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kbuild: support LLVM=1 to switch the default tools to Clang/LLVM

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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>
This commit is contained in:
Masahiro Yamada 2020-04-08 10:36:23 +09:00
parent 7e20e47c70
commit a0d1c951ef
4 changed files with 40 additions and 8 deletions
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5
Documentation/kbuild/kbuild.rst
View File

@ -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.

8
Documentation/kbuild/llvm.rst
View File

@ -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 \\

29
Makefile
View File

@ -399,8 +399,13 @@ HOST_LFS_CFLAGS := $(shell getconf LFS_CFLAGS 2>/dev/null)
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_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

6
tools/objtool/Makefile
View File

@ -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)