Port use_lld flag from gyp to gn.

build/toolchain/win/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/win/visual_studio_version.gni")
 import("//build/toolchain/goma.gni")
 import("//build/toolchain/toolchain.gni")
 
 # Should only be running on Windows.
 assert(is_win)
 
+declare_args() {
+  # Set to true to use lld, the LLVM linker.
+  use_lld = false
+}
+
 # Setup the Visual Studio state.
 #
 # Its arguments are the VS path and the compiler wrapper tool. It will write
 # "environment.x86" and "environment.x64" to the build directory and return a
 # list to us.
 gyp_win_tool_path =
     rebase_path("//tools/gyp/pylib/gyp/win_tool.py", root_build_dir)
 
 if (use_goma) {
   goma_prefix = "$goma_dir/gomacc.exe "
@@ skipping 27 matching lines @@
 #  environment: File name of environment file.
 template("msvc_toolchain") {
   if (defined(invoker.concurrent_links)) {
     concurrent_links = invoker.concurrent_links
   }
 
   env = invoker.environment
 
   cl = invoker.cl
 
+  if (use_lld) {
+    if (host_os == "win") {
+      lld_link = "lld-link.exe"
+    } else {
+      lld_link = "lld-link"
+    }
+    prefix = rebase_path("//third_party/llvm-build/Release+Asserts/bin",
+                         root_build_dir)
+
+    # lld-link includes a replacement for lib.exe that can produce thin
+    # archives and understands bitcode (for lto builds).
+    lib = "$prefix/$lld_link /lib /llvmlibthin"
+    link = "$prefix/$lld_link"
+  } else {
+    lib = "lib.exe"
+    link = "link.exe"
+  }
+
   toolchain(target_name) {
     # Make these apply to all tools below.
     lib_switch = ""
     lib_dir_switch = "/LIBPATH:"
 
     # Object files go in this directory.
     object_subdir = "{{target_out_dir}}/{{label_name}}"
 
     tool("cc") {
       rspfile = "{{output}}.rsp"
@@ skipping 45 matching lines @@
       }
       command = "$python_path gyp-win-tool asm-wrapper $env $ml {{defines}} {{include_dirs}} {{asmflags}} /c /Fo{{output}} {{source}}"
       description = "ASM {{output}}"
       outputs = [
         "$object_subdir/{{source_name_part}}.obj",
       ]
     }
 
     tool("alink") {
       rspfile = "{{output}}.rsp"
-      command = "$python_path gyp-win-tool link-wrapper $env False lib.exe /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
+      command = "$python_path gyp-win-tool link-wrapper $env False $lib /nologo /ignore:4221 /OUT:{{output}} @$rspfile"
       description = "LIB {{output}}"
       outputs = [
         # Ignore {{output_extension}} and always use .lib, there's no reason to
         # allow targets to override this extension on Windows.
         "{{target_out_dir}}/{{target_output_name}}.lib",
       ]
       default_output_extension = ".lib"
 
       # The use of inputs_newline is to work around a fixed per-line buffer
       # size in the linker.
       rspfile_content = "{{inputs_newline}}"
     }
 
     tool("solink") {
       dllname = "{{root_out_dir}}/{{target_output_name}}{{output_extension}}"  # e.g. foo.dll
       libname = "${dllname}.lib"  # e.g. foo.dll.lib
       rspfile = "${dllname}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /IMPLIB:$libname /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path gyp-win-tool manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:${dllname}.manifest"
       #command = "cmd /c $link_command && $manifest_command"
       command = link_command
 
       default_output_extension = ".dll"
       description = "LINK(DLL) {{output}}"
       outputs = [
         dllname,
@@ skipping 10 matching lines @@
 
       # The use of inputs_newline is to work around a fixed per-line buffer
       # size in the linker.
       rspfile_content = "{{libs}} {{solibs}} {{inputs_newline}} {{ldflags}}"
     }
 
     tool("solink_module") {
       dllname = "{{root_out_dir}}/{{target_output_name}}{{output_extension}}"  # e.g. foo.dll
       rspfile = "${dllname}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /DLL /OUT:$dllname /PDB:${dllname}.pdb @$rspfile"
 
       command = link_command
 
       default_output_extension = ".dll"
       description = "LINK_MODULE(DLL) {{output}}"
       outputs = [
         dllname,
       ]
 
       # The use of inputs_newline is to work around a fixed per-line buffer
       # size in the linker.
       rspfile_content = "{{libs}} {{solibs}} {{inputs_newline}} {{ldflags}}"
     }
 
     tool("link") {
       rspfile = "{{output}}.rsp"
 
-      link_command = "$python_path gyp-win-tool link-wrapper $env False link.exe /nologo /OUT:{{output}} /PDB:{{output}}.pdb @$rspfile"
+      link_command = "$python_path gyp-win-tool link-wrapper $env False $link /nologo /OUT:{{output}} /PDB:{{output}}.pdb @$rspfile"
 
       # TODO(brettw) support manifests
       #manifest_command = "$python_path gyp-win-tool manifest-wrapper $env mt.exe -nologo -manifest $manifests -out:{{output}}.manifest"
       #command = "cmd /c $link_command && $manifest_command"
       command = link_command
 
       default_output_extension = ".exe"
       description = "LINK {{output}}"
       outputs = [
         "{{root_out_dir}}/{{target_output_name}}{{output_extension}}",
@@ skipping 157 matching lines @@
 
   msvc_toolchain("winrt_x64") {
     environment = "environment.winrt_x64"
     cl = "${goma_prefix}\"${vc_bin_dir}/cl.exe\""
     is_clang = false
 
     toolchain_cpu = "x64"
     toolchain_os = current_os
   }
 }