build: Add Windows port of LLVM LTO config.

build/common.gypi

 # Copyright (c) 2012 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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 5986 matching lines @@
         ['CC', '<(make_clang_dir)/bin/clang-cl'],
       ],
     }],
     ['use_lld==1 and OS=="win"', {
       'make_global_settings': [
         # Limited to Windows because lld-link is the driver that is
         # compatible with link.exe.
         ['LD', '<(make_clang_dir)/bin/lld-link'],
       ],
     }],
+    ['clang==1 and use_lto==1 and OS=="win"', {
+      'make_global_settings': [
+        # llvm-lib is a replacement for lib.exe that understands bitcode.

[Nico, 2015/11/11 05:00:00]

we don't bundle this, do we? how big is it?

[pcc1, 2015/11/11 20:22:10]

Hm, since we do apparently bundle lld-link and lld-link supports the /lib flag
(which causes it to act like llvm-lib) we can just do that instead.

And since it turns out we can pass flags in $AR we might as well also pass
/llvmlibthin which causes lld-link to produce thin archives on Windows.

And since that's useful even when not doing LTO, it seems like a good idea to
use it all the time when use_lld==1.

I've done all that in the new patch set.

+        ['AR', '<(make_clang_dir)/bin/llvm-lib'],
+      ],
+    }],
     ['OS=="android" and clang==0', {
       # Hardcode the compiler names in the Makefile so that
       # it won't depend on the environment at make time.
       'make_global_settings': [
         ['CC', '<!(/bin/echo -n <(android_toolchain)/*-gcc)'],
         ['CXX', '<!(/bin/echo -n <(android_toolchain)/*-g++)'],
         ['CC.host', '<(host_cc)'],
         ['CXX.host', '<(host_cxx)'],
       ],
     }],
@@ skipping 63 matching lines @@
     }],
     ['use_lto==1 and clang==1', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
             'arflags': [
               '--plugin', '../../<(make_clang_dir)/lib/LLVMgold.so',
             ],
           }],
         ],
+        'msvs_settings': {
+          'VCCLCompilerTool': {
+            'AdditionalOptions': [
+              '-Xclang',
+              '-emit-llvm-bc',
+            ],
+          },
+        },
       },
     }],
     # Apply a lower LTO optimization level as the default is too slow.
     ['use_lto==1 and clang==1', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
             'ldflags': [
               '-Wl,--plugin-opt,O1',
             ],
           }],
           ['_toolset=="target" and _type!="static_library"', {
             'xcode_settings':  {
               'OTHER_LDFLAGS': [
                 '-Wl,-mllvm,-O1',
               ],
             },
           }],
         ],
+        'msvs_settings': {
+          'VCLinkerTool': {
+            'AdditionalOptions': [
+              '/opt:lldlto=1',
+            ],
+          },
+        },
       },
     }],
     ['use_lto==1 and clang==1 and target_arch=="arm"', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
             # Without this flag, LTO produces a .text section that is larger
             # than the maximum call displacement, preventing the linker from
             # relocating calls (http://llvm.org/PR22999).
             'ldflags': [
@@ skipping 189 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }