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

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Message-ID: <CAKwvOdkgtoNJuagT_uoNnDKcxHAjsvfWo9hQMP2OKz9_qpstNQ@mail.gmail.com>
Date:   Tue, 21 Aug 2018 09:57:11 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     joe@...ches.com
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>, dwmw@...zon.co.uk,
        LKML <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, Aug 21, 2018 at 9:33 AM Joe Perches <joe@...ches.com> wrote:
>
> 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?

Depends on the architecture and which kernel version/LTS branch you're
using.  I'm trying to backport fixes to LTS branches, but sometimes a
compiler upgrade is required.  I know that's not great, but I'm
actively trying to fix it.

> 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.
>

See commit e501ce9 ("x86: Force asm-goto").

$ git describe --contains e501ce9 | sed 's/~.*//'
v4.17-rc1

-- 
Thanks,
~Nick Desaulniers