kbuild: support LLVM=1 to switch the default tools to Clang/LLVM
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>
diff --git a/Makefile b/Makefile
index a2aadcf..d5c907c 100644
--- a/Makefile
+++ b/Makefile
@@ -399,8 +399,13 @@
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)
@@ -409,16 +414,28 @@
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