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Date:   Thu, 5 Apr 2018 09:24:01 +0200
From:   Peter Zijlstra <>
To:     Linus Torvalds <>
Cc:     Matthias Kaehlcke <>,
        Arnd Bergmann <>, Ingo Molnar <>,
        Linux Kernel Mailing List <>,
        Thomas Gleixner <>,
        Andrew Morton <>,
        James Y Knight <>,
        Chandler Carruth <>,
        Stephen Hines <>,
        Nick Desaulniers <>,
        Kees Cook <>,
        Guenter Roeck <>,
        Greg Hackmann <>,
        Greg Kroah-Hartman <>
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

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