[RFC] Android runtimes build demonstration

[smeenai]

Summary

This demonstrates a possible Android runtimes build setup once
https://reviews.llvm.org/D158476 lands and runtimes library searching
accounts for versioned Android triples. Meta uses a similar setup
internally (except for using a standalone CMake toolchain file instead
of wrapping the NDK's; I'm not actually sure which one I prefer.)

I'm putting this up as a demonstration of aspects of the runtime build
(in particular delegating to separate toolchain and cache files, which I
find to be much more manageable than putting everything in the runtimes
cache file). If the Android toolchain maintainers think it could be
generally useful, I'd be happy for a maintainer to commandeer it and
change it as they see fit, or make any suggested changes myself.

There's some existing Android cache and toolchain files:

• clang/cmake/caches/Android.cmake
• clang/cmake/caches/Android-stage2.cmake
• llvm/cmake/platforms/Android.cmake

As far as I can tell the toolchain file is pretty bit-rotted and the
cache files don't reflect the current NDK runtimes configuration (e.g.
the use of the __ndk1 inline namespace). I'm not changing them here,
but it'd be great if a maintainer could take a look and decide what to
do with them.

[smeenai]

The manual androideabi normalization is only needed when building the runtimes, not when using them:

$ for i in {21..32}; do printf '%s ' $i; bin/clang --print-file-name=libclang_rt.builtins.a -target armv7-none-linux-androideabi$i; done
21 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
22 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
23 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
24 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
25 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
26 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
27 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
28 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android21/libclang_rt.builtins.a
29 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android29/libclang_rt.builtins.a
30 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android29/libclang_rt.builtins.a
31 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android29/libclang_rt.builtins.a
32 /data/users/smeenai/llvm-project/build/Release/lib/clang/18/lib/armv7-none-linux-android29/libclang_rt.builtins.a

clang/cmake/caches/Android-libcxx.cmake

+# Most of this will be supplanted by https://reviews.llvm.org/D139147.
+set(LIBUNWIND_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBUNWIND_HIDE_SYMBOLS ON CACHE BOOL "")
+
+set(LIBCXXABI_ENABLE_SHARED OFF CACHE BOOL "")
+set(LIBCXXABI_USE_LLVM_UNWINDER ON CACHE BOOL "")
+
+set(LIBCXX_ENABLE_ABI_LINKER_SCRIPT OFF CACHE BOOL "")
+set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")
+set(LIBCXX_STATICALLY_LINK_ABI_IN_SHARED_LIBRARY ON CACHE BOOL "")
+
+set(LIBCXX_SHARED_OUTPUT_NAME c++_shared CACHE STRING "")
+set(LIBCXX_STATIC_OUTPUT_NAME c++_static CACHE STRING "")
+
+set(LIBCXX_ABI_NAMESPACE __ndk1 CACHE STRING "")

clang/cmake/caches/Android-runtimes.cmake

+if(NOT NDK_PATH)
+  message(FATAL_ERROR "NDK_PATH must point to NDK path")
+endif()
+
+set(LLVM_ENABLE_PROJECTS
+  clang
+  lld
+  CACHE STRING "")
+
+set(LLVM_ENABLE_RUNTIMES
+  compiler-rt
+  libunwind
+  libcxxabi
+  libcxx
+  CACHE STRING "")
+
+function(android_runtime arch api_level target)
+  if(arch STREQUAL armv7)
+    set(abi armeabi-v7a)
+  elseif(arch STREQUAL aarch64)
+    set(abi arm64-v8a)
+  elseif(arch STREQUAL i686)
+    set(abi x86)
+  elseif(arch STREQUAL x86_64)
+    set(abi x86_64)
+  else()
+    message(FATAL_ERROR "Invalid arch ${arch}")
+  endif()
+
+  set(toolchain_file "${CMAKE_SOURCE_DIR}/cmake/platforms/Android-runtimes.cmake")
+  foreach(prefix BUILTINS RUNTIMES)
+    set(${prefix}_${target}_ANDROID_ABI ${abi} CACHE STRING "")
+    set(${prefix}_${target}_ANDROID_PLATFORM "android-${api_level}" CACHE STRING "")
+    set(${prefix}_${target}_NDK_PATH "${NDK_PATH}" CACHE PATH "")
+    set(${prefix}_${target}_CMAKE_TOOLCHAIN_FILE "${toolchain_file}" CACHE FILEPATH "")
+  endforeach()
+
+  # This matches the NDK. It's problematic if you have emulated atomics
+  # used across shared libraries, but that should be very rare.
+  set(BUILTINS_${target}_COMPILER_RT_EXCLUDE_ATOMIC_BUILTIN OFF CACHE BOOL "")
+
+  set(RUNTIMES_${target}_CMAKE_CACHES
+    "${CMAKE_CURRENT_LIST_DIR}/Android-libcxx.cmake"
+    CACHE STRING "")
+endfunction()
+
+set(targets)
+foreach(arch armv7 aarch64 i686 x86_64)
+  # 21 is the minimum supported and 29 adds ELF TLS.
+  foreach(api_level 21 29)
+    set(target "${arch}-none-linux-android${api_level}")
+    android_runtime(${arch} ${api_level} ${target})
+    list(APPEND targets ${target})
+  endforeach()
+endforeach()
+
+set(LLVM_BUILTIN_TARGETS ${targets} CACHE STRING "")
+set(LLVM_RUNTIME_TARGETS ${targets} CACHE STRING "")

