Re: Linux 4.19 and GCC 9

From: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Ignat Korchagin <ignat@cloudflare.com>,
	Ivan Babrou <ivan@cloudflare.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	kernel-team <kernel-team@cloudflare.com>
Subject: Re: Linux 4.19 and GCC 9
Date: Mon, 10 Jun 2019 17:04:43 +0200	[thread overview]
Message-ID: <CANiq72maZcuMXQoSDPhY=+4UKUFPwbQLuGsqcoPX-feCerbBtw@mail.gmail.com> (raw)
In-Reply-To: <20190610144858.GA1481@kroah.com>

On Mon, Jun 10, 2019 at 4:49 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Mon, Jun 10, 2019 at 04:42:27PM +0200, Miguel Ojeda wrote:
> > I think Josh Poimboeuf added support for a few related things in GCC 8
> > (e.g. 13810435b9a7 ("objtool: Support GCC 8's cold subfunctions")).
>
> That commit is already in all stable releases, so does there need to be
> a gcc 9 specific one?

Ah, you are right. Hm... I recall discussing this at some point when
cleaning up for GCC 9, let me take a look...

> > I typically compile a bare-bones GCC for those things, it is quite quick.
>
> Pointers to how to do that is appreciated.  It's been years since I had
> to build gcc "from scratch".

We crossed emails -- I copy it here on-list in case someone else wants it :-)

mkdir ${BUILD_PATH} \
    && cd ${BUILD_PATH} \
    && ${REPO_PATH}/configure \
        --enable-languages=c,c++ \
        --disable-multilib \
        --disable-bootstrap \
        --disable-nls \
        --prefix=${INSTALL_PATH} \
    && time make \
    && time make install

The key thing is disabling bootstrap to have a single-pass compilation
-- that speeds up things a lot (although, of course, you get less
testing). I would guess you can also disable c++ for the kernel for
even a faster build. You can also use -j, but even without it it does
not really take long (1 hour? -- way less than LLVM anyway). The
installed size nowadays is about 1 GiB.

To get the actual code, I use the git mirror in github for speed
(although it is not that big anyway):

  git clone https://github.com/gcc-mirror/gcc

The tags you want to use look like:

  gcc-9_1_0-release

Also, the first time you run this, you may need some extra dependencies:

  libgmp-dev (maybe named gmp-devel)
  libmpfr-dev (mpfr-devel)
  libmpc-dev (libmpc-devel)

Unless your distro is quite old, I think they should simply work (a
few years ago I had to compile those manually, but nowadays I don't
seem to need them anymore).

Cheers,
Miguel