当WebRTC m84 遇上 Xcode 12.5

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在Mac上编译WebRTC m84时遇到编译错误，原因是Xcode 12.5的工具链不兼容。解决方案包括更改工具链为macos系统工具链，或者升级WebRTC的llvm-build工具链。通过本地编译llvm或下载预编译版本，可以成功解决编译问题。

文章目录

前言

最近，多位同事在 Mac 端编译 Android 平台的 WebRTC（m84版本）的时遇到了编译错误：

FAILED: clang_x64/obj/third_party/ijar/ijar/ijar.o 
../../third_party/llvm-build/Release+Asserts/bin/clang++ -MMD -MF clang_x64/obj/third_party/ijar/ijar/ijar.o.d -D_LIBCPP_HAS_NO_ALIGNED_ALLOCATION -DCR_XCODE_VERSION=1250 -DCR_CLANG_REVISION=\"n353803-99ac9ce7-1\" -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FORTIFY_SOURCE=2 -D__ASSERT_MACROS_DEFINE_VERSIONS_WITHOUT_UNDERSCORES=0 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -I../.. -Iclang_x64/gen -I../../third_party/zlib -fno-strict-aliasing -fstack-protector -fcolor-diagnostics -fmerge-all-constants -fcrash-diagnostics-dir=../../tools/clang/crashreports -Xclang -mllvm -Xclang -instcombine-lower-dbg-declare=0 -arch x86_64 -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -Xclang -fdebug-compilation-dir -Xclang . -no-canonical-prefixes -Wall -Werror -Wextra -Wimplicit-fallthrough -Wunreachable-code -Wthread-safety -Wunguarded-availability -Wno-missing-field-initializers -Wno-unused-parameter -Wno-c++11-narrowing -Wno-unneeded-internal-declaration -Wno-unused-private-field -Wno-unused-variable -Wno-undefined-var-template -Wno-ignored-pragma-optimize -Wno-implicit-int-float-conversion -Wno-final-dtor-non-final-class -Wno-builtin-assume-aligned-alignment -Wno-deprecated-copy -Wno-non-c-typedef-for-linkage -O2 -fno-omit-frame-pointer -gdwarf-4 -g2 -Xclang -debug-info-kind=constructor -isysroot ../../../../../../../../../../../Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk -mmacosx-version-min=10.10.0 -ftrivial-auto-var-init=pattern -fvisibility=hidden -Xclang -add-plugin -Xclang find-bad-constructs -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -std=c++14 -stdlib=libc++ -fno-exceptions -fvisibility-inlines-hidden -c ../../third_party/ijar/ijar.cc -o clang_x64/obj/third_party/ijar/ijar/ijar.o
In file included from ../../third_party/ijar/ijar.cc:21:
../../third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/stdlib.h:142:34: error: unknown type name 'ldiv_t'
inline _LIBCPP_INLINE_VISIBILITY ldiv_t div(long __x, long __y) _NOEXCEPT {
                                 ^
../../third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/stdlib.h:143:12: error: no member named 'ldiv' in the global namespace
  return ::ldiv(__x, __y);
         ~~^
../../third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/stdlib.h:146:34: error: unknown type name 'lldiv_t'
inline _LIBCPP_INLINE_VISIBILITY lldiv_t div(long long __x,

...

../../third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/wchar.h:157:93: error: cannot initialize a parameter of type 'wchar_t *' with an lvalue of type 'const wchar_t *'
wchar_t* __libcpp_wcsstr(const wchar_t* __s1, const wchar_t* __s2) {return (wchar_t*)wcsstr(__s1, __s2);}
                                                                                            ^~~~
../../third_party/llvm-build/Release+Asserts/bin/../include/c++/v1/wchar.h:140:38: note: passing argument to parameter '__s' here
      wchar_t* wcschr(      wchar_t* __s, wchar_t __c) {return __libcpp_wcschr(__s, __c);}
                                     ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
ninja: build stopped: subcommand failed.

