What's the use of LLVM in Android NDK Toolchains?

Question

What\'s the use of LLVM in Android NDK Toolchains?

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A little recap:

I was building my native project with Gradlew on Ubuntu, targeting

Answer 1

LLVM is the compiler (backend). The compiler used is Clang, which resides within the llvm directory. (LLVM is the name of the component of Clang that does the actual code generation, aka backend.)

Previously, the NDK used GCC as compiler. With GCC, each target architecture (arm, aarch64, x86 etc) had a separate copy of GCC built with that individual target configured. Clang/LLVM on the other hand can target any configured architecture with one single compiler executable. So with Clang, you'll save a bit of diskspace, avoiding to have many separate compiler executables. That's why there's only one copy of the llvm directory tree.

In NDK r17, you have both GCC and Clang compilers available; Clang is used by default but GCC is still available for projects that haven't been able to migrate to using Clang yet. In newer NDK versions, the old GCC is removed.

In the newer NDK versions, even if GCC is removed, the architecture specific directories like aarch64-linux-android-4.9 are still kept around, as the GNU binutils (minor tools used by the build process) are still used, and those also come in one copy per architecture (even though they technically might work across architectures).

And as for why building for e.g. arm also mentions x86_64; when you are running Clang or GCC, you are running an executable for your build computer which runs x86_64, hence the prebuilt/linux-x86_64 part of the paths.