From: Masahiro Yamada masahiroy@kernel.org
commit a0d1c951ef08ed24f35129267e3595d86f57f5d3 upstream.
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=1 to use Clang and LLVM tools instead of GCC and Binutils. You can pass it from the command line or as an environment variable.
Please note LLVM=1 does not turn on the integrated assembler. You need to pass LLVM_IAS=1 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 flag that switches both target and host tools:
https://lkml.org/lkml/2020/3/28/494 https://lkml.org/lkml/2020/4/3/43
Some items discussed, but not adopted:
- 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.
Signed-off-by: Masahiro Yamada masahiroy@kernel.org Reviewed-by: Nathan Chancellor natechancellor@gmail.com Tested-by: Nathan Chancellor natechancellor@gmail.com # build Tested-by: Nick Desaulniers ndesaulniers@google.com Reviewed-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- Documentation/kbuild/kbuild.rst | 5 +++++ Documentation/kbuild/llvm.rst | 8 ++++++-- Makefile | 29 +++++++++++++++++++++++------ tools/objtool/Makefile | 6 ++++++ 4 files changed, 40 insertions(+), 8 deletions(-)
--- 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. --- a/Documentation/kbuild/llvm.rst +++ b/Documentation/kbuild/llvm.rst @@ -47,8 +47,12 @@ example: LLVM Utilities --------------
-LLVM has substitutes for GNU binutils utilities. These can be invoked as -additional parameters to `make`. +LLVM has substitutes for GNU binutils utilities. Kbuild supports `LLVM=1` +to enable them. + + make LLVM=1 + +They can be enabled individually. The full list of the parameters:
make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \ OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \ --- a/Makefile +++ b/Makefile @@ -394,8 +394,13 @@ HOST_LFS_CFLAGS := $(shell getconf LFS_C HOST_LFS_LDFLAGS := $(shell getconf LFS_LDFLAGS 2>/dev/null) HOST_LFS_LIBS := $(shell getconf LFS_LIBS 2>/dev/null)
-HOSTCC = gcc -HOSTCXX = g++ +ifneq ($(LLVM),) +HOSTCC = clang +HOSTCXX = clang++ +else +HOSTCC = gcc +HOSTCXX = g++ +endif KBUILD_HOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes -O2 \ -fomit-frame-pointer -std=gnu89 $(HOST_LFS_CFLAGS) \ $(HOSTCFLAGS) @@ -404,16 +409,28 @@ KBUILD_HOSTLDFLAGS := $(HOST_LFS_LDFLAG 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 = $(CROSS_COMPILE)strip +endif PAHOLE = pahole LEX = flex YACC = bison --- a/tools/objtool/Makefile +++ b/tools/objtool/Makefile @@ -3,9 +3,15 @@ include ../scripts/Makefile.include include ../scripts/Makefile.arch
# always use the host compiler +ifneq ($(LLVM),) +HOSTAR ?= llvm-ar +HOSTCC ?= clang +HOSTLD ?= ld.lld +else HOSTAR ?= ar HOSTCC ?= gcc HOSTLD ?= ld +endif AR = $(HOSTAR) CC = $(HOSTCC) LD = $(HOSTLD)