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++ 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