WebRTC的Android平台编译需要用 mac 工具链编译 ijar 可执行文件，用于将java文件打包成 jar或将jar库进行拆解。经过对比判断，同事升级了Xcode 12.5(clang-1205.0.22.9)，而我本机仍是 Xcode 12.4(clang-1200.0.32.29)，WebRTC m84的工具链llvm-build(clang 11.0.0)。在同事把头文件目录 third_party/llvm-build/Release+Asserts/bin/…/include 重命名 include_bk后，编译通过，可以基本确定是工具链问题导致，毕竟众所周知，苹果的Xcode升级经常会有各种兼容问题。但该方法经验证，在 Xcode 12.4 环境会编译失败，即该解决方法并不能通用，需要查找新的解决方法。
所以，便有了以下两个预案：

1. 弃用 llvm-build 工具链，改用 macos 系统的工具链;
1. 升级 WebRTC 的llvm-build工具链。

接下来，我们一一尝试。

1 更改工具链

经过对编译脚本的分析，修改了以下几个文件，将 WebRTC m84的工具链从 llvm-build 切换到系统的 clang 工具链:

diff --git a/build/config/clang/BUILD.gn b/build/config/clang/BUILD.gn
old mode 100644
new mode 100755
index 960726e1e5..30df135499
--- a/build/config/clang/BUILD.gn
+++ b/build/config/clang/BUILD.gn
@@ -10,12 +10,12 @@ config("find_bad_constructs") {
 
     # The plugin is built directly into clang, so there's no need to load it
     # dynamically.
-    cflags += [
-      "-Xclang",
-      "-add-plugin",
-      "-Xclang",
-      "find-bad-constructs",
-    ]
+    # cflags += [
+    #   "-Xclang",
+    #   "-add-plugin",
+    #   "-Xclang",
+    #   "find-bad-constructs",
+    # ]
 
     if (is_linux || is_android || is_fuchsia) {

diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
old mode 100644
new mode 100755
index d166b82928..4c2cc7443d
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1114,22 +1114,22 @@ config("compiler_deterministic") {
 
 config("clang_revision") {
   if (is_clang && clang_base_path == default_clang_base_path) {
-    update_args = [
-      "--print-revision",
-      "--verify-version=$clang_version",
-    ]
-    if (llvm_force_head_revision) {
-      update_args += [ "--llvm-force-head-revision" ]
-    }
-    clang_revision = exec_script("//tools/clang/scripts/update.py",
-                                 update_args,
-                                 "trim string")
+    # update_args = [
+    #   "--print-revision",
+    #   "--verify-version=$clang_version",
+    # ]
+    # if (llvm_force_head_revision) {
+    #   update_args += [ "--llvm-force-head-revision" ]
+    # }
+    # clang_revision = exec_script("//tools/clang/scripts/update.py",
+    #                              update_args,
+    #                              "trim string")
 
     # This is here so that all files get recompiled after a clang roll and
     # when turning clang on or off. (defines are passed via the command line,
     # and build system rebuild things when their commandline changes). Nothing
     # should ever read this define.
-    defines = [ "CR_CLANG_REVISION=\"$clang_revision\"" ]
+    # defines = [ "CR_CLANG_REVISION=\"$clang_revision\"" ]
   }
 }

 @@ -1441,6 +1441,7 @@ config("default_warnings") {
     cflags += [
       # TODO(thakis): Consider -Wloop-analysis (turns on
       # -Wrange-loop-analysis too).
+      "-Wno-range-loop-analysis",
 
       # This warns on using ints as initializers for floats in
       # initializer lists (e.g. |int a = f(); CGSize s = { a, a };|),
@@ -1456,6 +1457,7 @@ config("default_warnings") {
       # add by vinton.liu on 20210226
       "-Wno-unused-private-field",
       "-Wno-unused-variable",
+      "-Wno-unknown-warning-option",
     ]
 
     # use_xcode_clang only refers to the iOS toolchain, host binaries use
@@ -2287,10 +2289,10 @@ config("symbols") {
     }
 
     if (is_clang && !is_nacl && !use_xcode_clang) {
-      cflags += [
-        "-Xclang",
-        "-debug-info-kind=constructor",
-      ]
+      # cflags += [
+      #   "-Xclang",
+      #   "-debug-info-kind=constructor",
+      # ]
     }
 
     if (use_debug_fission && !is_nacl && !is_android) {

diff --git a/build/toolchain/mac/BUILD.gn b/build/toolchain/mac/BUILD.gn
old mode 100644
new mode 100755
index add57c71bb..46c82968bf
--- a/build/toolchain/mac/BUILD.gn
+++ b/build/toolchain/mac/BUILD.gn
@@ -92,6 +92,7 @@ template("mac_toolchain") {
       prefix = rebase_path("$clang_base_path/bin/", root_build_dir)
     }
 
+    prefix = ""
     _cc = "${prefix}clang"
     _cxx = "${prefix}clang++"

