[PATCH 4.19 33/37] kbuild: support LLVM=1 to switch the default tools to Clang/LLVM

Greg Kroah-Hartman
2020-09-25T12:54:51Z

From: Masahiro Yamada <masahiroy@kernel.org>

commit a0d1c951ef08ed24f35129267e3595d86f57f5d3 upstream.

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>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
[nd: conflict in exported vars list from not backporting commit
e83b9f55448a ("kbuild: add ability to generate BTF type info for vmlinux")]
[nd: hunk against Documentation/kbuild/kbuild.rst dropped due to not backporting
commit cd238effefa2 ("docs: kbuild: convert docs to ReST and rename to *.rst")]
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Documentation/kbuild/llvm.rst | 8 ++++++--
Makefile | 29 +++++++++++++++++++++++------
tools/objtool/Makefile | 6 ++++++
3 files changed, 35 insertions(+), 8 deletions(-)

--- 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 \\
--- a/Makefile
+++ b/Makefile
@@ -358,8 +358,13 @@ HOST_LFS_CFLAGS := $(shell getconf LFS_C
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)
@@ -368,16 +373,28 @@ KBUILD_HOSTLDFLAGS := $(HOST_LFS_LDFLAG
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
LEX = flex
YACC = bison
AWK = awk
--- a/tools/objtool/Makefile
+++ b/tools/objtool/Makefile
@@ -7,9 +7,15 @@ ARCH := x86
endif

# 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)