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

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Date:   Tue, 21 Aug 2018 09:33:37 -0700
From:   Joe Perches <joe@...ches.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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@...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

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.