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@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 -- 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/20200403051709.22407-1-masahiroy%40kernel.org.
