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Date: Tue, 7 Apr 2020 13:39:13 -0700
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
clang-built-linux <clang-built-linux@...glegroups.com>,
Jonathan Corbet <corbet@....net>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Michal Marek <michal.lkml@...kovi.net>,
Peter Zijlstra <peterz@...radead.org>,
Linux Doc Mailing List <linux-doc@...r.kernel.org>,
LKML <linux-kernel@...r.kernel.org>,
Matthias Männich <maennich@...gle.com>,
Sandeep Patil <sspatil@...gle.com>
Subject: Re: [PATCH v2 2/2] kbuild: support LLVM=1 to switch the default tools
to Clang/LLVM
On Tue, Apr 7, 2020 at 11:44 AM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> 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 <masahiroy@...nel.org>
> Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
> Tested-by: Nathan Chancellor <natechancellor@...il.com> # build
> Tested-by: Nick Desaulniers <ndesaulniers@...gle.com>
Thanks for the series!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.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@...t 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)
> --
--
Thanks,
~Nick Desaulniers
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