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Steps to reproduce:
1) Create a new Android app project with native C++ support
2) Add optimization flag "-Ofast" to the list of cppFlags specified in android.defaultConfig.externalNativeBuild.cmake.cppFlags in the app's build.gradle file.
3) Build your app. The native code will compile with "-O0" for debug builds and "-O2" or "-Os"  (depending on architecture) for release builds instead of the specified "-Ofast". This can be seen by invoking "ninja -v" manually which results in the following compiler invocation:

clang++  --target=aarch64-none-linux-android --gcc-toolchain=/Users/xxxx/Library/Android/sdk/ndk-bundle/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64 --sysroot=/Users/xxxx/Library/Android/sdk/ndk-bundle/sysroot  -Dnative_lib_EXPORTS -isystem /Users/xxxx/Library/Android/sdk/ndk-bundle/sources/cxx-stl/gnu-libstdc++/4.9/include -isystem /Users/xxxxx/Library/Android/sdk/ndk-bundle/sources/cxx-stl/gnu-libstdc++/4.9/libs/arm64-v8a/include -isystem /Users/xxxxx/Library/Android/sdk/ndk-bundle/sources/cxx-stl/gnu-libstdc++/4.9/include/backward -isystem /Users/xxxxx/Library/Android/sdk/ndk-bundle/sysroot/usr/include/aarch64-linux-android -D__ANDROID_API__=23 -g -DANDROID -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -Wa,--noexecstack -Wformat -Werror=format-security  -std=c++11 -Ofast -O2 -DNDEBUG  -fPIC -MD -MT CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o -MF CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o.d -o CMakeFiles/native-lib.dir/src/main/cpp/native-lib.cpp.o -c /Users/fr810/Development/MyApplication/app/src/main/cpp/native-lib.cpp

The compiler is invoked with "-Ofast -O2", i.e. -Ofast is immediately overridden by "-O2".

Gradle Version: 3.3
Android plugin version: 2.3.3
Android Studio Version: 2.3.3
Compile SDK Version: 26
Build Tools Version: 26
NDK version: 15.1.4119039