Hi Nathan, On Fri, Apr 3, 2020 at 5:57 PM Nathan Chancellor <natechancellor@xxxxxxxxx> wrote: > > 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. I will add the following info to llvm.rst: `LLVM=1` does not turn on the LLVM integrated assembler, so you still need assembler from GNU binutils. You can pass `AS=clang` to use the integrated assembler, but it is experimental as of writing. > > 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 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200403085719.GA9282%40ubuntu-m2-xlarge-x86. -- Best Regards Masahiro Yamada
