React-native: [META] Build with Clang (Android)

[dulmandakh]

For Discussion

The purpose of this issue is to create a "single point of truth" regarding the efforts related to building React Native using Clang and against libc++, in order to coordinate and recap all the efforts connected to this subject.

Background

Android NDK is about to drop GCC and GNU STL support in favor of Clang and libc++ in version 18. Below are notable changes in NDK

Version 11

• GCC in the NDK is now deprecated in favor of Clang. We strongly recommend switching to Clang.

Version 13

• GCC is no longer supported. It will not be removed from the NDK just yet, but is no longer receiving backports. It cannot be removed until after libc++ has become stable enough to be the default, as some parts of gnustl are still incompatible with Clang. It will likely be removed after that point.
• NDK_TOOLCHAIN_VERSION now defaults to Clang.

Version 14

• This release ends active support for GCC

Version 16

• libc++ is out of beta and is now the preferred STL in the NDK. Starting in r17, libc++ is the default STL for CMake and standalone toolchains. If you manually selected a different STL, we strongly encourage you to move to libc++. For more details, see this blog post.

Version 17

• GCC is no longer supported. It will be removed in NDK r18.
• libc++ is now the default STL for CMake and standalone toolchains
  gnustl and stlport are deprecated and will be removed in NDK r18.

Documentations

• Clang Migration Notes - https://android.googlesource.com/platform/ndk/+/master/docs/ClangMigration.md
• C++ Library Support - https://developer.android.com/ndk/guides/cpp-support

TODO

• Use CMake to build C++ code
• Upgrade Folly (https://github.com/facebook/react-native/issues/20302)
• Compile RN using Clang
• Compile JSC and RN using Clang against libc++, because gnustl and libc++ ABI are incompatible

How to

Compile with Clang

Open ReactAndroid/src/main/jni/Application.mk and remove or comment out NDK_TOOLCHAIN_VERSION

Compile with against libc++

Open ReactAndroid/src/main/jni/Application.mk and set APP_STL := c++_shared

Related issues

• Roadmap of future Android https://github.com/facebook/react-native/issues/19297
• Upgrading JSC - https://github.com/facebook/react-native/issues/19737

Related PRs

• Bump NDK toolchain to GCC 4.9 and generalize C++ compiler flags (merged https://github.com/facebook/react-native/commit/ccdd450b1284b73bee80a9709c864816cbfc1108) https://github.com/facebook/react-native/pull/19945
• Bump Android NDK version to r17b (merged https://github.com/facebook/react-native/commit/6117a6c7205c969f93d39ba02e0583881572d5fa) https://github.com/facebook/react-native/pull/20357

[dulmandakh]

I was able to compile RN with NDK r17, and will create a PR once https://github.com/facebook/react-native/pull/19945 is merged.

[gengjiawen]

Amazing job，thanks.

[dulmandakh]

If I understood correctly, iOS builds with clang agains libc++ and with almost the same native dependencies. So it won't be a big problem to build with Clang, but to use libc++ we may need to build JSC against it. Please correct me if wrong

[dulmandakh]

I'm not that proficient in C++ and don't know much about RN internals, therefore cannot make changes to C++ code to make it compile with Clang. @hramos could you please find someone from Facebook to help with it.

[dulmandakh]

NDK r17b landed https://github.com/facebook/react-native/commit/6117a6c7205c969f93d39ba02e0583881572d5fa

[dulmandakh]

RN master now use clang