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Message-ID: <20200406112220.GB126804@google.com>
Date: Mon, 6 Apr 2020 13:22:20 +0200
From: Matthias Maennich <maennich@...gle.com>
To: Masahiro Yamada <masahiroy@...nel.org>
Cc: linux-kbuild@...r.kernel.org,
Nick Desaulniers <ndesaulniers@...gle.com>,
Nathan Chancellor <natechancellor@...il.com>,
clang-built-linux@...glegroups.com,
Jonathan Corbet <corbet@....net>,
Michal Marek <michal.lkml@...kovi.net>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: support 'LLVM' to switch the default tools to
Clang/LLVM
On Fri, Apr 03, 2020 at 02:17:09PM +0900, Masahiro Yamada 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' 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++
What about HOSTLD ? I saw recently, that setting HOSTLD=ld.lld is not
yielding the expected result (some tools, like e.g. fixdep still require
an `ld` to be in PATH to be built). I did not find the time to look into
that yet, but I would like to consistently switch to the llvm toolchain
(including linker and possibly more) also for hostprogs.
Cheers,
Matthias
>
> 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@...nel.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@...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 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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