Commit a0d1c951 authored by Masahiro Yamada's avatar Masahiro Yamada
Browse files

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: default avatarMasahiro Yamada <masahiroy@kernel.org>
Reviewed-by: default avatarNathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Tested-by: default avatarNick Desaulniers <ndesaulniers@google.com>
Reviewed-by: default avatarNick Desaulniers <ndesaulniers@google.com>
parent 7e20e47c

Documentation/kbuild/kbuild.rst

+5 −0
Original line number Diff line number Diff line
@@ -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.

Documentation/kbuild/llvm.rst

+6 −2
Original line number Diff line number Diff line
@@ -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 \\

Makefile

+23 −6
Original line number Diff line number Diff line
@@ -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)



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

tools/objtool/Makefile

+6 −0
Original line number Diff line number Diff line
@@ -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)