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SubjectRe: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM
On Mon, Apr 6, 2020 at 8:22 PM 'Matthias Maennich' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> 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.
> >
> >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
> >
> > 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++
>
> What about HOSTLD ? I saw recently, that setting HOSTLD=ld.lld is not
> yielding the expected result (some tools, like e.g. fixdep still require
> an `ld` to be in PATH to be built). I did not find the time to look into
> that yet, but I would like to consistently switch to the llvm toolchain
> (including linker and possibly more) also for hostprogs.


HOSTLD=ld.lld worked for me, but HOSTCC=clang did not.



HOSTCC=clang without CC=clang fails to build objtool.

The build system of objtool is meh. :(


HOSTCC scripts/mod/sumversion.o
HOSTLD scripts/mod/modpost
CALL scripts/checksyscalls.sh
CALL scripts/atomic/check-atomics.sh
DESCEND objtool
error: unknown warning option '-Wstrict-aliasing=3'; did you mean
'-Wstring-plus-int'? [-Werror,-Wunknown-warning-option]
HOSTCC /home/masahiro/workspace/linux-kbuild/tools/objtool/fixdep.o
HOSTLD /home/masahiro/workspace/linux-kbuild/tools/objtool/fixdep-in.o
LINK /home/masahiro/workspace/linux-kbuild/tools/objtool/fixdep
CC /home/masahiro/workspace/linux-kbuild/tools/objtool/exec-cmd.o
CC /home/masahiro/workspace/linux-kbuild/tools/objtool/help.o
CC /home/masahiro/workspace/linux-kbuild/tools/objtool/pager.o






> Cheers,
> Matthias
>
> >
> > 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
> >
> >--
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>
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--
Best Regards
Masahiro Yamada

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