Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Thu, 5 Apr 2018 09:24:01 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Matthias Kaehlcke <mka@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        James Y Knight <jyknight@google.com>,
        Chandler Carruth <chandlerc@google.com>,
        Stephen Hines <srhines@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@google.com>,
        Guenter Roeck <groeck@chromium.org>,
        Greg Hackmann <ghackmann@google.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17
Message-ID: <20180405072401.GO4043@hirez.programming.kicks-ass.net>
References: <20180404191724.GF87376@google.com>
 <CAK8P3a1VFWYBymd3AeG1DwW++tCGsP9+vw6t8XSGHby2jmQPqA@mail.gmail.com>
 <20180404205848.GG87376@google.com>
 <CAK8P3a2tf3v6seq=OeyBakZ1ZXYzzL=tY=aHvKYv-GtD7sWeTA@mail.gmail.com>
 <20180404214639.GH87376@google.com>
 <CA+55aFwp8vRmf5p09nbunp6DfO+YvvWLj=FsU8KSW0ZwF6u80w@mail.gmail.com>
 <20180404221744.GI87376@google.com>
 <CA+55aFz+RcyPOTs3WM-FcwoQtgaotcBVqswma7qZ-3O2Fv__9Q@mail.gmail.com>
 <20180404233111.GJ87376@google.com>
 <CA+55aFyHzKg3GzTdW4Z+xC5u2FZ8HnduNFi3PYVTGoKCqpR6Dg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFyHzKg3GzTdW4Z+xC5u2FZ8HnduNFi3PYVTGoKCqpR6Dg@mail.gmail.com>
User-Agent: Mutt/1.9.3 (2018-01-21)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Apr 04, 2018 at 05:05:25PM -0700, Linus Torvalds wrote:
> for some reason the test_bit() case looks like
> this:
> 
>   #define test_bit(nr, addr)                      \
>         (__builtin_constant_p((nr))             \
>          ? constant_test_bit((nr), (addr))      \
>          : variable_test_bit((nr), (addr)))
> 
> which is much more straightforward anyway. I'm not quite sure why we
> did it that odd way anyway, but I bet it's just "hysterical raisins"
> along with the test_bit() not needing inline asm at all for the
> constant case.

I always assumed BT was a more expensive instruction than AND with
immediate.