linux-kernel - Re: [GIT PULL] x86/build changes for v4.17

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Message-ID: <20180405072401.GO4043@hirez.programming.kicks-ass.net>
Date:   Thu, 5 Apr 2018 09:24:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        James Y Knight <jyknight@...gle.com>,
        Chandler Carruth <chandlerc@...gle.com>,
        Stephen Hines <srhines@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Guenter Roeck <groeck@...omium.org>,
        Greg Hackmann <ghackmann@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [GIT PULL] x86/build changes for v4.17

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.