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/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index 510f38d..2d1fc03 100644 (file)
--- 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 4507085..c776b6e 100644 (file)
--- 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 \\

diff --git a/Makefile b/Makefile
index a2aadcf..d5c907c 100644 (file)
--- a/Makefile
+++ b/Makefile
@@ -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

diff --git a/tools/objtool/Makefile b/tools/objtool/Makefile
index ee08aef..f591c4d 100644 (file)
--- 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)