经修改后编译验证，android 及 mac 平台代码都可以正常编译通过。

2 更新工具链

WebRTC 提供了一个可以自行下载并编译llvm-build的脚本(tools/clang/scripts/build.py)，但这个脚本是会随WebRTC的版本不断更新的，这个脚本所在目录 tools 是在下载WebRTC代码执行 gclient runhooks的时候才下载的，它不在 WebRTC 的源码中，而是在 Chromium 项目内，源码目录在 https://chromium.googlesource.com/chromium/src/+/refs/heads/man/tools，我们将这个路径的源码打包下载解压:

unzip src-refs_heads_main-tools.tar.gz

在此方案中，我们又可以通过两种方式来更新工具链：

本地编译 llvm 工程；
直接下载预编译的 llvm 工具链文件。

2.1 本地编译 llvm

本地下载并编译llvm的方式耗时会比较长，因为要将llvm的代码全部clone到本地，然后再编译。

首先，将 src-refs_heads_main-tools/clang 目录替换到现有 WebRTC 的 tools/clang：

rm -rf src/tools/clang
cp -rf src-refs_heads_main-tools/clang src/tools/

执行 build.py 脚本，下载llvm并编译：

cd src
python tools/clang/scripts/build.py --without-fuchsia --use-system-cmake

编译完成后，可以将llvm-build内多余的临时文件删除，仅保留必要的include,libs和bin内必要的命令即可。

最后，修改 toolchain.gni 内 clang的版本号：

diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 552ceb67e6..06c8958fb9 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -38,7 +38,7 @@ if (generate_linker_map) {
 declare_args() {
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
-    clang_version = "11.0.0"
+    clang_version = "13.0.0"
   }
 }

具体版本号，可能每次都不一样，可以通过命令 clang --version 查看:

cd src/third_party/llvm-build/Release+Asserts/bin
./clang --version

输出版本号类似如下：

clang version 13.0.0 (https://github.com/llvm/llvm-project/ 5dad3d1ba9ad01152be21e94cfbbfb31659ea3e1)
Target: x86_64-apple-darwin20.4.0
Thread model: posix
InstalledDir: <webrtc path>/third_party/llvm-build/Release+Asserts/bin/.

2.2 下载预编译 llvm

预编译的llvm 在google仓库内，所以你懂的。
在此方式中，最主要的是对比新旧 update.py 的下载方法，然后修改到本地 update.py或尝试替换。
在此以当前下载的 tools (2021-05-13) 文件为例。

首先，修改本地 update.py 如下：

diff --git a/tools/clang/scripts/update.py b/tools/clang/scripts/update.py
index 83e0ede51f..970703da61 100755
--- a/tools/clang/scripts/update.py
+++ b/tools/clang/scripts/update.py
@@ -37,13 +37,12 @@ import zipfile
 # Do NOT CHANGE this if you don't know what you're doing -- see
 # https://chromium.googlesource.com/chromium/src/+/master/docs/updating_clang.md
 # Reverting problematic clang rolls is safe, though.
-CLANG_REVISION = '99ac9ce7016d701b43b8f0c308dc3463da57d983'
+CLANG_REVISION = 'llvmorg-13-init-9881-g5dad3d1b'
 CLANG_SVN_REVISION = 'n353803'
 CLANG_SUB_REVISION = 1
 
-PACKAGE_VERSION = '%s-%s-%s' % (CLANG_SVN_REVISION, CLANG_REVISION[:8],
-                                CLANG_SUB_REVISION)
-RELEASE_VERSION = '11.0.0'
+PACKAGE_VERSION = '%s-%s' % (CLANG_REVISION, CLANG_SUB_REVISION)
+RELEASE_VERSION = '13.0.0'
 
 
 CDS_URL = os.environ.get('CDS_CLANG_BUCKET_OVERRIDE',

执行 update.py 脚本，下载相关工具:

python tools/clang/scripts/update.py --host-os=mac
python tools/clang/scripts/update.py --host-os=mac --package=lld_mac
python tools/clang/scripts/update.py --host-os=mac --package=objdump

最后，修改 toolchain.gni 内 clang的版本号：

diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 552ceb67e6..06c8958fb9 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -38,7 +38,7 @@ if (generate_linker_map) {
 declare_args() {
   if (is_clang) {
     # Clang compiler version. Clang files are placed at version-dependent paths.
-    clang_version = "11.0.0"
+    clang_version = "13.0.0"
   }
 }