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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
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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.