linux-kernel - Re: [PATCH 04/13] lib: introduce BITS

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Date:   Tue, 16 Mar 2021 09:35:35 +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 04/13] lib: introduce BITS_{FIRST,LAST} macro

On 16/03/2021 02.54, Yury Norov wrote:
> BITMAP_{LAST,FIRST}_WORD_MASK() in linux/bitmap.h duplicates the
> functionality of GENMASK(). The scope of BITMAP* macros is wider
> than just bitmaps. This patch defines 4 new macros: BITS_FIRST(),
> BITS_LAST(), BITS_FIRST_MASK() and BITS_LAST_MASK() in linux/bits.h
> on top of GENMASK() and replaces BITMAP_{LAST,FIRST}_WORD_MASK()
> to avoid duplication and increase the scope of the macros.
> 
> This change doesn't affect code generation. On ARM64:
> scripts/bloat-o-meter vmlinux.before vmlinux
> add/remove: 1/2 grow/shrink: 2/0 up/down: 17/-16 (1)
> Function                                     old     new   delta
> ethtool_get_drvinfo                          900     908      +8
> e843419@...2_0001309d_7f0                      -       8      +8
> vermagic                                      48      49      +1
> e843419@...5_000138bb_f68                      8       -      -8
> e843419@...9_00012bce_198c                     8       -      -8

[what on earth are those weird symbols?]


> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 7f475d59a097..8c191c29506e 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -37,6 +37,12 @@
>  #define GENMASK(h, l) \
>  	(GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
>  
> +#define BITS_FIRST(nr)		GENMASK((nr), 0)
> +#define BITS_LAST(nr)		GENMASK(BITS_PER_LONG - 1, (nr))
> +
> +#define BITS_FIRST_MASK(nr)	BITS_FIRST((nr) % BITS_PER_LONG)
> +#define BITS_LAST_MASK(nr)	BITS_LAST((nr) % BITS_PER_LONG)

I don't think it's a good idea to propagate the unusual closed-range
semantics of GENMASK to those wrappers. Almost all C and kernel code
uses the 'inclusive lower bound, exclusive upper bound', and I'd expect
BITS_FIRST(5) to result in a word with five bits set, not six. So I
think these changes as-is make the code much harder to read and understand.

Regardless, please add some comments on the valid input ranges to the
macros, whether that ends up being 0 <= nr < BITS_PER_LONG or 0 < nr <=
BITS_PER_LONG or whatnot.

It would also be much easier to review if you just redefined the
BITMAP_LAST_WORD_MASK macros etc. in terms of these new things, so you
wouldn't have to do a lot of mechanical changes at the same time as
introducing the new ones - especially when those mechanical changes
involve adding a "minus 1" everywhere.

Rasmus