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

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Message-ID: <da893e74179ec301ef41d7d240224d2eb824d2a8.camel@perches.com>
Date:   Tue, 21 Aug 2018 10:22:45 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.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, 2018-08-21 at 09:57 -0700, Nick Desaulniers wrote:
> 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.

Can you enumerate these dependencies please.

> > What features are not supported by the minimum clang version?

And enumerate this too please.

Uglifying the compiler-gcc file to support
clang features that can be overridden in
compiler-clang.h instead is not a good way
to fix compilation for clang.

But still, whatever features in the various
clang versions that need accommodation also
need listing out in compiler-clang.h, not in
compiler-gcc.h

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

Thanks, but I know this.

The question remains, if clang can't compile
v4.17, why does it immediately matter for v4.19?

Why wouldn't overriding the clang __GNUC_<foo>
#defines in compiler-gcc.h work acceptably with
adding whatever is necessary to compiler-clang.h?

I'll experiment with a compiler-clang.h patch.