Re: [PATCH v2 2/2] kbuild: support LLVM=1 to switch the default tools to Clang/LLVM

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Apr 7, 2020 at 11:44 AM Masahiro Yamada <masahiroy@kernel.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@kernel.org>
> Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
> Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
> Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Thanks for the series!
Reviewed-by: Nick Desaulniers <ndesaulniers@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)
> --

-- 
Thanks,
~Nick Desaulniers