linux-kernel - Re: [PATCH 06/13] lib: extend the scope of small_const

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Date:   Tue, 16 Mar 2021 09:56:49 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org
Cc:     linux-m68k@...ts.linux-m68k.org, linux-arch@...r.kernel.org,
        linux-sh@...r.kernel.org, Alexey Klimov <aklimov@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>, David Sterba <dsterba@...e.com>,
        Dennis Zhou <dennis@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Jianpeng Ma <jianpeng.ma@...el.com>,
        Joe Perches <joe@...ches.com>,
        John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rich Felker <dalias@...c.org>,
        Stefano Brivio <sbrivio@...hat.com>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>
Subject: Re: [PATCH 06/13] lib: extend the scope of small_const_nbits() macro

On 16/03/2021 02.54, Yury Norov wrote:
> find_bit would also benefit from small_const_nbits() optimizations.
> 
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  include/asm-generic/bitsperlong.h | 9 +++++++++
>  include/linux/bitmap.h            | 3 ---
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
> index 3905c1c93dc2..96032f4f908f 100644
> --- a/include/asm-generic/bitsperlong.h
> +++ b/include/asm-generic/bitsperlong.h
> @@ -23,4 +23,13 @@
>  #define BITS_PER_LONG_LONG 64
>  #endif
>  
> +#define SMALL_CONST(n) (__builtin_constant_p(n) && (unsigned long)(n) < BITS_PER_LONG)
> +
> +/*
> + * The valid number of bits for a bitmap to be small enough, or in other words,
> + * fit into a single machine word is 1 to BITS_PER_LONG inclusively. 0 is not a
> + * valid number for size, and most probably a sing of error.
> + */
> +#define small_const_nbits(n) SMALL_CONST((n) - 1)
> +

I still don't see the point of introducing SMALL_CONST and still don't
like the much-too-generic-name - especially since AFAICT you don't
actually use it anywhere outside the definition of small_const_nbits()?

>  #endif /* __ASM_GENERIC_BITS_PER_LONG */
> diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> index adf7bd9f0467..bc13a890ecc1 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -224,9 +224,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
>   * so make such users (should any ever turn up) call the out-of-line
>   * versions.
>   */

You added another comment near its new definition, but the left-behind
comment in bitmap.h is now somewhat confusing, no? I suggest expanding
your new comment a bit so it's clear why we're interested in whether a
bitmap is known at compile-time to consist of exactly one word:

/*
small_const_nbits(n) is true precisely when it is known at compile-time
that BITMAP_SIZE(n) is 1, i.e. 1 <= n <= BITS_PER_LONG. This allows
various bit/bitmap APIs to provide a fast inline implementation. Bitmaps
of size 0 are very rare, and a compile-time-known-size 0 is most likely
a sign of error. They will be handled correctly by the bit/bitmap APIs,
but using the out-of-line functions, so that the inline implementations
can unconditionally dereference the pointer(s).
*/

> -#define small_const_nbits(nbits) \
> -	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
> -
>  static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
>  {
>  	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
> 

Rasmus