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Subject[PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM
Date
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' to use Clang and LLVM tools instead of
GCC and Binutils. You can pass LLVM=1 from the command line or as an
environment variable. Then, Kbuild will use LLVM toolchains in your
PATH environment.

Please note LLVM=1 does not turn on the LLVM integrated assembler.
You need to explicitly pass AS=clang 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
switch (https://lkml.org/lkml/2020/3/28/494).

Some items in the discussion:

- 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.

- HOSTCC, HOSTCXX, etc.

We can switch the host compilers in the same way:

ifneq ($(LLVM),)
HOSTCC = clang
HOSTCXX = clang++
else
HOSTCC = gcc
HOSTCXX = g++
endif

This may the right thing to do, but I could not make up my mind.
Because we do not frequently switch the host compiler, a counter
solution I had in my mind was to leave it to the default of the
system.

HOSTCC = cc
HOSTCXX = c++

Many distributions support update-alternatives to switch the default
to GCC, Clang, or whatever, but reviewers were opposed to this
approach. So, this commit does not touch the host tools.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---

Documentation/kbuild/kbuild.rst | 5 +++++
Documentation/kbuild/llvm.rst | 5 +++++
Makefile | 20 ++++++++++++++++----
3 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/Documentation/kbuild/kbuild.rst b/Documentation/kbuild/kbuild.rst
index 510f38d7e78a..2d1fc03d346e 100644
--- 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 d6c79eb4e23e..4602369f6a4f 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -55,6 +55,11 @@ additional parameters to `make`.
READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
HOSTLD=ld.lld

+You can use a single switch `LLVM=1` to use LLVM utilities by default (except
+for building host programs).
+
+ make LLVM=1 HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar HOSTLD=ld.lld
+
Getting Help
------------

diff --git a/Makefile b/Makefile
index c91342953d9e..6db89ecdd942 100644
--- a/Makefile
+++ b/Makefile
@@ -409,16 +409,28 @@ 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
--
2.17.1
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