Building libc++ for sysroots in llvm-toolchain

[Bryan Chan]

I was trying to build the latest code in llvm-toolchain to get a relatively standalone toolchain, but ran into a problem with libc++. While libc++.so.1 is built correctly for the host (i.e. out/install/linux-x86/clang-dev/lib64/libc++.so.1), only stubs are generated under the sysroot directories, which cannot actually be used for linking C++ programs that use the STL. Judging by the comment inside toolchain/llvm_android/do_build.py, it seems that libc++ is not built since NDK r15 because strtod_l is missing (http://b/64037266).

What is the correct way to build libc++ for arm and aarch64 sysroots?

Bryan

[Pirama Arumuga Nainar]

libc++ for the device is built as part of the NDK.  See https://android.googlesource.com/platform/ndk/+/master/docs/Building.md for instructions to build the NDK.

(+danalbert, in case I'm wrong :)

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[Bryan Chan]

Thanks, that's what I figured as well. I started syncing the ndk-release-r21 branch a few hours ago... it's still cloning!

[Bryan Chan]

I have synced the ndk-release-r21 branch and ran checkbuild.py to get a usable NDK toolchain with sysroot, under out/linux/android-ndk-r21d/. How do I go about modifying Clang and LLVM now? The ndk-release-r21 branch only contains prebuilt binaries and no obvious way to edit or rebuild Clang/LLVM. If I try to use the sysroot and lib directories from the NDK with the compiler I built from the llvm-toolchain branch, C++ programs fail to link with errors like this:

ld.lld: error: undefined symbol: std::__1::__basic_string_common<true>::__throw_length_error() const

Bryan

[Bryan Chan]

I found the relevant documentation: 

https://android.googlesource.com/platform/ndk/+/master/docs/Toolchains.md

Sorry for the noise. Let me try again.

[Bryan Chan]

I built llvm-r353983 after my modification, and then unpacked the clang-dev tarball in the NDK prebuilts directory as per the instructions in Toolchains.md. checkbuild.py ran successfully. But the resulting toolchain is unusable; it seems that clang cannot find the sysroot automatically when invoked with -target aarch64-linux-android21, and ends up using the host system's headers. What did I do wrong? How do people hack Clang/LLVM inside the NDK, especially when we need to work with an older version than tip of trunk?

[Dan Albert]

If it's older enough it's probably missing several of the patches that make it work without manually configuring sysroots and whatnot. That's a pretty old compiler, so that's probably what happened.

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[Bryan Chan]

Thanks Dan. That's exactly what happened. I was trying to use the AOSP 10 compiler as my baseline, not realizing that NDK r21 had moved on to a newer version of Clang/LLVM 9. Checking out llvm-r365631 allowed me to make more progress.