Linker error when enabling Link Time Optimization in NDK

Issue

When I add the flag -flto to my NDK C++ project the linker emits the following error: "Optimization level must be between 0 and 3", even though my optimization level is explicitly set to 3 via -O3.

Does anyone know how to solve this?

The compiler flags are passed via Gradle which, as I understand it, should pass the flags to both the Clang compiler and linker. When I remove the -flto flag everything works fine.

Notes:

• I'm using NDK 19.2 (latest version at the time I write this).

• I also get the warning "clang++.exe: warning: argument unused during compilation: '-Wa,--noexecstack' [-Wunused-command-line-argument]" which I do not have if I compile without link time optimizations.

Solution

Two parts to the answer:

The error is caused by https://github.com/android-ndk/ndk/issues/721. Clang's LTO plugin just doesn't accept -Os or -Oz. This is a bug.

Okay, I might be really stupid, I suppose between 0 and 3 means 1 or 2 :)

It's actually because you can't use the generic cppFlags to set optimization levels. That corresponds to CMAKE_CXX_FLAGS, and you need to set these in CMAKE_CXX_FLAGS_DEBUG and CMAKE_CXX_FLAGS_RELEASE (and/or the C flavors of those). CMake has its own defaults in those variables and the command line is built as ${CMAKE_CXX_FLAGS} ${CMAKE_CXX_FLAGS_RELEASE}, so your -O3 is being overridden by the default.

Answered By - Dan Albert