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SubjectRe: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM
Hi Masahiro,

On Fri, Apr 03, 2020 at 02:17:09PM +0900, Masahiro Yamada wrote:
> As Documentation/kbuild/llvm.rst implies, building the kernel with a
> full set of LLVM tools gets very verbose and unwieldy.
>
> Provide a single switch 'LLVM' to use Clang and LLVM tools instead of
> GCC and Binutils. You can pass LLVM=1 from the command line or as an
> environment variable. Then, Kbuild will use LLVM toolchains in your
> PATH environment.
>
> Please note LLVM=1 does not turn on the LLVM integrated assembler.
> You need to explicitly pass AS=clang to use it. When the upstream
> kernel is ready for the integrated assembler, I think we can make
> it default.

I agree this should be the default but I think it should probably be
called out somewhere in the documentation as well since users might not
expect to have to have a cross assembler installed.

> We discussed what we need, and we agreed to go with a simple boolean
> switch (https://lkml.org/lkml/2020/3/28/494).
>
> Some items in the discussion:
>
> - LLVM_DIR
>
> When multiple versions of LLVM are installed, I just thought supporting
> LLVM_DIR=/path/to/my/llvm/bin/ might be useful.
>
> CC = $(LLVM_DIR)clang
> LD = $(LLVM_DIR)ld.lld
> ...
>
> However, we can handle this by modifying PATH. So, we decided to not do
> this.
>
> - LLVM_SUFFIX
>
> Some distributions (e.g. Debian) package specific versions of LLVM with
> naming conventions that use the version as a suffix.
>
> CC = clang$(LLVM_SUFFIX)
> LD = ld.lld(LLVM_SUFFIX)
> ...
>
> will allow a user to pass LLVM_SUFFIX=-11 to use clang-11 etc.,
> but the suffixed versions in /usr/bin/ are symlinks to binaries in
> /usr/lib/llvm-#/bin/, so this can also be handled by PATH.
>
> - HOSTCC, HOSTCXX, etc.
>
> We can switch the host compilers in the same way:
>
> ifneq ($(LLVM),)
> HOSTCC = clang
> HOSTCXX = clang++
> else
> HOSTCC = gcc
> HOSTCXX = g++
> endif

I would personally like to see this but I do not have the strongest
opinion.

> This may the right thing to do, but I could not make up my mind.
> Because we do not frequently switch the host compiler, a counter
> solution I had in my mind was to leave it to the default of the
> system.
>
> HOSTCC = cc
> HOSTCXX = c++
>
> Many distributions support update-alternatives to switch the default
> to GCC, Clang, or whatever, but reviewers were opposed to this
> approach. So, this commit does not touch the host tools.
>
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
>
> Documentation/kbuild/kbuild.rst | 5 +++++
> Documentation/kbuild/llvm.rst | 5 +++++
> Makefile | 20 ++++++++++++++++----
> 3 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
> index 510f38d7e78a..2d1fc03d346e 100644
> --- a/Documentation/kbuild/kbuild.rst
> +++ b/Documentation/kbuild/kbuild.rst
> @@ -262,3 +262,8 @@ KBUILD_BUILD_USER, KBUILD_BUILD_HOST
> These two variables allow to override the user@host string displayed during
> boot and in /proc/version. The default value is the output of the commands
> whoami and host, respectively.
> +
> +LLVM
> +----
> +If this variable is set to 1, Kbuild will use Clang and LLVM utilities instead
> +of GCC and GNU binutils to build the kernel.
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index d6c79eb4e23e..4602369f6a4f 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -55,6 +55,11 @@ additional parameters to `make`.
> READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
> HOSTLD=ld.lld
>
> +You can use a single switch `LLVM=1` to use LLVM utilities by default (except
> +for building host programs).
> +
> + make LLVM=1 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
> +
> Getting Help
> ------------
>
> diff --git a/Makefile b/Makefile
> index c91342953d9e..6db89ecdd942 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -409,16 +409,28 @@ KBUILD_HOSTLDFLAGS := $(HOST_LFS_LDFLAGS) $(HOSTLDFLAGS)
> KBUILD_HOSTLDLIBS := $(HOST_LFS_LIBS) $(HOSTLDLIBS)
>
> # Make variables (CC, etc...)
> -LD = $(CROSS_COMPILE)ld
> -CC = $(CROSS_COMPILE)gcc
> CPP = $(CC) -E
> +ifneq ($(LLVM),)
> +CC = clang
> +LD = ld.lld
> +AR = llvm-ar
> +NM = llvm-nm
> +OBJCOPY = llvm-objcopy
> +OBJDUMP = llvm-objdump
> +READELF = llvm-readelf
> +OBJSIZE = llvm-size
> +STRIP = llvm-strip
> +else
> +CC = $(CROSS_COMPILE)gcc
> +LD = $(CROSS_COMPILE)ld
> AR = $(CROSS_COMPILE)ar
> NM = $(CROSS_COMPILE)nm
> -STRIP = $(CROSS_COMPILE)strip
> OBJCOPY = $(CROSS_COMPILE)objcopy
> OBJDUMP = $(CROSS_COMPILE)objdump
> -OBJSIZE = $(CROSS_COMPILE)size
> READELF = $(CROSS_COMPILE)readelf
> +OBJSIZE = $(CROSS_COMPILE)size
> +STRIP = $(CROSS_COMPILE)strip
> +endif
> PAHOLE = pahole
> LEX = flex
> YACC = bison
> --
> 2.17.1
>

I have verified that the variables get their correct value with LLVM=1
and that they are still overridable.

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build

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