[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file #1693

rprichard · 2022-03-31T05:39:01Z

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

The text was updated successfully, but these errors were encountered:

rprichard · 2022-04-01T00:54:58Z

@jomof

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

rprichard · 2022-04-01T02:18:06Z

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

DanAlbert · 2022-04-21T21:26:04Z

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

DanAlbert · 2022-04-21T21:27:27Z

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 · 2022-04-22T20:31:15Z

Workaround is in r23 build 8486889.

r24 still needs a cherry-pick.

DanAlbert · 2022-06-02T16:45:42Z

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).

Previously, the NDK used the new toolchain file for CMake 3.21 and up. With the new toolchain file, CMAKE_CXX_FLAGS overrides the set of flags the NDK provides, but we want the flags to be appended instead, which is the legacy toolchain file's behavior. Bug: android/ndk#1693 Test: checkbuild.py && run_tests.py Change-Id: I241bcfbccc0b71a84616d78d68992c3856e42b85 (cherry picked from commit 61b77040e2cf383acef2d17c500a6196a9a35e48) Merged-In: I241bcfbccc0b71a84616d78d68992c3856e42b85

Bug: android/ndk#1693 Test: n/a Change-Id: Ic785a42e414c5976c027b62075d382047620090b (cherry picked from commit 341f8f600c25990ccdf0abfc1fa213a7e017f2e3) Merged-In: Ic785a42e414c5976c027b62075d382047620090b

Android NDK r25 defaults to legacy toolchain file which does not work for xsystem4 build. android/ndk#1693

See NDK issue for detail: android/ndk#1693

Includes a few tweaks needed to build bpftools with a recent NDK: - Libcxx prebuilts are no longer included in sources/cxx-stl [1], so use the sysroot path instead. - NDK now defaults to android-legacy.toolchain.cmake [2], irrespective of CMake version. With the legacy toolchain some env variables we set such as LDFLAGS aren't used, leading to build errors. Force the use of new toolchain file and switch to CXXFLAGS as opposed to CMAKE_CXX_FLAGS to avoid overriding toolchain defaults [3]. Test: make NDK_PATH=/opt/ndk/android-ndk-r25c bpftools-min [1] https://android-review.googlesource.com/c/platform/ndk/+/2051541 [2] https://android-review.googlesource.com/c/platform/ndk/+/2064269 [3] android/ndk#1693

rprichard added the bug label Mar 31, 2022

DanAlbert mentioned this issue Apr 13, 2022

[BUG] new toolchain file needs workaround for CMake AR identification bug #1698

Closed

DanAlbert added this to Triaged in r23c via automation Apr 13, 2022

DanAlbert added this to Triaged in r25 via automation Apr 13, 2022

DanAlbert added this to Triaged in r24b via automation Apr 13, 2022

DanAlbert moved this from Triaged to Needs cherry-pick in r23c Apr 21, 2022

DanAlbert moved this from Triaged to Needs cherry-pick in r24b Apr 21, 2022

DanAlbert moved this from Triaged to Merged in r25 Apr 21, 2022

DanAlbert moved this from Needs cherry-pick to Merged in r23c Apr 22, 2022

DanAlbert mentioned this issue May 31, 2022

M1 CMake #1545

Closed

DanAlbert closed this as completed Jun 2, 2022

DaydreamCoding mentioned this issue Jul 28, 2022

[BUG] NDK-r25 CMAKE_BUILD_TYPE=MinSizeRel(-Os) CMAKE_BUILD_TYPE=Release(null), NDK-r22b get MinSizeRel(-Os) Release(-O2) #1740

Closed

asuka-mio mentioned this issue Aug 9, 2022

determine cmake version Ehviewer-Overhauled/Ehviewer#172

Closed

kichikuou added a commit to kichikuou/xsystem4-android that referenced this issue Oct 8, 2022

CMake: Set ANDROID_USE_LEGACY_TOOLCHAIN_FILE=OFF

1a30905

Android NDK r25 defaults to legacy toolchain file which does not work for xsystem4 build. android/ndk#1693

aa889788 mentioned this issue Feb 4, 2023

The NDK default to include android-legacy.toolchain.cmake microsoft/vcpkg#29438

Closed

dg0yt mentioned this issue Mar 23, 2023

[libpq] build failure [android] microsoft/vcpkg#30327

Closed

jcar87 mentioned this issue Apr 3, 2023

Add docs for tools.android:cmake_legacy_toolchain conan-io/docs#3146

Merged

juan-lunarg mentioned this issue Apr 11, 2023

android: Support ANDROID_USE_LEGACY_TOOLCHAIN_FILE KhronosGroup/Vulkan-ValidationLayers#5622

Merged

juan-lunarg added a commit to KhronosGroup/Vulkan-ValidationLayers that referenced this issue Apr 11, 2023

android: Support ANDROID_USE_LEGACY_TOOLCHAIN_FILE

41af143

See NDK issue for detail: android/ndk#1693

tnovak mentioned this issue May 8, 2023

Add support for NDK r25 facebookexperimental/ExtendedAndroidTools#44

Merged

cortinico mentioned this issue Jun 23, 2023

Build React Native Shared Objects with Stack Smash Protection facebook/react-native#38018

Closed

rocka mentioned this issue Jan 24, 2024

Don't use Android NDK legacy CMake toolchain fcitx5-android/prebuilder#21

Draft

brainos233 mentioned this issue May 16, 2024

Use new toolchain file in android ndk to make compiler flags effective renpy/renpy-build#132

Merged

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[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file #1693

[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file #1693

[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file #1693

[BUG] -DCMAKE_CXX_FLAGS overrides default Android compiler flags with new toolchain file #1693

Comments

Description

Affected versions

Canary version

Host OS

Host OS version

Affected ABIs

Build system

Other build system

minSdkVersion

Device API level