linux-kernel - Re: [PATCH] kbuild: support 'LLVM' to switch the default tools to Clang/LLVM

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Date:   Sun, 5 Apr 2020 20:14:42 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Jonathan Corbet <corbet@....net>,
        Michal Marek <michal.lkml@...kovi.net>,
        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] kbuild: support 'LLVM' to switch the default tools to
 Clang/LLVM

On 2020-04-06, Masahiro Yamada wrote:
>On Mon, Apr 6, 2020 at 8:55 AM 'Fangrui Song' via Clang Built Linux
><clang-built-linux@...glegroups.com> wrote:
>>
>> On 2020-04-06, Masahiro Yamada wrote:
>> >On Sat, Apr 4, 2020 at 3:24 AM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>> >>
>> >> On Thu, Apr 2, 2020 at 10:17 PM 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' 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.
>> >>
>> >> Having this behavior change over time may be surprising.  I'd rather
>> >> that if you want to not use the integrated assembler, you explicitly
>> >> negate it, or just don't use the LLVM=1 syntax, ie. `make CC=clang
>> >> LD=ld.lld ...`.
>> >>
>> >> We could modify how `-no-integrated-as` is chosen when LLVM=1.
>> >>
>> >> make LLVM=1 LLVMIA=0 ... # add `-no-integrated-as`
>> >> # what the flag is doesn't really matter to me, something shorter might be nice.
>> >> make LLVM=1 # use all LLVM tools
>> >>
>> >> Since we got rid of $(AS), it would be appropriate to remove/change it
>> >> there, since no one really relies on AS=clang right now. (We do have 1
>> >> of our 60+ CI targets using it, but we can also change that trivially.
>> >> So I think we have a lot of freedom to change how `-no-integrated-as`
>> >> is set.
>> >>
>> >> This could even be independent of this patch.
>> >
>> >
>> >I also thought a boolean flag is preferred.
>> >
>> >AS=clang will not live long anyway, and
>> >I hesitated to break the compatibility
>> >for the short-term workaround.
>> >
>> >But, if this is not a big deal, I can
>> >replace AS=clang with LLVMIA=1.
>>
>> My mere complaint is that it may be difficult to infer the intention (integrated
>> assembler) from the abbreviation "IA" in "LLVMIA" :/
>>
>> Something with "AS" in the name may be easier for a user to understand,
>> e.g. CLANG_AS or LLVM_AS.
>
>I see 'llvm-as' in my PATH,
>but it is a different kind of tool, right?
>(converter from LLVM assembler *.ll to LLVM bit code *.bc)
>
>So, I thought "LLVM_AS" might be confusing.

You are right. llvm-as converts a textual form of LLVM IR (.ll) to
a binary form bitcode (.bc). LLVM_AS is confusing. CLANG_AS/CLANGAS might be
suitable.

   clang a.c '-###' => clang -cc1     # like gcc invokes cc1
   clang a.s '-###' => clang -cc1as   # this invokes the integrated assembler