f5334c7477
commit 7e20e47c70f810d678d02941fa3c671209c4ca97 upstream. The 'AS' variable is unused for building the kernel. Only the remaining usage is to turn on the integrated assembler. A boolean flag is a better fit for this purpose. AS=clang was added for experts. So, I replaced it with LLVM_IAS=1, breaking the backward compatibility. Suggested-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> (cherry-picked from commit 7e20e47c70f810d678d02941fa3c671209c4ca97) Change-Id: I1dcf901b2543ee6a95cfffddd2ced50b0a1ae14c
87 lines
3.0 KiB
Plaintext
87 lines
3.0 KiB
Plaintext
Building Linux with Clang/LLVM
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==============================
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This document covers how to build the Linux kernel with Clang and LLVM
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utilities.
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About
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-----
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The Linux kernel has always traditionally been compiled with GNU
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toolchains such as GCC and binutils. Ongoing work has allowed for
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[Clang](https://clang.llvm.org/) and [LLVM](https://llvm.org/) utilities
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to be used as viable substitutes. Distributions such as
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[Android](https://www.android.com/),
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[ChromeOS](https://www.chromium.org/chromium-os), and
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[OpenMandriva](https://www.openmandriva.org/) use Clang built kernels.
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[LLVM is a collection of toolchain components implemented in terms of
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C++ objects](https://www.aosabook.org/en/llvm.html). Clang is a
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front-end to LLVM that supports C and the GNU C extensions required by
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the kernel, and is pronounced "klang," not "see-lang."
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Clang
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-----
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The compiler used can be swapped out via [CC=]{.title-ref} command line
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argument to [make]{.title-ref}. [CC=]{.title-ref} should be set when
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selecting a config and during a build.
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> make CC=clang defconfig
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>
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> make CC=clang
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Cross Compiling
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---------------
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A single Clang compiler binary will typically contain all supported
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backends, which can help simplify cross compiling.
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> ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
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[CROSS\_COMPILE]{.title-ref} is not used to prefix the Clang compiler
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binary, instead [CROSS\_COMPILE]{.title-ref} is used to set a command
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line flag: [--target <triple>]{.title-ref}. For example:
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> clang --target aarch64-linux-gnu foo.c
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LLVM Utilities
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--------------
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LLVM has substitutes for GNU binutils utilities. These can be invoked as
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additional parameters to [make]{.title-ref}.
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> make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\
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> OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \\
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> READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
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> HOSTLD=ld.lld
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Currently, the integrated assembler is disabled by default. You can pass
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`LLVM_IAS=1` to enable it.
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Getting Help
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------------
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- [Website](https://clangbuiltlinux.github.io/)
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- [Mailing
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List](https://groups.google.com/forum/#!forum/clang-built-linux):
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<clang-built-linux@googlegroups.com>
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- [Issue Tracker](https://github.com/ClangBuiltLinux/linux/issues)
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- IRC: \#clangbuiltlinux on chat.freenode.net
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- [Telegram](https://t.me/ClangBuiltLinux): \@ClangBuiltLinux
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- [Wiki](https://github.com/ClangBuiltLinux/linux/wiki)
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- [Beginner
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Bugs](https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22)
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Getting LLVM
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------------
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- <http://releases.llvm.org/download.html>
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- <https://github.com/llvm/llvm-project>
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- <https://llvm.org/docs/GettingStarted.html>
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- <https://llvm.org/docs/CMake.html>
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- <https://apt.llvm.org/>
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- <https://www.archlinux.org/packages/extra/x86_64/llvm/>
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- <https://github.com/ClangBuiltLinux/tc-build>
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- <https://github.com/ClangBuiltLinux/linux/wiki/Building-Clang-from-source>
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- <https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/>
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