[question] How to build LLVM toolchain for the NDK locally for Windows x86_64.

[arpit07531]

Background

We want to port the following fix from the upstream LLVM to our NDK r21, as we are getting build failures because of this bug.
https://reviews.llvm.org/D83772

We use NDK r21 release on windows 10 64 bit machines for development. I want to sync to the LLVM source code corresponding to NDK r21 release, apply the above fix, generate binaries for Windows, and replace the clang++.exe front end binary in our NDK with the locally generated one. I want to just update the clang++.exe binary instead of updating the whole NDK toolchain, because we build multiple apps using the same toolchain, and it takes a lot of effort and testing for upgrading the NDK. And also this fix is not yet ported to the LLVM-project repository of the NDK.

Current clang version in the toolchain :

>clang++.exe --version
Android (6317467 based on r365631c1) clang version 9.0.8 (https://android.googlesource.com/toolchain/llvm-project e0caee08e5f09b374a27a676d04978c81fcb1928) (based on LLVM 9.0.8svn)
Target: x86_64-w64-windows-gnu
Thread model: posix
InstalledDir: D:\bin

Steps that I am following :

• git clone https://android.googlesource.com/toolchain/llvm-project

• git checkout e0caee08e5f09b374a27a676d04978c81fcb1928

• Make the source change - as per https://reviews.llvm.org/D83772

• cmake -G "Visual Studio 15 2017" -A x64 -Thost=x64 -DLLVM_ENABLE_PROJECTS="clang" -DPYTHON_HOME=C:\python38 e:\tmp\llvm-project-ndk\llvm-project\llvm

• Open LLVM.sln in Visual studio and build x64 release for clang.

Using the above steps I am able to generate clang++.exe binary successfully.

>clang++.exe --version
clang version 9.0.0 (https://android.googlesource.com/toolchain/llvm-project e0caee08e5f09b374a27a676d04978c81fcb1928)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: E:\tmp\llvm-project-ndk\build2\Release\bin

Problem

I am a little doubtful whether this locally generated clang++.exe binary is compatible with the rest of the toolchain, and whether it has all the fixes that are there in r21 release because I am seeing the following discrepancies in the binary.

• clang++.exe --version shows the version of this binary as 9.0.0, while the one in r21 release shows the version as 9.0.8

• clang++.exe --version shows the Target as x86_64-pc-windows-msvc, while the original r21 release binary shows target as x86_64-w64-windows-gnu

• Size of the NDK r21 release clang++.exe binary is 66,419 KB, while that of my locally generated binary is 69325 KB.

• In the r21 release clang++.exe references the default headers from directory - ../lib64/clang/9.0.8/include while the locally generated binary references headers from ../lib/clang/9.0.0/include. Even if I ignore the difference in the path of the includes, there are slight differences in the headers generated on my machine, and the ones that ship with NDK release. So I am really doubtful whether I should use the NDK shipped headers or the new ones generated in my build even if I take care of the includes path.

Questions

• Are the Windows LLVM binaries in NDK built on Windows with msvc or on windows with mingw or cross compiled on Linux? Also how can I get the target type of the binary to match - x86_64-w64-windows-gnu. Can someone please share the exact build steps to generate LLVM toolchain for the NDK release for Windows x86-64 locally. I hope it should fix the includes path issue also.

• How can I sync to the LLVM source code that is built into the r21 release. If I try to match the commit hash, I get the binary of version 9.0.0, while the one in NDK r21 release is 9.0.8.

• If somehow it's not possible to generate a local build without introducing new issues, As an option of last resort is it possible for the NDK team to trigger a build in the build machines with this fix and share a drop? We are really blocked on this issue and need to somehow work around this quickly.

I will highly appreciate any help.

[pirama-arumuga-nainar]

We cross-compile the Windows clang from Linux. Here are the instructions that should work.

$ mkdir llvm-toolchain && cd llvm-toolchain
$ repo init -u https://android.googlesource.com/platform/manifest -b llvm-toolchain
$ cp $NDK_CLANG_PATH/manifest_6317467.xml .repo/manifests
$ repo init -u manifest_6317467.xml
$ repo sync -c

# cherry-pick necessary patch into toolchain/llvm-project

$ python toolchain/llvm_android/build.py --no-build=linux

The built clang++.exe would be in out/install/windows-x86/clang-dev/bin.

PS: These instructions are partially from here. It's just missing the part about copying the manifest. We'll get the docs updated.

[arpit07531]

@pirama-arumuga-nainar Thanks a lot. I have got a perfectly working binary of exactly same size and signature using these instructions. These instructions worked like a charm. :)