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;
MIME-Version: 1.0
In-Reply-To: <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>
 <20180405072401.GO4043@hirez.programming.kicks-ass.net>
From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 5 Apr 2018 09:43:40 -0700
Message-ID: <CA+55aFzJ9iv6S2hczHjPS3kckWOrhfWvjNADdjivO6N7u5YNhQ@mail.gmail.com>
Subject: Re: [GIT PULL] x86/build changes for v4.17
To:     Peter Zijlstra <peterz@infradead.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>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Apr 5, 2018 at 12:24 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> I always assumed BT was a more expensive instruction than AND with
> immediate.

Oh, absolutely. That's why we do all those "depending on immediate or not".

The reason I brought that case up is that "test_bit()" and "set_bit()"
do this "is it constant" test COMPLETELY DIFFERENTLY.

The test_bit() one is arguably much more legible, and easier to
understand. And it so happens that clang will see that it's constant
because it's a macro (well, unless that macro is then used in an
inline function).

The set_bit() pattern looks completely different, and doesn't have
that abstraction of "constant_set_bit()" vs "variable_set_bit()", like
test_bit() does.

THAT was why I pointed it out - we do different things otherwise
similar operations.

Not because it would be odd that we do different things for the
"constant bit number" vs "variable bit number".

               Linus