[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file

[rprichard]

Description

The NDK automatically switches from the legacy CMake toolchain file to the new toolchain file if the CMake version is new enough. There are some differences in the way the two modes operate regarding the -DCMAKE_${LANG}_FLAGS variable that may have been unintentional.

With the legacy toolchain file, the CMAKE_${LANG}_FLAGS were appended to a set of default flags. With the new toolchain file, setting CMAKE_${LANG}_FLAGS instead overrides the default flags.

The result is that projects that upgrade to a newer CMake (or which start passing -DCMAKE_${LANG}_FLAGS) lose the default compiler flags.

This situation is made worse with Gradle. The Android Studio template for a new NDK project adds these build.gradle lines:

        externalNativeBuild {
            cmake {
                cppFlags ''
            }
        }

cppFlags is turned into a command-line argument setting CMAKE_CXX_FLAGS. However, when it's empty, the Android Gradle Plugin (AGP) instead omits the -DCMAKE_CXX_FLAGS=... argument. That's fine for the legacy toolchain, but with the new toolchain file, it means that adding a new argument to the cppFlags '' also suppresses many default flags. (With the new toolchain file, passing -DCMAKE_CXX_FLAGS= also overrides the defaults, but AGP isn't passing that.)

This behavioral change broke a project that was passing -std=... in cppFlags to set the C++ dialect. After upgrading CMake, the compiler stopped building with -ffunction-sections and -fdata-sections, which bloated the app's shared object. (For this particular project, -std=... was the only thing in cppFlags, so I think it was fixed by switching to CXX_STANDARD.)

Example:

# Test project
cat >CMakeLists.txt <<EOF
project(hello)
cmake_minimum_required(VERSION 3.18.1)
add_executable(hello hello.cpp)
EOF
cat >hello.cpp <<EOF
int main() {}
EOF

---

Legacy toolchain file (without -DCMAKE_CXX_FLAGS):

$ rm -fr out && cmake -GNinja -Bout \
  -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_TOOLCHAIN_FILE=/x/android-ndk-r24/build/cmake/android.toolchain.cmake \
  -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=ON \
  && ninja -Cout -v
...
/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=armv7-none-linux-androideabi19 \
  --sysroot=/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
  -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 \
  -march=armv7-a -mthumb -Wformat -Werror=format-security \
  -Os -DNDEBUG -fPIE \
  -MD -MT CMakeFiles/hello.dir/hello.cpp.o \
  -MF CMakeFiles/hello.dir/hello.cpp.o.d \
  -o CMakeFiles/hello.dir/hello.cpp.o \
  -c /x/cmaketest/hello.cpp

Legacy toolchain file (with -DCMAKE_CXX_FLAGS):

$ rm -fr out && cmake -GNinja -Bout \
  -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_TOOLCHAIN_FILE=/x/android-ndk-r24/build/cmake/android.toolchain.cmake \
  -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=ON \
  -DCMAKE_CXX_FLAGS=-std=c++17 \
  && ninja -Cout -v
...
/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=armv7-none-linux-androideabi19 \
  --sysroot=/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
  -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 \
  -march=armv7-a -mthumb -Wformat -Werror=format-security \
  -std=c++17 -Os -DNDEBUG -fPIE \
  -MD -MT CMakeFiles/hello.dir/hello.cpp.o \
  -MF CMakeFiles/hello.dir/hello.cpp.o.d \
  -o CMakeFiles/hello.dir/hello.cpp.o \
  -c /x/cmaketest/hello.cpp

These default flags come from ANDROID_COMPILER_FLAGS in $NDKr24/build/cmake/android-legacy.toolchain.cmake:

  -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
  -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 \
  -march=armv7-a -mthumb -Wformat -Werror=format-security`

---

New toolchain file (without -DCMAKE_CXX_FLAGS):

$ rm -fr out && cmake -GNinja -Bout \
  -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_TOOLCHAIN_FILE=/x/android-ndk-r24/build/cmake/android.toolchain.cmake \
  -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=OFF \
  && ninja -Cout -v
...
/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=armv7-none-linux-androideabi19 \
  --sysroot=/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
  -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 \
  -march=armv7-a -mthumb -Wformat -Werror=format-security \
  -fexceptions -frtti -stdlib=libc++ -Os -DNDEBUG -fPIE \
  -MD -MT CMakeFiles/hello.dir/hello.cpp.o \
  -MF CMakeFiles/hello.dir/hello.cpp.o.d \
  -o CMakeFiles/hello.dir/hello.cpp.o \
  -c /x/cmaketest/hello.cpp

New toolchain file (with -DCMAKE_CXX_FLAGS):

$ rm -fr out && cmake -GNinja -Bout \
  -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DCMAKE_TOOLCHAIN_FILE=/x/android-ndk-r24/build/cmake/android.toolchain.cmake \
  -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=OFF \
  -DCMAKE_CXX_FLAGS=-std=c++17 \
  && ninja -Cout -v
...
/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=armv7-none-linux-androideabi19 \
  --sysroot=/x/android-ndk-r24/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -std=c++17 -Os -DNDEBUG -fPIE \
  -MD -MT CMakeFiles/hello.dir/hello.cpp.o \
  -MF CMakeFiles/hello.dir/hello.cpp.o.d \
  -o CMakeFiles/hello.dir/hello.cpp.o \
  -c /x/cmaketest/hello.cpp

Adding -DCMAKE_CXX_FLAGS suppresses these flags:

  -DANDROID -fdata-sections -ffunction-sections -funwind-tables \
  -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 \
  -march=armv7-a -mthumb -Wformat -Werror=format-security \
  -fexceptions -frtti -stdlib=libc++

---

Using the new toolchain file results in explicit -fexceptions and -frtti arguments, which were not passed with the legacy toolchain file. This also means that:

• With the new toolchain file, passing -DCMAKE_CXX_FLAGS=... suppresses the flags produced by -DANDROID_CPP_FEATURES="no-exceptions no-rtti"
• With the legacy toolchain file, the -DCMAKE_CXX_FLAGS=... flags are appended to those resulting from ANDROID_CPP_FEATURES.

Affected versions

r23, r24, r25, Canary

Canary version

n/a

Host OS

Linux, Mac, Windows

Host OS version

any

Affected ABIs

armeabi-v7a, arm64-v8a, x86, x86_64

Build system

CMake

Other build system

No response

minSdkVersion

n/a

Device API level

n/a

[rprichard]

@jomof

@DanAlbert proposed having new versions of AGP pass -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=ON to avoid this behavior change on CMake upgrade.

[rprichard]

I opened https://gitlab.kitware.com/cmake/cmake/-/issues/23378 against upstream CMake.

[DanAlbert]

Not actually in the r25 branch, but we'll do a full update of r25 from master so marking done for that release.

[DanAlbert]

https://android-review.googlesource.com/c/platform/ndk/+/2064269 is the "fix". We're back to the legacy toolchain by default until CMake can support this.

[DanAlbert]

Workaround is in r23 build 8486889.

r24 still needs a cherry-pick.

[DanAlbert]

r24 is not an LTS so it will lose support when r25 ships in early July. Due to a lack of bandwidth in our dependencies, it won't be possible to ship r24b before the end of the support window. Closing since this is already fixed in r25 and r23c.

If you are using r24 and do not want to encounter this bug, do not upgrade to CMake 3.21 or newer (or pass -DANDROID_USE_LEGACY_TOOLCHAIN_FILE=TRUE to CMake).