[PATCH v2 1/2] kbuild: replace AS=clang with LLVM_IA=1

[Masahiro Yamada]

The 'AS' variable is unused for building the kernel. Only the remaining
usage is to turn on the integrated assembler. A boolean flag is a better
fit for this purpose.

AS=clang was added for experts. So, I replaced it with LLVM_IA=1,
breaking the backward compatibility.

Suggested-by: Nick Desaulniers <ndesau...@google.com>
Signed-off-by: Masahiro Yamada <masa...@kernel.org>
---

Changes in v2:
- new patch

Documentation/kbuild/llvm.rst | 5 ++++-
Makefile | 2 +-
2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index eefbdfa3e4d9..2b40afa58049 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -50,11 +50,14 @@ LLVM Utilities
LLVM has substitutes for GNU binutils utilities. These can be invoked as
additional parameters to `make`.

- make CC=clang AS=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\
+ make CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm STRIP=llvm-strip \\
OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump OBJSIZE=llvm-size \\
READELF=llvm-readelf HOSTCC=clang HOSTCXX=clang++ HOSTAR=llvm-ar \\
HOSTLD=ld.lld

+Currently, the integrated assembler is disabled by default. You can pass
+LLVM_IA=1 to enable it.
+
Getting Help
------------

diff --git a/Makefile b/Makefile
index 1b2691057cb5..f9beb696d6d3 100644
--- a/Makefile
+++ b/Makefile
@@ -538,7 +538,7 @@ endif
ifneq ($(GCC_TOOLCHAIN),)
CLANG_FLAGS += --gcc-toolchain=$(GCC_TOOLCHAIN)
endif
-ifeq ($(if $(AS),$(shell $(AS) --version 2>&1 | head -n 1 | grep clang)),)
+ifneq ($(LLVM_IA),1)
CLANG_FLAGS += -no-integrated-as
endif
CLANG_FLAGS += -Werror=unknown-warning-option
--
2.17.1

[Masahiro Yamada]

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_IA=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 <masa...@kernel.org>
Reviewed-by: Nathan Chancellor <natecha...@gmail.com>
Tested-by: Nathan Chancellor <natecha...@gmail.com> # build
Tested-by: Nick Desaulniers <ndesau...@google.com>
---

Changes in v2:
- Switch host compilers as well as requested

Documentation/kbuild/kbuild.rst | 5 +++++
Documentation/kbuild/llvm.rst | 8 ++++++--
Makefile | 29 +++++++++++++++++++++++------
tools/objtool/Makefile | 6 ++++++
4 files changed, 40 insertions(+), 8 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 2b40afa58049..81f915c02c4c 100644
--- 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 f9beb696d6d3..ffc1d2c618a7 100644
--- 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 ee08aeff30a1..f591c4d1b6fe 100644
--- 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)
--
2.17.1

[Nathan Chancellor]

On Wed, Apr 08, 2020 at 03:43:35AM +0900, Masahiro Yamada wrote:
> The 'AS' variable is unused for building the kernel. Only the remaining
> usage is to turn on the integrated assembler. A boolean flag is a better
> fit for this purpose.
>
> AS=clang was added for experts. So, I replaced it with LLVM_IA=1,
> breaking the backward compatibility.
>
> Suggested-by: Nick Desaulniers <ndesau...@google.com>
> Signed-off-by: Masahiro Yamada <masa...@kernel.org>

Reviewed-by: Nathan Chancellor <natecha...@gmail.com>

[Nick Desaulniers]

On Tue, Apr 7, 2020 at 11:44 AM Masahiro Yamada <masa...@kernel.org> wrote:
>
> The 'AS' variable is unused for building the kernel. Only the remaining
> usage is to turn on the integrated assembler. A boolean flag is a better
> fit for this purpose.
>
> AS=clang was added for experts. So, I replaced it with LLVM_IA=1,
> breaking the backward compatibility.
>
> Suggested-by: Nick Desaulniers <ndesau...@google.com>
> Signed-off-by: Masahiro Yamada <masa...@kernel.org>

Thank you for this series!
Reviewed-by: Nick Desaulniers <ndesau...@google.com>

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--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

On Tue, Apr 7, 2020 at 11:44 AM Masahiro Yamada <masa...@kernel.org> wrote:
>

Thanks for the series!
Reviewed-by: Nick Desaulniers <ndesau...@google.com>

--
Thanks,
~Nick Desaulniers