linux-kernel - Re: [tip:x86/mce] x86/bitops: Move BIT

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Message-ID: <CA+55aFx9pgAQEFuNyANpHA6OcBL8LyHwknPUOb4=ysjYd4cnwQ@mail.gmail.com>
Date:	Wed, 23 May 2012 09:40:40 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Borislav Petkov <borislav.petkov@....com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, frank.arnold@....com,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mce] x86/bitops: Move BIT_64() for a wider use

On Wed, May 23, 2012 at 9:19 AM, Borislav Petkov
<borislav.petkov@....com> wrote:
>
> Actually we need a BIT() macro that works both
> on 32- and 64-bit. But that won't be that easy:

We could use __builtin_choose_expr(), but that *only* works with constants.

So we could do this:

  static inline unsigned long bit(unsigned int x)
  {
      return 1ul << x;
  }

  static inline u64 bit64(unsigned int x)
  {
      return 1ull << x;
  }

  #define BIT(x) \
    __builtin_choose_expr((x) < 8*sizeof(unsigned long), bit(x), bit64(x))

but then you *have* to use a plain constant for the BIT() macro.
Anything else will error out in a big way. Non-constant users would
have to be modified to use bit() and bit64() instead.

And no, I tested. You apparently can't do

  #define __is_longlongshift(x) \
        (__builtin_constant_p(x) && (x) < 8*(sizeof(long)))

because while that is a compile-time constant expression, it's not
"constant enough" for __builtin_choose_expr().

                   Linus
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