llvm/cmake/platforms/Android-runtimes.cmake

+# A wrapper around the NDK's CMake toolchain that uses the just-built
+# compiler instead of the NDK's and tweaks flags to work with a
+# partially built toolchain, for the runtimes build.
+
+if(NOT NDK_PATH)
+  message(FATAL_ERROR "NDK_PATH must point to NDK path")
+endif()
+
+# These are passed down from the runtimes build.
+set(ORIGINAL_C_COMPILER "${CMAKE_C_COMPILER}")
+set(ORIGINAL_CXX_COMPILER "${CMAKE_CXX_COMPILER}")
+
+include("${NDK_PATH}/build/cmake/android.toolchain.cmake")
+
+set(CMAKE_C_COMPILER "${ORIGINAL_C_COMPILER}")
+set(CMAKE_CXX_COMPILER "${ORIGINAL_CXX_COMPILER}")
+
+list(APPEND CMAKE_TRY_COMPILE_PLATFORM_VARIABLES NDK_PATH)
+
+# We won't have libunwind available since we're building it. CMake's ABI
+# checks fail without this.
+# FIXME: compiler-rt uses --unwindlib=none already; does that just need
+# to be more comprehensive, or is the "Detecting compiler ABI info"
+# check ingrained enough in CMake that only the toolchain file can
+# influence it?
+string(APPEND CMAKE_EXE_LINKER_FLAGS " --unwindlib=none")
+string(APPEND CMAKE_MODULE_LINKER_FLAGS " --unwindlib=none")
+string(APPEND CMAKE_SHARED_LINKER_FLAGS " --unwindlib=none")
+
+# libatomic is provided by compiler-rt and our just-built compiler
+# doesn't have the NDK's dummy libatomic available.
+string(REPLACE "-latomic" "" CMAKE_C_STANDARD_LIBRARIES_INIT "${CMAKE_C_STANDARD_LIBRARIES_INIT}")
+string(REPLACE "-latomic" "" CMAKE_CXX_STANDARD_LIBRARIES_INIT "${CMAKE_CXX_STANDARD_LIBRARIES_INIT}")
+
+# Fortify doesn't play well with -ffreestanding, which is used heavily
+# by sanitizers, because e.g. it prevents Clang's built-in limits.h from
+# reaching out to the NDK's and supplying SSIZE_MAX.
+# FIXME: should this be in compiler-rt instead?
+string(REPLACE "-D_FORTIFY_SOURCE=2" "" CMAKE_ASM_FLAGS "${CMAKE_ASM_FLAGS}")
+string(REPLACE "-D_FORTIFY_SOURCE=2" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+string(REPLACE "-D_FORTIFY_SOURCE=2" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
+
+# Clang canonicalizes androideabi as just android, so we need to mimic
+# that to get the correct per-target installation directory.
+# FIXME: remove this once https://reviews.llvm.org/D140925 lands
+string(REPLACE "androideabi" "android" CMAKE_ASM_COMPILER_TARGET "${CMAKE_ASM_COMPILER_TARGET}")
+string(REPLACE "androideabi" "android" CMAKE_C_COMPILER_TARGET "${CMAKE_C_COMPILER_TARGET}")
+string(REPLACE "androideabi" "android" CMAKE_CXX_COMPILER_TARGET "${CMAKE_CXX_COMPILER_TARGET}")