linux-kernel - Re: [PATCH 3/6] bitops: define gen_test_bit() the same way as the rest of functions

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Date:   Mon, 6 Jun 2022 17:19:42 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Alexander Lobakin <alexandr.lobakin@...el.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Richard Henderson <rth@...ddle.net>,
        Matt Turner <mattst88@...il.com>,
        Brian Cain <bcain@...cinc.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>,
        "David S. Miller" <davem@...emloft.net>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-alpha@...r.kernel.org, linux-hexagon@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
        linux-sh@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] bitops: define gen_test_bit() the same way as the
 rest of functions

On Mon, Jun 06, 2022 at 01:49:04PM +0200, Alexander Lobakin wrote:
> Currently, the generic test_bit() function is defined as a one-liner
> and in case with constant bitmaps the compiler is unable to optimize
> it to a constant. At the same time, gen_test_and_*_bit() are being
> optimized pretty good.
> Define gen_test_bit() the same way as they are defined.
> 
> Signed-off-by: Alexander Lobakin <alexandr.lobakin@...el.com>

Regardless of whether compilers prefer this, I think it's nicer to have the
structure consistent with the rest of the functions, so FWIW:

Acked-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  include/asm-generic/bitops/generic-non-atomic.h | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/include/asm-generic/bitops/generic-non-atomic.h b/include/asm-generic/bitops/generic-non-atomic.h
> index 7a60adfa6e7d..202d8a3b40e1 100644
> --- a/include/asm-generic/bitops/generic-non-atomic.h
> +++ b/include/asm-generic/bitops/generic-non-atomic.h
> @@ -118,7 +118,11 @@ gen___test_and_change_bit(unsigned int nr, volatile unsigned long *addr)
>  static __always_inline int
>  gen_test_bit(unsigned int nr, const volatile unsigned long *addr)
>  {
> -	return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1)));
> +	const unsigned long *p = (const unsigned long *)addr + BIT_WORD(nr);
> +	unsigned long mask = BIT_MASK(nr);
> +	unsigned long val = *p;
> +
> +	return !!(val & mask);
>  }
>  
>  #endif /* __ASM_GENERIC_BITOPS_GENERIC_NON_ATOMIC_H */
> -- 
> 2.36.1
>