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@xxxxxxxxxx> > --- > > 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@xxxxxxxxx> Tested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> # build
