linux-kernel - Re: [PATCH] scripts/Makefile.clang: default to LLVM

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Message-ID: <CAK7LNATWm94K=UcSNn88PyPST38rfe_31vLLpDf=ERPnKnBvVA@mail.gmail.com>
Date:   Fri, 6 Aug 2021 00:15:29 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Khem Raj <raj.khem@...il.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Jonathan Corbet <corbet@....net>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:SIFIVE DRIVERS" <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH] scripts/Makefile.clang: default to LLVM_IAS=1

On Tue, Aug 3, 2021 at 8:43 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>
> LLVM_IAS=1 controls enabling clang's integrated assembler via
> -integrated-as. This was an explicit opt in until we could enable
> assembler support in Clang for more architecures. Now we have support
> and CI coverage of LLVM_IAS=1 for all architecures except a few more
> bugs affecting s390 and powerpc.
>
> This commit flips the default from opt in via LLVM_IAS=1 to opt out via
> LLVM_IAS=0.  CI systems or developers that were previously doing builds
> with CC=clang or LLVM=1 without explicitly setting LLVM_IAS must now
> explicitly opt out via LLVM_IAS=0, otherwise they will be implicitly
> opted-in.
>
> This finally shortens the command line invocation when cross compiling
> with LLVM to simply:
>
> $ make ARCH=arm64 LLVM=1
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1434
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Note: base is:
> https://lore.kernel.org/lkml/20210802183910.1802120-1-ndesaulniers@google.com/
>
>  Documentation/kbuild/llvm.rst | 14 ++++++++------
>  Makefile                      |  2 +-
>  arch/riscv/Makefile           |  2 +-
>  scripts/Makefile.clang        |  6 +++---
>  4 files changed, 13 insertions(+), 11 deletions(-)
>
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index f8a360958f4c..16712fab4d3a 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -60,17 +60,14 @@ They can be enabled individually. The full list of the parameters: ::
>           OBJCOPY=llvm-objcopy OBJDUMP=llvm-objdump 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_IAS=1`` to enable it.
> +Currently, the integrated assembler is enabled by default. You can pass
> +``LLVM_IAS=0`` to disable it.
>
>  Omitting CROSS_COMPILE
>  ----------------------
>
>  As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
>
> -Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
> -``--prefix=<path>`` to search for the GNU assembler and linker.
> -
>  If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
>  from ``ARCH``.
>
> @@ -78,7 +75,12 @@ That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
>
>  For example, to cross-compile the arm64 kernel::
>
> -       make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +       make ARCH=arm64 LLVM=1
> +
> +If ``LLVM_IAS=0`` is specified, ``CROSS_COMPILE`` is also used to derive
> +``--prefix=<path>`` to search for the GNU assembler and linker. ::
> +
> +       make ARCH=arm64 LLVM=1 LLVM_IAS=0 CROSS_COMPILE=aarch64-linux-gnu-
>
>  Supported Architectures
>  -----------------------
> diff --git a/Makefile b/Makefile
> index 444558e62cbc..b24b48c9ebb7 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -845,7 +845,7 @@ else
>  DEBUG_CFLAGS   += -g
>  endif
>
> -ifneq ($(LLVM_IAS),1)
> +ifeq ($(LLVM_IAS),0)
>  KBUILD_AFLAGS  += -Wa,-gdwarf-2
>  endif
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index bc74afdbf31e..807f7c94bc6f 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -41,7 +41,7 @@ endif
>  ifeq ($(CONFIG_LD_IS_LLD),y)
>         KBUILD_CFLAGS += -mno-relax
>         KBUILD_AFLAGS += -mno-relax
> -ifneq ($(LLVM_IAS),1)
> +ifeq ($(LLVM_IAS),0)
>         KBUILD_CFLAGS += -Wa,-mno-relax
>         KBUILD_AFLAGS += -Wa,-mno-relax
>  endif



Please drop these two hunks.

I will apply my patch instead.
https://lore.kernel.org/patchwork/patch/1472580/






> diff --git a/scripts/Makefile.clang b/scripts/Makefile.clang
> index 1f4e3eb70f88..3ae63bd35582 100644
> --- a/scripts/Makefile.clang
> +++ b/scripts/Makefile.clang
> @@ -22,12 +22,12 @@ else
>  CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
>  endif # CROSS_COMPILE
>
> -ifeq ($(LLVM_IAS),1)
> -CLANG_FLAGS    += -integrated-as
> -else
> +ifeq ($(LLVM_IAS),0)
>  CLANG_FLAGS    += -no-integrated-as
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
>  CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
> +else
> +CLANG_FLAGS    += -integrated-as
>  endif
>  CLANG_FLAGS    += -Werror=unknown-warning-option
>  KBUILD_CFLAGS  += $(CLANG_FLAGS)
>
> base-commit: d7a86429dbc691bf540688fcc8542cc20246a85b
> prerequisite-patch-id: 0d3072ecb5fd06ff6fd6ea81fe601f6c54c23910
> prerequisite-patch-id: 2654829756eb8a094a0ffad1679caa75a4d86619
> prerequisite-patch-id: a51e7885ca2376d008bbf146a5589da247806f7b
> --
> 2.32.0.554.ge1b32706d8-goog
>


When we negate a flag that is enabled by default,
which is a common way?
 - set it to '0'
 - set is to empty


So, I was wondering if we should support
not only LLVM_IAS=0 but also LLVM_IAS=.

What do you think?


-- 
Best Regards
Masahiro Yamada