Cross building from x64 to arm64 fails compiling using clang

[AnInternetTroll]

Describe the Bug

When cross building from x64 to arm64 the process fails as clang "is not able to compile a simple test program."

Steps to Reproduce

In the dotnet/dotnet repo on version v8.0.7 the following command should cause the issue

ROOTFS_DIR=/usr/aarch64-linux-gnu bash ./build.sh --clean-while-building --online --source-repository https://github.com/dotnet/dotnet --source-version 8be139ddde52d33e24c7d82f813248ff9fc54b97 -- -p:TargetArchitecture=arm64 -p:CrossBuild=true

The full void template is available on github (very much WIP)

Other Information

The relevant output log of the build process can be found here
(I have omitted the void-related stuff, such as installing dependencies, as well as the prep.sh output. If it's relevant I can paste it as well)

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[omajid]

Some relevant bits from the log:

...
        The C compiler

          "/usr/bin/clang-17"

        is not able to compile a simple test program.

        It fails with the following output:
...
          /usr/bin/clang-17   -fstack-clash-protection -D_FORTIFY_SOURCE=2 -O2 -pipe -march=armv8-a  -g -I/usr/aarch64-linux-gnu/usr/include -ffile-prefix-map=/builddir/dotnet-sdk-8.0.7=.  -MD -MT CMakeFiles/cmTC_9081e.dir/testCCompiler.c.o -MF CMakeFiles/cmTC_9081e.dir/testCCompiler.c.o.d -o CMakeFiles/cmTC_9081e.dir/testCCompiler.c.o -c /builddir/dotnet-sdk-8.0.7/src/runtime/artifacts/source-build/self/src/artifacts/obj/coreclr/linux.x64.Release/CMakeFiles/CMakeScratch/TryCompile-NWEw92/testCCompiler.c
          error: unknown target CPU 'armv8-a'
          note: valid target CPU values are: nocona, core2, penryn, bonnell, atom, silvermont, slm, goldmont, goldmont-plus, tremont, nehalem, corei7, westmere, sandybridge, corei7-avx, ivybridge, core-avx-i, haswell, core-avx2, broadwell, skylake, skylake-avx512, skx, cascadelake, cooperlake, cannonlake, icelake-client, rocketlake, icelake-server, tigerlake, sapphirerapids, alderlake, raptorlake, meteorlake, sierraforest, grandridge, graniterapids, graniterapids-d, emeraldrapids, knl, knm, k8, athlon64, athlon-fx, opteron, k8-sse3, athlon64-sse3, opteron-sse3, amdfam10, barcelona, btver1, btver2, bdver1, bdver2, bdver3, bdver4, znver1, znver2, znver3, znver4, x86-64, x86-64-v2, x86-64-v3, x86-64-v4

Could it be that the compiler being used can't target arm64 and the compiler that can target arm64 isn't being picked up?

[AnInternetTroll]

Yeah, that seems to be the case. It looks as though void's clang can't cross compile, and they use cross-aarch64-linux-gnu instead as a compiler. If I install clang then dotnet's build script tries to use it and fails. If I don't install it then dotnet's build script will complain it can't find clang. In both cases the $CC variable is set to cross-aarch64-linux-gnu

I don't know how dependent dotnet is on clang, but would it be possible to use something else? Ideally though an env var or flag?

[omajid]

I haven't done any cross-builds for arm64 myself, but when I compare with ppc64le cross builds (which I do build):

I set some env vars:

• CROSSCOMPILE=1
• ROOTFS_DIR=/
• PKG_CONFIG_PATH=/usr/lib/powerpc64le-linux-gnu/pkgconfig

My build command is then literally:

./build.sh --source-only --online --use-mono-runtime /p:TargetArchitecture=ppc64le /p:PortableBuild=true

[AnInternetTroll]

I can't get this to work. This kinda got it closer though, but unknown target CPU 'armv8-a'

Here is a full log with the output of env as well if it helps: dotnet-sdk-aarch64-2.txt

As well as the template used: template

[MichaelSimons]

[Triage] This appears as a discussion with no actionable changes necessary within source-build. Converting to a discussion. Please open follow-up issues if any source changes are needed.