linux-kernel - Re: [PATCH] compiler-gcc: get back Clang build

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Date:   Wed, 22 Aug 2018 02:07:40 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will.deacon@....com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] compiler-gcc: get back Clang build

2018-08-22 1:33 GMT+09:00 Joe Perches <joe@...ches.com>:
> On Tue, 2018-08-21 at 15:48 +0900, Masahiro Yamada wrote:
>> Commit cafa0010cd51 ("Raise the minimum required gcc version to 4.6")
>> missed the fact that <linux/compiler-gcc.h> is included by Clang
>> as well as by GCC.
>>
>> Clang actually defines __GNUC__, __GNUC_MINOR__, __GNUC_PATCHLEVEL__
>> and it looks like GCC 4.2.1.
>>
>>   $ scripts/gcc-version.sh -p clang
>>   040201
>>
>> If you try to build the kernel with Clang, you will get the
>> "Sorry, your compiler is too old - please upgrade it."
>> followed by a bunch of "unknown attribute" warnings.
>>
>> Add !defined(__clang__) to the minimum version check.
>>
>> Also, revive the version test blocks for versions >= 4.2.1
>> in order to disable features not supported by Clang.
>
> What is the minimum clang version required to compile the kernel?
> What features are not supported by the minimum clang version?
>
> On my system, using clang
>
> $ clang -v
> clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
>
> and
>
> $ git checkout v4.16 ; make clean ; make CC=clang defconfig ; make CC=clang
> HEAD is now at 0adb32858b0b... Linux 4.16
>
> is successful
>
> but
>
> $ git checkout v4.17 ; make clean ; make CC=clang defconfig ; make CC=clang
> HEAD is now at 29dcea88779c... Linux 4.17
> arch/x86/Makefile:184: *** Compiler lacks asm-goto support..  Stop.
> arch/x86/Makefile:184: *** Compiler lacks asm-goto support..  Stop.
>

You cannot build x86 because asm-goto support is missing in clang.

How about building arm or arm64?

$ make ARCH=arm CC=clang CROSS_COMPILE=arm-linux-gnueabihf- defconfig all





-- 
Best Regards
Masahiro Yamada