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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Thu, 20 Jun 2019 13:25:36 -0700
Message-ID: <CAKwvOd=twuZAAyKsBRSeJEFuQZGdyTw+=JAwmJugUhV+bppdtg@mail.gmail.com>
Subject: Re: [PATCH] gen_compile_command: Add support for separate
 KBUILD_OUTPUT directory
To:     Doug Anderson <dianders@google.com>
Cc:     Matthias Kaehlcke <mka@chromium.org>,
        Tom Roeder <tmroeder@google.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Raul E Rangel <rrangel@chromium.org>,
        Tom Hughes <tomhughes@chromium.org>,
        Ryan Case <ryandcase@chromium.org>,
        Yu Liu <yudiliu@google.com>,
        Nathan Chancellor <natechancellor@gmail.com>
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On Thu, Jun 20, 2019 at 1:13 PM Doug Anderson <dianders@google.com> wrote:
> On Thu, Jun 20, 2019 at 12:53 PM Nick Desaulniers
> <ndesaulniers@google.com> wrote:
> >
> > I do miss Doug's Kbuild caching patches' speedup.
>
> You actually get quite a bit of this by grabbing a new version of
> ccache (assuming you use ccache).  :-P  You still have to pay the
> penalty (twice) for all the options that are tested that the compiler
> _doesn't_ support, but at least you get the cache for the commands
> that the compiler does support.

Hello darkness my old friend:
https://nickdesaulniers.github.io/blog/2018/06/02/speeding-up-linux-kernel-builds-with-ccache/
Man, that post has not aged well.  Here's what we do now:
https://github.com/ClangBuiltLinux/continuous-integration/blob/45ab5842a69cb0c72d27d34e73b0599ec2a0e2ed/driver.sh#L227-L245

> Specifically, make sure you have a ccache with:
>
>     * https://github.com/ccache/ccache/pull/365
>     * https://github.com/ccache/ccache/pull/370

Oh! Interesting finds and thanks for the pointers.  Did these make it
into a release version of ccache, yet? If so, do you know which
version?

> I still have it in my thoughts to avoid the penalty for options that
> the compiler doesn't support but haven't had time to work on it
> recently.

It had better not be autoconf! (Hopefully yet-to-be-written GNU C
extensions can support feature detection via C preprocessor)
-- 
Thanks,
~Nick Desaulniers