Messages in this thread | | | Date | Mon, 6 Apr 2020 13:22:20 +0200 | From | Matthias Maennich <> | Subject | Re: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM |
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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.
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 > >-- >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@googlegroups.com. >To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20200403051709.22407-1-masahiroy%40kernel.org.
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