clang_base_path != "") {
+    cl = "$clang_base_path/bin/clang-cl"
+  }
+
+  if (use_goma) {
+    cl = "$goma_dir/gomacc $cl"
+  }
+
+  tool("cc") {
+    command = "$cl /nologo /showIncludes /Fo{{output}} /c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_c}}"
+    depsformat = "msvc"
+    description = "CC {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
+    ]
+  }
+
+  tool("cxx") {
+    command = "$cl /nologo /showIncludes /Fo{{output}} /c {{source}} {{defines}} {{include_dirs}} {{cflags}} {{cflags_cc}}"
+    depsformat = "msvc"
+    description = "CXX {{output}}"
+    outputs = [
+      "{{source_out_dir}}/{{target_output_name}}.{{source_name_part}}.obj",
+    ]
+  }
+
+  tool("alink") {
+    command = "lib /nologo {{arflags}} /out:{{output}} {{inputs}}"
+    description = "LIB {{output}}"
+    outputs = [
+      "{{output_dir}}/{{target_output_name}}.lib",
+    ]
+    default_output_dir = "{{root_out_dir}}/lib"
+  }
+
+  tool("solink") {
+    dllfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    libfile = "$dllfile.lib"
+    command = "link /nologo /dll {{ldflags}} /out:$dllfile /implib:$libfile {{libs}} /pdb:$dllfile.pdb {{inputs}}"
+    description = "LINK $dllfile"
+    link_output = libfile
+    depend_output = libfile
+    runtime_outputs = [ dllfile ]
+    outputs = [
+      dllfile,
+      libfile,
+    ]
+    default_output_extension = ".dll"
+    restat = true
+
+    # Put dlls next to the executables in bin/ on Windows, since Windows
+    # doesn't have a configurable rpath. This matches initialization of
+    # module_dir to bin/ in AddLLVM.cmake's set_output_directory().
+    default_output_dir = "{{root_out_dir}}/bin"
+  }
+
+  # Plugins for opt and clang and so on don't work in LLVM's Windows build
+  # since the code doesn't have export annotations, but there are a few
+  # standalone loadable modules used for unit-testing LLVM's dynamic library
+  # loading code.
+  tool("solink_module") {
+    dllfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    command = "link /nologo /dll {{ldflags}} /out:$dllfile {{libs}} /pdb:$dllfile.pdb {{inputs}}"
+    description = "LINK_MODULE $dllfile"
+    outputs = [
+      dllfile,
+    ]
+    runtime_outputs = outputs
+    default_output_extension = ".dll"
+
+    # No default_output_dir, all clients set output_dir.
+  }
+
+  tool("link") {
+    outfile = "{{output_dir}}/{{target_output_name}}{{output_extension}}"
+    command = "link /nologo {{ldflags}} /out:$outfile {{libs}} /pdb:$outfile.pdb {{inputs}}"
+    description = "LINK $outfile"
+    outputs = [
+      outfile,
+    ]
+    default_output_extension = ".exe"
+
+    # Setting this allows targets to override the default executable output by
+    # setting output_dir.
+    default_output_dir = "{{root_out_dir}}/bin"
+  }
+
+  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"
+    description = "COPY {{source}} {{output}}"
+  }
+
+  tool("stamp") {
+    command = "cmd /c type nul > {{output}}"
+    description = "STAMP {{output}}"
+  }
+}

llvm/utils/gn/build/toolchain/compiler.gni

+declare_args() {
+  # Whether to use goma (https://chromium.googlesource.com/infra/goma/client/)
+  use_goma = false
+
+  # Set this to a clang build directory. If set, that clang is used as compiler.
+  # goma only works with compiler binaries it knows about, so useful both for
+  # using a goma-approved compiler and for compiling clang with a locally-built
+  # clang in a different build directory.
+  clang_base_path = ""
+}
+
+declare_args() {
+  # Set if the host compiler is clang.  On by default on Mac or if
+  # clang_base_path is set.
+  is_clang = host_os == "mac" || clang_base_path != ""
+}

llvm/utils/gn/tree/llvm/lib/Demangle/BUILD.gn

+static_library("Demangle") {
+  output_name = "LLVMDemangle"
+
+  sources = [
+    "ItaniumDemangle.cpp",
+    "MicrosoftDemangle.cpp",
+    "MicrosoftDemangleNodes.cpp",
+  ]
+}